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DECEMBER 2024 ISSN 1030-2662 12 9 771030 266001 $ 00* NZ $1390 The VERY BEST DIY Projects! 13 INC GST INC GST Compact HiFi headphone Amplifier 🎵 🎵 🎵 THE PICO COMPUTER using a Raspberry Pi Pico 1W into 16Ω 3.5mm & 6.5mm headphone jack Class-AB operating mode 9-12V AC plugpack Undersea Communications The vast underwater fibre-optic cable network. Capacitor Discharger A great piece of gear for safely discharging small & large capacitors. Raspberry Pi Pico 2 We review the newest Raspberry Pi Pico 2 which is priced at around $8 each. ...and even more inside this issue Win a DHO-924S Oscilloscope see page 9 for details and enter before December 13th for a chance to win! www.jaycar.com.au Contents Vol.37, No.12 December 2024 14 Undersea Communications Undersea cables carry nearly all international internet traffic, making them one of the most important parts of the internet. As we explain, there is a lot more to them than you may realise! By Dr David Maddison, VK3DSM Communications technology 38 Precision Electronics, Part 2 This series covers the basics of precision electronics design. Building on from last month, we aim to improve the precision of our example circuit. In doing so, we investigate many features of precision op amps. By Andrew Levido Electronic design 62 Raspberry Pi Pico 2 The new Raspberry Pi Pico 2 is priced around $8 and uses an RP2350 micro. Compared to the previous iteration, it boasts double the RAM, faster clock speeds and other features like a hardware random number generator. Review by Tim Blythman Microcontroller board 98 MicroBee 256TC Restoration The MicroBee 256TC is a computer kit from 1987 and the last of the original MicroBee computers. This article details the work needed to restore one of these old devices. By Don Peterson Vintage computers UNDERSEA COMMUNICATIONS DATA TRANSMISSION & POWER CABLES Feature, Page 14 Project, Page 33 Capacitor Discharger Discharg er Precision Electronics Part 2 – Page 38 2 Editorial Viewpoint 5 Mailbag 13 Subscriptions 26 Circuit Notebook 52 Mini Projects 86 Online Shop 88 Serviceman’s Log 94 Vintage Electronics 109 Ask Silicon Chip 78 Variable Speed Drive Mk2, Part 2 111 Market Centre Our Variable Speed Drive can drive single-phase shaded pole or permanent split capacitor induction motors, as well as three-phase 230V induction motors, up to 1.5kW. We cover the construction, testing and how to use it. By Andrew Levido Motor speed control project 112 Advertising Index 112 Notes & Errata 33 Capacitor Discharger Safely discharge capacitors, both large and small, including capacitors used to store rectified mains (up to ~400V DC). It is especially helpful when servicing switch-mode power supplies and valve gear. By Andrew Levido Safety equipment project 44 Compact HiFi Headphone Amp Our new Headphone Amplifier is easy to build, fairly priced and most importantly, sounds great! It’s powered via a 9-12V AC plugpack and delivers up to 0.9W into 8Ω, 1W into 16Ω and 140mW into 600Ω. Part 1 by Nicholas Vinen Audio project 66 The Pico Computer Turn a Raspberry Pi Pico into a standalone computer with a USB keyboard and HDMI monitor. All the Pico’s I/O pins are broken out on a separate header, making it easy to use for controlling other circuitry. By Tim Blythman Computer project 1. Regulated negative supply with a 555 2. Four-cell voltage monitor 3. Over-temperature alarm 4. HT supply generator 1. Automatic night light 2. WiFi weather logger Dallas Arbiter Fuzz Face guitar pedal by Brandon Speedie SILICON SILIC CHIP www.siliconchip.com.au Publisher/Editor Nicholas Vinen Technical Editor John Clarke – B.E.(Elec.) Technical Staff Bao Smith – B.Sc. Tim Blythman – B.E., B.Sc. Advertising Enquiries (02) 9939 3295 adverts<at>siliconchip.com.au Regular Contributors Allan Linton-Smith Dave Thompson David Maddison – B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Geoff Graham Associate Professor Graham Parslow Dr Hugo Holden – B.H.B, MB.ChB., FRANZCO Ian Batty – M.Ed. Phil Prosser – B.Sc., B.E.(Elec.) Cartoonist Louis Decrevel loueee.com Founding Editor (retired) Leo Simpson – B.Bus., FAICD Silicon Chip is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 626 922 870. ABN 20 880 526 923. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Subscription rates (Australia only) 6 issues (6 months): $70 12 issues (1 year): $130 24 issues (2 years): $245 Online subscription (Worldwide) 6 issues (6 months): $52.50 12 issues (1 year): $100 24 issues (2 years): $190 For overseas rates, see our website or email silicon<at>siliconchip.com.au * recommended & maximum price only Editorial office: Unit 1 (up ramp), 234 Harbord Rd, Brookvale, NSW 2100. Postal address: PO Box 194, Matraville, NSW 2036. Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: 24-26 Lilian Fowler Pl, Marrickville 2204 2 Silicon Chip Editorial Viewpoint Printer ink costs more than gold! Many printer companies have been milking their customers for years. Did you realise that many printers, especially ink jets, are sold below their cost? The goal is to lock you into buying their overpriced ink, making you spend more (a lot more) in the long run. I bet if this was made clear to prospective buyers, they would be a lot less interested in those ‘cheap’ printers. If you open up a $30+ printer cartridge for an entrylevel printer, you might find a few millilitres of ink, if that. If you calculate the cost per weight, it’s more than gold! This has given rise to a large third-party ink industry. Third-party cartridges can be a fraction of the cost of the ‘official’ ones and, in my experience, work just as well – at least for day-to-day tasks like printing letters, invoices, bills, contracts etc. But the printer companies do everything they can to make it impossible for you to use those cartridges. For years now, they have been incorporating encryption chips in their cartridges to prevent third parties from making compatible devices. It hasn’t really worked, but they certainly have tried. This is one of the reasons that if you have a printer, and it works, you should never ‘upgrade’ its firmware. Most firmware ‘upgrades’ for printers are actually just attempts to block your use of third-party ink, and it should be your choice which ink you use in the printer you paid for. I have ignored the “please upgrade the firmware” messages on our printers for years now and have thankfully had no problem using third-party ink. Until recently, for our home printer, I was paying $8 for a full set of four CMYK cartridges, including delivery! The price has now gone up to about $12, but it’s still a great deal compared to – I kid you not – $150 for the printer brand equivalent. What a ripoff! Who would even consider paying that much? You’d have to be desperate! That’s for a set of cartridges that would last you maybe a couple of months of modest usage. If you do a lot of printing and want to use an ink jet, you’re better off buying one of the ink tank printers. They cost more up-front, of course, but the ink lasts a really long time (years), there’s no ink ‘DRM’ and even the genuine ink is not that expensive. So they are a good option, although I hear that they are not without their problems. Apparently, Brother laser printers are a good choice, especially if you don’t need colour. Thermal label printers aren’t much better, except the ‘DRM’ isn’t on the ink (because there is none), it’s on the paper. Older label printers are actually worth a lot of money on the second-hand market (and are hard to find!) because you can use any paper you want, and again, the third-party paper is a fraction the cost of the paper you get from the original brand. I sincerely hope our label printers don’t pack it in because I would refuse to pay the extortionate prices that those companies charge for their labels. Sure, the quality of the originals is a little better than the third-party ones, but when you’re using them as shipping labels, who cares? We pay $60 for 8 rolls of 220 labels for our printer, which works out to 3.4¢ per label – that seems reasonable. The original brand labels have an RRP of $60 for a single roll of 220, or over 27¢ per label. Again, who would pay that? That’s a higher cost per unit than the packaging we put the stickers on! I might pay double the price for genuine labels, but eight times as much? Forget it! We have two interesting Vintage articles this month. Next issue, we’ll be back to the usual Vintage Radio. by Nicholas Vinen Australia's electronics magazine siliconchip.com.au The key to unrestricted access Explore millions of components for your next design Although a trek to this active volcanic island is strictly prohibited, you have unlimited access to millions of electronic components, from well over a thousand leading brands engineers know and trust. Discover the hottest components for design. 03 9253 9999 | australia<at>mouser.com MAILBAG your feedback Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd has the right to edit, reproduce in electronic form, and communicate these letters. This also applies to submissions to “Ask Silicon Chip”, “Circuit Notebook” and “Serviceman’s Log”. Mains wiring colours should be standardised worldwide I was reading the comments in Mailbag about old German wiring using red for Neutral and Earth and connecting them together at the outlet. It’s a good job that they discontinued that idea. Very strange that they would use red for something benign when red is more of a colour to show danger, like Australian wiring using red for Active. Then you have wiring in the US where they use white for Neutral and black for Active and only use red for the second Active. I have come across some imported equipment with white wire for Neutral and black for Active. This is more common than you’d think. It’s unfortunate that there isn’t an international standard for wiring colours, so that, no matter where you go, the wiring would all be done in the same colours. Bruce Pierson, Dundathu, Qld. Comment: we agree that Active should be a bright colour to warn you! Even the brown for Active scheme originating from Europe in IEC 60446 that we now use is a bit of an odd choice, but at least it’s unusual to see low-voltage wiring in brown. We can’t say the same for black. A worldwide standard would be a good idea. Maybe it will happen around the time the USA switches to metric! Simulating fluid dynamics with SPICE I was interested in your remarks in the November 2024 editorial about analogies between fluidic and electrical systems. In some past design work I did in building phaco machines for cataract extraction, I created some original research to explain the fluidic systems in these machines. I had to model the properties of the plastic (elastic tubing), the elastic nature of the eye (analogous to capacitance) and flow resistances related to both laminar and turbulent flow in the small apertures of the system. I also had to consider the effect of fluidic inertia, analogous to inductance. ourPCB LOCAL SERVICE <at> OVERSEAS PRICES AUSTRALIA PCB Manufacturing Full Turnkey Assembly Wiring Harnesses Solder Paste Stencils small or large volume orders premium-grade wiring low cost PCB assembly laser-cut and electropolished Instant Online Buying of Prototype PCBs www.ourpcb.com.au siliconchip.com.au Australia's electronics magazine 0417 264 974 December 2024  5 In cataract surgery, the fluid fed to the eye is from a bottle gravity feed via a 1/8-inch (3.175mm) internal diameter plastic irrigation tubing and a needle or annulus around the phaco needle, to enter the eye’s anterior chamber. The fluid, along with fragments of the natural lens, are extracted from the eye via the phaco needle’s mouth and 1/16-inch (~1.6mm) internal diameter tubing to the machine’s vacuum pump. The pumps are either peristaltic or venturi types. The probe’s phaco needle oscillates at around 20– 40kHz (using piezo crystals) to fragment the lens. Otherwise, the flow would be blocked by lens fragments that are too big to be aspirated into the phaco needle’s mouth. The problem is that during the operation, the flow stops and starts at times due to occlusion and then the occlusion breaking, with lens fragments at the phaco needle’s mouth. That leads to instability of the volume of the eye and the anterior chamber area where the surgeon is working. That can result in significant tissue damage in some cases, with the phaco needle lacerating the lens capsule or iris. The instability relates to the elastic nature of the structures, the inertia of the fluid and the limitation of the irrigation pathway (flow resistances). The thing was, there was no hydraulic modelling software in existence to show the pressure conditions in the eye’s anterior chamber during flow occlusions or flow-starting events in cataract surgery. So I decided to model the entire scenario as its electronic equivalent circuit. I was able to convert all the physical parameters like the flow resistances of the needles & tubing, the compliance of the plastic tubing and the eye, the properties of the machine’s pump etc to electrical equivalents. I could then simulate the entire thing in a SPICE engine (Anasoft in the UK gave me permission to use it for the application). As far as I know, it had never been done before for this system, and it led to important insights to help set the machine’s parameters and irrigation bottle height for safer surgery and a more stable anterior chamber. One interesting finding was that fluid-filled elastic tubing behaves exactly as a coaxial transmission line does with applied transients, with a characteristic velocity and reflection effects when an impedance bump is encountered. This is because the elastic wall tubing has compliance (capacitance) and fluid inertia (inductance) distributed along its length. Similar phenomena have been noted in hydraulics and are referred to as the ‘water hammer’ effect. I wouldn’t expect that Silicon Chip readers would want to read the whole thing, but they might be interested in some parts of it. The whole paper can be downloaded from www.worldphaco.com/uploads/APHACOBOOK.pdf Also, regarding the article on Maxwell’s Equations in the same issue; at that point in history, around 1865, light was ill understood. However, the speed of light had been measured previously by experimentation. Maxwell’s equations led to the calculation of the speed of the electromagnetic wave, which turned out to exactly match the known speed of light. To quote from one book (Principles of Electricity by Page & Adams, 1931), in the section on Maxwell’s equations, “The inference that light itself is electromagnetic in character is inescapable”. Maxwell’s equations basically 6 Silicon Chip lifted the lid on light and finally explained what it was. At the time, that fact was a monumental scientific discovery. Dr Hugo Holden, Buddina, Qld. Unbuilt kits to give away Since May 1948, I have read and enjoyed learning from the predecessor magazines and now Silicon Chip, and have built some of the projects. I bought the kits for the LP Doctor, the Studio Preamp (including the chassis) and the Hifi Headphone Amplifier. I have not opened the packages, and I would like to give them to anyone who would find them useful; hopefully, someone in Adelaide. Les Howard, Coromandel Valley, SA. Comment: if any readers are interested, please send us an email and we’ll pass it on to Les. OLED Clock & Timer upgrade suggestion Congratulations on yet another ingenious creation. I have spent some time studying the article. I always find each issue interesting and informative. The reason I am writing to you is to propose a change to the way the countdown timer operates, which I think would broaden the appeal of the device. Optionally, after the countdown interval has been specified and the countdown has commenced, when the countdown interval is exhausted and the chime sounds, the countdown interval is automatically restored, and the countdown starts again. It will thus produce a continuous sequence of chimes separated by the countdown interval with no intervention required. For example, if the countdown interval is set to 60 seconds, a train of chimes spaced one minute apart would be generated. The sequence could be interrupted (and recommenced) by depressing the ‘down’ button, as currently is the case. I realise implementing this would require adding a flag to the countdown timer setting screen for ‘continuous’ or ‘once only’ and this does not have a lot of spare space, as well as requiring a fair bit of programming change to the timer logic. I have an old and soon-to-fail sports watch with this countdown function, and find it very handy when exercising. David Jane, Umina Beach, NSW. Comment: That is quite a good idea. We will investigate if it is feasible to add this feature. If so, it could appear as a brief Circuit Notebook entry. Praise for the base model MG4 I’m writing in response to Julian Edgar’s review of the MG4 XPower EV in the October 2024 issue (siliconchip. au/Article/16670). At only $31k now, I’ve found the base model MG4, with its 320km range, adequate for rural use. The saved $30k makes a good down-payment on an off-grid solar power system. In the first six months. I’ve put a MWh of 100% off-grid fossil-free energy into the EV, for around 5000km of highway driving. The rear-wheel drive, low centre of gravity, and wide stable stance makes it a delight to drive, and zooming along on pure planet-saving photons – Oh what a feeling! A nephew said, “It’s like a spaceship”. It does have a galactic windscreen, and techy dashboard screens. The second driver’s screen is a must, I feel. Australia's electronics magazine siliconchip.com.au FREE Download Now! Introducing DaVinci Resolve 19 Edit and color correct video with the same software used by Hollywood, for free! DaVinci Resolve is Hollywood’s most popular software! Now it’s easy to create feature film quality videos by using professional color correction, editing, audio and visual effects. Because DaVinci Resolve is free, you’re not locked into a cloud license so you won’t lose your work if you stop paying a monthly fee. There’s no monthly fee, no embedded ads and no user tracking. Creative Color Correction Editing, Color, Audio and Effects! 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Learn the basics for free then get more creative control with our accessories! so you are learning advanced skills used in TV and film� www.blackmagicdesign.com/au Learn More! NO SUBSCRIPTIONS • NO ADS • NO USER TRACKING • NO AI TRAINING “Beware! The Loop”, a book by Jim Sinclair on the what-if time travel was possible Tom Marsden, aka “Time Warp Tommy”, is asked to investigate the circumstances surrounding the disappearance of a military scientist experimenting with time travel in a small country in the middle of Asia. What he finds will shock you! Time Warp Tommy is asked to explain how the world’s greatest expert on time travel has built a time machine, climbed into it and disappeared into a grey haze. With only two co-workers left behind and a pile of hand-written notes and diagrams, Time Warp Tommy must devise a way in which the experiment can be safely ended. With the help of his highly intelligent daughter Emily, what they discover will lead them into a web of drama and intrigue, danger and distrust. When the co-workers are charged with their superior’s murder, Time Warp Tommy must explain the science to the judges in order to save their lives. But time is running out. Beware! The Loop has many twists and turns, facts and figures that inspires your imagination. Purchase it for just $5.50: https://moonglowpublishing. com.au/store/p48/bewarethe-loop-jim-sinclair Beware! The Loop is available as an EPUB, MOBI and PDF RRP $5.50 | available as an EPUB, MOBI and PDF 8 Silicon Chip E-ISBN 9780645945669 The base model’s LiFePO4 battery is not only safer than the Li-ion type of the more expensive models, but it has no 20%-80% state-of-charge (SoC) restriction. I charge it to 100% almost all the time, as is the norm for LiFePO4. Notably, the 64kWh battery of the middle model is only 51kWh at 80% SoC – exactly the same as the base model at 100%. And the radar auto-separation from leading car in cruise control mode is well tuned for the base model. A light (164kg) 2m trailer makes the most of the limited 500kg towing capacity, and makes up for the MG4 not being a ute. I can still go down the paddock for firewood, 117mm ground clearance permitting. The intelligent speed limit mode can save thousands in speeding fines, but school zones in the middle of the day can then be an irritation. With 27kW of solar panels, 24kW of inverters and 46kWh of DIY LiFePO4 battery banks, off-grid charging of the EV at 7.2kW has been effortless, even in the depths of winter. Adding 43km of range per hour is fine at home – a 64km town trip is made up by the time the groceries are stowed and a load of washing done. OK, the “surplus solar” smarts of the EV charger don’t work, despite a firmware upgrade, but manual modulation with the phone app suffices in practice, given the substantial house battery. Apropos environmental footprint, figures from Renew magazine suggest I have to drive 23,000km on fossil-free photons to recoup the extra energy debt of EV manufacture – that’s another two years. I kept the previous car for 24 years, and EVs last longer. When grid-scale batteries and a bit more solar and wind free the grid of fossil fuels, that penalty calculation zeroes out, as manufacture becomes emissions-free. I have yet to try a fast charger. Where do you find one that takes credit card? In Gippsland? (I won’t have payment guff on my phone, thank you very much.) But battery life is unlikely to be a concern, at least for LiFePO4. There are increasing reports of over 80% residual range after half a million km. The battery is very likely to outlast the car. Tech advancement will add extra range to new models, so delaying purchase a year or two can only get you more for less (and there will be more fast chargers – the critical bottleneck now). There are a couple of photos of my off-grid solar system and MG4 in the second story from the ABC at www.abc. net.au/news/103679116 Erik Christiansen, Munro, Vic. Comment: the LiFePO4 battery certainly should make it a more attractive option for those concerned about either the safety or longevity of a Li-ion battery, and the price is pretty attractive. Mystery of French TV pioneers solved I noticed that in your series on the History of Electronics (October-December 2023; siliconchip.au/Series/404), you listed the dates of birth and death of most of the inventors. However, in the second part on page 25, the dates for French television pioneer Georges Rignoux was listed as ~1885-unknown and no dates were listed for his partner, A. Fournier. I did some research and found a French language document that may help solve this mystery, and also that of Fournier’s full name. This is the document I found: www. persee.fr/doc/acths_1764-7355_2009_act_132_2_1642 Australia's electronics magazine siliconchip.com.au Emona You can win one of FIVE new Rigol DHO-924S ultra-portable 12-bit digital storage oscilloscopes, each valued at $1592.80. Simply click on the link below and complete the form to enter the competition. Hurry, enter now! Competition closes December 13, 2024. WIN 1 OF 5 RIGOL 250MHZ DSOS To enter visit: www.emona.com.au/win-rigol Rigol DHO-924S features:  250MHz bandwidth, four channels  High vertical resolution: 12 bits  Real-time sampling rate up to 1.25GS/s  Maximum memory depth: 50Mpts  Built-in 25MHz Arbitrary Waveform Generator  7-inch (18cm) multi-touch screen  USB, LAN and HDMI interfaces  16 digital channels standard (requires optional probes) All eligible entries also receive a discount coupon for any Rigol online purchases from the Emona website, valid until 31st of January, 2025. Contest open to residents of Australia and New Zealand only. Terms & conditions apply. See website linked above for details. That paper is from the La Rochelle region in France, where Georges Rignoux was a researcher in electricity. It includes photos of both of them, and indicates he was born in 1882 and died in 1944 (in the footnotes on page 82). Fournier’s given name was actually Auguste, and he was born in 1864 according to the footnote on page 81. His date of death is unknown. Alan Winstanley, North Lincolnshire, UK. Idea of a global electric power grid I feel I must comment regarding the solar/wind vs nuclear power letter. “The main argument raised against solar is that the sun doesn’t always shine”. That is not quite right. Solar energy is available 24/7, because at any one time, half of the Earth is lit by the sun. Here is the interesting bit. If we could tie all the power grids of the world together, solar-electric power would then be available 24/7. No new technology needs to be developed; HV undersea cables are readily available, as are power devices to do the switching from DC to 50 or 60Hz, bidirectionally as required. No nuclear, no ugly, noisy wind turbines or expensive wasteful battery’s. Just standard, almost off-theshelf technology. I suppose that politics will prevent such a solution from being implemented. Dick Powell, via email. Comment: such a project would have serious engineering problems to overcome, such as high transmission losses and potential grid instability. Still, it may be feasible. While there could be political challenges, we think the engineering challenges would need to be adequately considered first. Recommendations for portable computer shopping I have not been in the market to purchase a new laptop computer for some time. My old computer’s cooling fan failed, and in researching a new machine, I noticed a few things that may be of interest to readers in a similar situation. Many laptops now have soldered-in RAM. There are no slots to expand memory in future. Many modern laptops also don’t have an HDMI port to connect to an external monitor. You need to use a USB-C to HDMI adaptor or use a monitor that supports video over USB-C (DisplayPort). Most laptops no longer have an easily replaceable battery, either. The battery is built-in and difficult to replace, if it is even possible. Most new laptops (except the ones from Apple) also come preinstalled with Windows 11, which requires you to establish a Microsoft account to operate, although there are apparently ways around that, but you have to research them. I also don’t want to have to perform endless Windows updates, which eventually render the computer slow and unusable due to ‘bloating’. Nor do I want AI, now built into Windows and which can’t ever be fully disabled, telling me what to say or do next. This sort of thing doesn’t happen with GNU/Linux and further motivates me to migrate to that operating system, or at least create a dual boot system. Finally, you can’t ‘have it all’. It is almost impossible now to get the full feature set you want in one machine. Compromises have to be made. For example, I wanted an 10 Silicon Chip Australia's electronics magazine siliconchip.com.au siliconchip.com.au Australia's electronics magazine December 2024  11 OLED screen, but had to settle on an IPS LCD screen, as no OLED notebook had all the other features I wanted. In the end, I manage to purchase a machine with most of what I wanted, including expandable slotted memory, a spare SSD port, an HDMI port, a touchscreen that is rotatable 360° (a so-called ‘2-in-1’ laptop) and a micro SD card reader. For those interested, the laptop I finally purchased was a Lenovo ThinkBook 14 2-in-1 Gen 4 14-inch WUXGA Touch Core Ultra 7 16GB RAM 512GB SSD model. Dr David Maddison, Toorak, Vic. Comment: Lenovo computers also have a pretty good reputation for repairability. Xenon timing light project identified I noticed the final item in the “Ask Silicon Chip” section of the October 2024 issue was for information about a xenon timing light project. It was in the June 1974 edition of ETI. I built it and still have it gathering dust in my shed. Incidentally, I used a ferrite core from a TV EHT transformer for the pickup transducer. Rod Lovel, Wareemba, NSW. Comment: thanks for the information! We likely didn’t find it because it was suggested we look for it between 1980 and 1982 (we did look as far back as the late 1970s but not the early 1970s). Funnily enough, searching our ETI index for “xenon” yields the June 1974 issue as #50 out of 54 results; apparently, the word only appeared once in the whole issue, in an unrelated ad. The project was simply named “Ignition timing light”. Upgrading a ZC1 Mk2 Communications Receiver I’m responding to Dr Hugo Holden’s excellent article on the old ZC1 wartime transceiver in the October 2024 issue (siliconchip.au/Article/16680). I have written before on one of his earlier articles on the ZC1 Mk2. The ZC1 Mk2 was my first ham gear when I was first licensed in Fiji in 1962. I asked around and received lots of advice on improvements, ultimately completing the following. 1. I replaced the vibrator power supply with a conventional mains supply. 2. I replaced the 6V6 valves with metal octal 6L6 types, which increased the power output. 3. I greatly increased the modulation percentage by putting a 1kW resistor in the HT input to the PA 6L6 and bypassed it with a 100nF capacitor. 4. I replaced all the old-style capacitors. 5. A few years later, I replaced all the old wiring with PVC-insulated stuff, rewiring the whole thing. 6. I changed the output circuit to a conventional pi configuration. At the time, I was serving in the RNZAF where these Mk2s were still in occasional use for field exercises up to about 1964. The higher modulation depth gave the sets a greatly improved long-distance range and better readability. I took my own set up to PNG with me in 1970, where for about four years, I used it as a test transmitter for proving wideband HF aerial designs. It was a great and very rugged set which survived its many journeys on New Zealand railways! Dave Brewster, Lake Cathie, NSW. SC 12 Silicon Chip Australia's electronics magazine siliconchip.com.au Subscribe to NOVEMBER 2024 ISSN 1030-2662 11 The VERY BEST DIY Projects ! 9 771030 266001 $13 00* NZ $13 90 INC GST INC GST FlexIDIce Dice up to 100 Random cards Variable Speed Drive heads or tails for induction motors Australia’s top electronics magazine Surf Sound Simulator Silicon Chip is one of the best DIY electronics magazines in the world. Each month is filled with a variety of projects that you can build yourself, along with features on a wide range of topics from in-depth electronics articles to general tech overviews. Published in Silicon Chip If you have an active subscription you receive 10% OFF orders from our Online Shop (siliconchip.com.au/Shop/)* Rest of World New Zealand Australia * does not include the cost of postage Length Print Combined Online 6 months $70 $80 $52.50 1 year $130 $150 $100 2 years $245 $280 $190 6 months $82.50 $92.50 1 year $155 $175 2 years $290 $325 6 months $100 $110 1 year $195 $215 All prices are in Australian dollars (AUD). Combined subscriptions include both the printed magazine and online access. 2 years $380 $415 Prices are valid for month of issue. Try our Online Subscription – now with PDF downloads! FlexiDice; November 2024 Surf Sound Simulator; November 2024 Micromite Explore-40; October 2024 Compact OLED Clock; September 2024 An online issue is perfect for those who don’t want too much clutter around the house and is the same price worldwide. Issues can be viewed online, or downloaded as a PDF. To start your subscription go to siliconchip.com.au/Shop/Subscribe UNDERSEA COMMUNICATIONS DATA TRANSMISSION & POWER CABLES We don’t hear much about undersea communications cables, even though they carry around 99% of international internet traffic. Without undersea cables, the internet as we know it would not exist. By Dr David Maddison, VK3DSM U ndersea communications cables are the most invisible and yet one of the most important parts of the internet. Compared to alternatives such as satellites, cables are much cheaper and offer much lower latency (delays due to the time the signal takes). As of June 2024, there were 600 active or planned submarine communications cables and 1.4 million kilometres of cables in service (see Fig.1). The lengths range from 131km for the CeltixConnect-1 (CC-1) from Dublin, Ireland to Holyhead, United Kingdom to 20,000km for the Asia-­America Gateway (AAG) Cable System from the United States to various places in Asia and the Pacific. There are usually multiple cables connecting each country to provide 14 Silicon Chip redundancy in case of accidental or deliberate damage. Communications cables carry not only internet traffic including video but also telephone calls and private computer networks. Undersea communications cables originate with the first undersea telegraph cables. There are also undersea power-­ c arrying cables. Undersea telegraph cables Before there was significant (or any) radio traffic, there was an extensive network of undersea telegraph cables. Fig.2 shows the Eastern Telegraph Company’s international telegraph network in 1901. On the 12th of December, 1901, Marconi conducted the first transatlantic Australia's electronics magazine radio transmission from Cornwall (UK) to Newfoundland (Canada), using a 150m-long kite-supported antenna for reception. Marconi established a commercial service for ships at sea in 1904 and a transatlantic radio-telegraph service in 1907. However, that service was not reliable for many years. Thus, there was still a demand for cabled telegraph services in the early 1900s. Today, there is still competition for communications between optical fibre and wireless services, including via satellites. Land-based cables were uninsulated and suspended between poles, but subsea cables must be insulated. Few suitable materials were known in the early 1800s. siliconchip.com.au In 1843, Michael Faraday sent samples of the natural rubber-like material gutta-percha from a tree of the same name from Singapore to London for testing. In 1845, Sir Charles Wheatstone suggested it be used to insulate a cable between Dover and Calais. The cable was laid in 1850 and was successful. The first attempt at laying a telegraph cable across the Atlantic was in 1858; it was ultimately unsuccessful. It was laid between Ireland and Newfoundland and worked extremely slowly for a few weeks before being destroyed by applying too high a voltage (2000V) to it in an attempt to speed it up. The problem was that signals were ‘smeared out’ at the receiving end, significantly reducing the transmission rate, as subsequent signals would interfere with prior signals. This was due to cable capacitance. The cable acts as a long, thin capacitor, with one electrode being the conductive seawater on the outside and the other the central conductor. This meant the transmission rate had to be dramatically reduced to receive intelligible signals. The speed was so slow that a 99-word transmission between Queen Victoria and President James Buchanan took 16.5 hours, or ten minutes per word. Incidentally, in a classic engineering error, two cables were ordered from two suppliers and were provided with cable twists running in opposite directions. This would have made splicing them impossible, so a special bracket was improvised to hold the wires. Fig.1: just some of the current submarine cables worldwide. Source: www. submarinecablemap.com Transmission line theory In those early years, transmission line theory, shown in Fig.3, was poorly understood. In 1855, the future Lord Kelvin (William Thomson) made some theoretical progress and developed a model that predicted the poor performance of the 1858 cable. However, that did not lead to a complete understanding because he only considered capacitance and resistance but not inductance in the cable. Although Lord Kelvin was involved in that cable project, his concerns were not heeded due to internal company squabbles. He wanted a thicker cable. Nevertheless, he developed a highly sensitive mirror galvanometer to detect signals on the cable (see Fig.4). Morse dots and dashes were siliconchip.com.au Fig.2: undersea and overland telegraph cables of the Eastern Telegraph Company, the largest cable company in the world in 1901. Source: www. zmescience.com/other/great-pics/map-undersea-cables-18112010 Fig.3: an electrical model of a transmission line, such as an undersea telegraph cable, with resistive (R), conductive (G), capacitive (C) & inductive (L) components Australia's electronics magazine December 2024  15 represented by negative or positive pulses rather than pulses of differing duration. In 1876, Oliver Heaviside revolutionised the understanding of transmission lines and published the first of his papers on analysing the propagation of signals in cables. They included the ‘telegrapher’s equations’: δ/δx V(x, t) = -L δ/δt I(x, t) − RI(x, t) δ/δx I(x, t) = -C δ/δt V(x, t) − GV(x, t) These use resistance, conductance, inductance and current to predict voltage and current distributions in transmission lines as a function of distance and time. They are derived from Maxwell’s equations. More transatlantic cables A second cable was laid in 1865, but it broke over halfway across and could not be recovered after numerous attempts. A third cable was laid in 1866, which was successful. The 1865 cable was also retrieved and repaired, so there were two cables in service. Remarkably, although it took several attempts, the 1865 cable was recovered with a grappling hook at a depth of 4km. The line speed was decent at seven words per minute, much faster than the 1858 cable. Like the 1858 cable, these were laid between Ireland and Newfoundland. The effective line speed was further improved with Julius Wilhelm Gintl’s development of duplex transmission Fig.4: Thomson’s mirror galvanometer could detect extremely small currents. Source: https://w.wiki/AoEY 16 Silicon Chip in 1872, which allowed two messages to be sent simultaneously in different directions. In 1874, Thomas Edison invented quadruplex transmission, which allowed four messages to be sent simultaneously on one cable, two in each direction. Fig.5 shows manufacturer samples of the 1858, 1865 and 1866 cables. Each cable had a thicker version for the shore and continental shelf sections and a thinner version for the deep ocean. Note that there was no repeater technology in this period (as used routinely today), so a signal needed to travel the entire length of the cable without being amplified or having its waveform conditioned in any way. That made the feat of transoceanic communications even more formidable. In 1866, a transatlantic telegraph message cost US$10 per word with a ten-word minimum. Back then, $100 was 10 weeks’ pay for a skilled worker. That is equivalent to US$2000 or $3000 today – for a ten-word message! The first undersea telegraph cable connecting Australia ● The first undersea telegraph cable connecting the Australian mainland to Tasmania was built in 1859. It had numerous problems and was abandoned in 1861. Another cable was installed in 1869, running from Cape Shanck, Vic to Low Head, Tas. ● In 1871, the first cable connecting Australia to the rest of the world was installed from Darwin to Singapore via Java (see Figs.6 & 7). It was described at the time thus (siliconchip. au/link/abyz): The cable consists of seven small copper wires—a central one, with the six twisted round it. It is insulated by gutta-percha, over this is a coating of tarred hemp, then a sheathing of galvanised iron wire, with an outside covering of tarred hemp. The deep sea portion is three-quarters of an inch in diameter, the intermediate one inch, and the shore ends (twenty miles in length) three inches in diameter. There is much information about this cable at siliconchip.au/link/abz0 ● In 1876, the first undersea cable was laid between Australia and New Zealand. ● In 1889, a third international link was laid from Broome, WA to Batavia (Jakarta). ● In 1891, a cable was laid from Bundaberg, Qld to Gomen (New Caledonia). ● In 1901, another cable was run from the Cocos-Keeling Islands to Perth, part of the global “Red Route” cable through British territories. ● In 1902, a cable was added from Southport, Qld to Canada via Fiji and Norfolk Island. For more information on the Southport cable, see the telegraph display in The Gold Coast Historical Museum Fig.5: a manufacturer’s sample case of products for the 1858, 1865 and 1866 Atlantic Cables manufactured by Glass, Elliot, and Co. They merged into Telegraph Construction and Maintenance Co. Source: https://atlantic-cable. com/Article/AtlanticCables Australia's electronics magazine siliconchip.com.au (www.gcmuseum.com.au) at 8 Elliot St, Surfers Paradise. You can see the remains of the cable hut of the Pacific Cable Station at Cable Park, Main Beach Parade, Main Beach, Gold Coast City. The Cable Station operated from 1902 to 1962. The All Red Line The All Red Line was a system of telegraph lines and undersea cables that linked most countries of the British Empire (Fig.8). The colour red was the traditional colour used on maps to indicate British Empire countries and colonies. It was built because the UK had security concerns about a vital cable network with landfalls that were not on territory they controlled. The first successful part of the cable was from Ireland to Newfoundland, Canada in 1866. The network was completed in 1902 with a final trans-Pacific cable from British Columbia, Canada to Fanning Island (then part of the UK and roughly in the middle of the Pacific Ocean). That section of the cable was funded by the UK, Canada, New Zealand, New South Wales, Victoria and Queensland. Australia’s first connection to the cable was from Darwin to Singapore via Java in 1871. Fig.6: a portion of the original Darwin to Java cable recovered from the Timor Sea in 2016. Source: https://digital-classroom.nma.gov.au/images/section-portdarwin-java-underwater-telegraph-cable-1871-72 Fig.7: bringing the cable to shore at Darwin in 1871. Source: www.pastmasters. org.au/overland-telegraph-amp-undersea-cables.html Cable circuits Telegraph cables generally had one central conductor. The return current path of single-core telegraph cables was through the sea; although sea water is not nearly as conductive as copper, the cross-section is high, so the resistance is low. At the low frequency of Morse transmission, such an arrangement worked satisfactorily. The currents involved in transoceanic telegraphy were extremely small and susceptible to many forms of landbased electrical interference. Therefore, the Earth electrodes for cables were run many kilometres out to sea to minimise such interference (see siliconchip.au/link/abz1 for further information). Fig.8: the All Red Line of telegraph cables connecting the British Empire, built between 1866 and 1902. Source: https://w.wiki/AoEZ Increasing telegraph speed One way of increasing the speed of a telegraph cable was to wrap the inner conductor with mu-metal, which is typically used today for magnetic shielding. Mu-metal was invented in 1923 and was used to provide inductive loading of subsea telegraph cables (see Fig.9) to compensate for the siliconchip.com.au Fig.9: “Loaded cable” as used on part of the Pacific cable route to increase transmission speed between England and Australia: (a) conductor made of copper; (b) continuous winding of “mumetal” wire; (c) gutta-percha insulation; (d) inner wrapping of jute; (e) sheathing of steel wires; (f) coating of composition; (g) outer wrapping of jute with external coating. Source: https://atlantic-cable.com/Cables/1902PacificGB Australia's electronics magazine December 2024  17 plastic jacket dielectric insulator metallic shield centre core Fig.10: the structure of a typical coaxial cable. A subsea cable has many more layers of insulation, reinforcement and armour. Source: https://w.wiki/AoEa Fig.12: how the repeaters were powered for the first transatlantic communications cable, TAT-1. cable’s capacitance. This enabled a much greater transmission rate. For example, in 1926, the busiest part of the Pacific cable from Fiji to Vancouver was duplicated with this ‘loaded cable’, increasing the transmission from 200 to 1000 letters per minute. Telephony through subsea cables Fig.11: a cross-section of TAT-1 coaxial cable. Source: https://w.wiki/ AoEb 18 Silicon Chip Single-wire subsea telegraph cables with Earth returns are unsuitable for voice because the attenuation is too great at higher frequencies due to cable inductance and capacitance. The signal was distorted and the cables were also too susceptible to interference. In 1877, Alexander Graham Bell attempted to make a telephone call over the Atlantic telegraph cable but the experiment failed. One attempt to resolve such problems was to ‘pupinise’ (named after Michael Pupin) a subsea cable. This involved adding inductors (loading coils) at regular intervals along it with balanced pairs of wires to increase its inductance, thus offsetting its capacitance. This method also allowed the use of thinner, cheaper wires. This technique was independently discovered by George Campbell at AT&T and Michael Pupin at Columbia University, based on Oliver Heaviside’s theory. Still, there were limits to the distance over which this technique was effective. A pupinised cable was laid across Lake Constance in Switzerland in 1906, and in 1910, such a cable was laid across Chesapeake Bay with 17 pairs of conductors. Pupinised cables had problems; the waterproofing materials available at the time were inadequate, and bulges in the cable where the inductors were installed mechanically weakened it. Continuous loading, with no cutoff frequency, was a superior method of Australia's electronics magazine solving the same problems as pupinisation. A project to install a continuously loaded transatlantic cable was underway in the 1930s, but it was abandoned during the Great Depression. By the late 1930s, repeaters and multiplexing provided more capacity on the same number of circuits at a lower cost, so cable loading was no longer necessary. Transatlantic radiotelephony A transatlantic radiotelephony service was also established in 1927. It charged US$45 for three minutes, equivalent to about US$800 or $1200 today. Thus, plenty of financial incentives existed to develop a cheaper service, but certain technological advances were required. Such advances included synthetic polyethylene insulation to replace rubber and gutta-percha from 1947 and reliable vacuum tubes for repeaters and coaxial cable. Modern coaxial cable was patented in 1929, although Nikola Tesla obtained a similar patent in 1894. Coaxial subsea telephone cables Coaxial cables have an inner conductor plus a shield around the outside (see Fig.10). They can carry high-­frequency signals with low losses and are therefore suitable for many telephone circuits and/or data/video. Coaxial cables are superior to single or multiple conductors in subsea cables. The first transatlantic telegraph cables (from 1858) were coaxial, but transmission line theory was not fully developed at the time, so they could not operate at high speeds. The first modern subsea coaxial cable was laid in 1936 and ran 300km between Apollo Bay near Melbourne and Stanley, Tas. It carried six siliconchip.com.au Perspex Bar Supervisory Directional Filter Unit (removed) Power Bridge & Separating Equaliser Filter Amplifier Valves Directional Brass Resistor Filter Cylinder Box Housing Cable Centre Gland Cover Armour Wires Sea Cable Conductor Bridge & Power Armour Equaliser Separating Watertight Cable Diaphragm Bulkhead Seal Wire Clamp (removed) Filter Gland Fig.13: a cutaway of the repeaters used for TAT-1. Source: https://collection.sciencemuseumgroup.org.uk/objects/co33321/ submerged-repeater-for-tat-1-1956-amplifier telephone circuits, at least a dozen telegraph circuits and an 8.5kHz broadcast channel. For further information, see siliconchip.au/link/abz2 In 1956, the first intercontinental transatlantic coaxial cable, TAT-1 (Transatlantic No. 1), was installed (see Fig.11). It carried 35 telephone channels, with a 36th channel carrying 22 telegraph lines (used by Telex). There were two separate cables, one for each direction, each 41mm in diameter. TAT-1 used valve (vacuum tube) repeaters to boost and condition the signals. Each repeater had three valves. Valves were specially developed for this: the 6P12 for the shallow water portion and the 175HQ for the deepsea portion. The repeaters were at 69km intervals and were 2.74m long, 73mm in diameter and flexible so they could be wound over the cable drum – see Fig.13. Power was supplied via the cable (see Fig.12). Each repeater unit was unidirectional to minimise size, so it was compatible with cable-laying equipment while also minimising the effect of stray capacitance and inductance. For more details, see siliconchip.au/link/ abz3 and siliconchip.au/link/abz4 From 1963, TAT-1 carried the original primary circuit for the famous “Moscow–Washington hotline”. The original bandwidth of TAT-1 was 4kHz per phone channel, but it was reduced to 3kHz to allow for a total of 48 channels. Three additional channels were added using a carrier-­ suppressed ‘Type C’ modulation scheme (siliconchip.au/link/abz5). In 1960, a Time-Assignment Speech Interpolation (TASI) system was implemented on the cable, increasing the number of speech circuits to 72. TASI uses the idle time on calls to carry additional calls. For more information on TASI, see siliconchip. au/link/abz6 TAT-1 was in operation until 1978. siliconchip.com.au The valve repeaters proved extremely reliable, and the cable might still be in use had it not become obsolete due to its low bandwidth. Australia’s first submarine telephone cable The first subsea coaxial cable for telephony connecting Australia to the world was the COMPAC cable, which began service in 1963. It connected to Canada via New Zealand, Fiji and Hawaii, as shown in Fig.14. A microwave link across Canada and the transatlantic CANTAT cable connected it to the UK. It provided 80 two-way telephone channels or 1760 teleprinter circuits, including leased lines. The cable was 32mm in diameter in the offshore sections. A video from 1963 about the project, “80 Channels Under The Sea”, can be viewed at is at https://youtu. be/m1sfMjTyjPo Before the COMPAC cable, Australia had operated an international radio telephone service since the 30th of April 1930. People had to rely on booking a radiotelephone call, which was transmitted by HF radio and could only be made at particular times of day, depending upon atmospheric conditions. Optical fibre cables The next major development beyond submarine coaxial cables was optical fibre cables. Optical fibres for communications are made of high-­ purity glass that can transmit data via pulses of laser light at one or more frequencies. Light stays within the fibre due to total internal reflection. Optical fibres offer many advantages. The data rate achievable is many times faster than over coaxial cable, and the signal loss is lower. Fibre is immune to electrical interference and harder to intercept by hostile actors. More optical fibres can be inserted into an undersea cable (or anywhere) than coaxial cables, as they are much smaller in diameter and weigh less. Fig.14: a COMPAC cable map from Voices Through The Deep (1963), NZ Post Office. Source: https://heritageetal.blogspot.com/2020/09/the-many-lives-of-emervyn-taylors.html Australia's electronics magazine December 2024  19 Fig.15: a cross-section of a submarine optical fibre communications cable. The copper or aluminium tube is both for protection and to carry power, while the petroleum jelly provides lubrication. Original source: https://w.wiki/7ojk Fig.16 shows the basic elements of an individual ‘single-mode’ optical fibre for communications cables, while Fig.15 shows a bundle of optical fibres incorporated into an undersea communications cable. Single-mode fibre is typically used for long-distance communications cables as it can support a longer distance (up to 50 times more than multimode) and a higher data rate. However, it is more expensive and requires a light source with a narrow spectral width. Multi-mode fibre is cheaper but more suitable for short-to-­ mediumrange applications. The first undersea optical fibre was TAT-8, a transatlantic cable that opened in 1988 and retired in 2002. It had a capacity of 280Mb/s, equivalent to 4000 voice circuits. It contained two working fibres plus a spare. TAT-8 had repeaters every 67km. Wavelength division multiplexing (WDM) is used in modern cables to increase the bandwidth by utilising multiple laser wavelengths (colours), up to 30, over a single fibre instead of a single wavelength (see Fig.17). An older optical fibre cable may be able to be retrofitted with WDM terminal equipment to increase its capacity. Optical fibre repeaters (Fig.18) contain optical amplifiers and circuitry to condition and reform the signal. DC power to repeaters is provided via the cable, usually between 3kV and 15kV. The current for a 10kV supply might be 1.65A, meaning an incredible 16.5kW of power is running through the cable. One end of the cable is typically supplied with a positive voltage, the other with a negative voltage, resulting in a virtual Earth in the middle of the cable. The return current is through the seawater. A recent development (2021) is NEC’s multicore fibre. This refers to individual fibres that have four instead of just one optical pathway (see Figs.20 & 19). This quadruples the number of channels through an individual cable compared to a conventional cable of the same diameter. Fig.17: the principle of wavelength division multiplexing (WDM), as used on modern optical fibre communications cables. A ‘mux’ is a multiplexer, while a ‘demux’ is a demultiplexer. 20 Silicon Chip Fig.16: the structure of a typical single-mode optical fibre. This is an individual fibre with protection, not a complete communications cable. Original source: https://w.wiki/33S5 Information on the bandwidth of modern optic fibre cables is hard to come by. Still, the 6605km transatlantic MAREA cable with eight fibre pairs (owned by Microsoft, Meta and Telxius) is said to be rated at 224 terabits per second (224Tb/s). Google’s 15,000km West African Equiano cable with 12 fibre pairs is said to carry 150Tb/s. Modern fibre optic cables are 17-21mm in diameter, except on the continental shelf (typically to a depth of 1500m), where they are 40-50mm due to additional armouring against sea life and abrasion from storms etc. Different cable configurations are possible depending on the level of protection needed; see Fig.21. Additional protection may be provided by burying the cable in shallower areas. Reliability and redundancy Communications cables and repeaters have to be very tough and strong to withstand the bending of the cable as it is loaded, then unloaded and installed. Fig.18: an NEC repeater for the 9400km-long Trans-Asia cable as it goes into the sea. Source: www.nec.com/en/case/ asia_direct_cable Australia's electronics magazine siliconchip.com.au while Amazon is a major capacity buyer or part owner of 4 cables. Many of these cables are shown in Fig.1 (see siliconchip.au/link/abzf). Undersea cable manufacturers Fig.19: an LW-series optical fibre cable from OCC Corporation using 32 of NEC’s multicore optical fibres. It is 17mm in diameter, designed for depths up to 8km and can carry 15kV DC to power repeaters. Source: www.occjp. com/en/products/seabed/sc500.html Consider the tensile loading from the weight of several kilometres of cable as it hangs from the ship (possibly during rough seas) during laying and possible retrieval for cable repairs. The cable may be laid as deep as 8000m, such as in the Japan Trench, where the pressure is 800 atmospheres or 826kg/cm2. The temperature at the bottom of the ocean is around 4°C. Also, the cables have to be armoured to protect against certain marine life. Cables also have to be 100% reliable; no one wants to have to retrieve a cable that has a fault due to a quality control failure. Cables typically have redundant components in the repeaters that can be switched on if required, along with one or more redundant fibres. Who owns undersea cables? Apart from telecommunications companies and investors, about 1% of cables are owned by government entities. The Big Tech giants, Amazon, Alphabet (Google), Meta (Facebook) and Microsoft, own or have interests in many cables. After all, these companies are responsible for about 70% of internet traffic combined. Their business models rely on ample internet capacity. Google owns 17 cables outright and is part owner of an additional 16. Meta (Facebook) is a part owner or major capacity buyer in 15 cables and owns one outright. Microsoft is a part owner or major capacity buyer of 6 cables, siliconchip.com.au Companies that manufacture undersea cables include: ● SubCom LLC (www.subcom.com) ● Alcatel Submarine Networks (www.asn.com) ● HMN Technologies Co Ltd (www. hmntech.com) ● NEC (www.nec.com/en/global/ prod/nw/submarine) Components are made by Corning, General Cable and Norddeutsche Seekabelwerke. Fig.20: regular optical fibre (left) and NEC multicore optical fibre (right). 1000µm = 1mm. Original source: NEC – siliconchip.au/link/abzd Protection of cables by international law An international convention protects undersea cables: the Convention for the Protection of Submarine Telegraph Cables. This was brought into effect in 1884 and remains in force. It makes it an offence to damage submarine cables and outlines who is responsible in the event of accidental damage. The Australian colonies signed in 1885 (SA, Vic), 1886 (Qld) and 1888 (NSW, Tas & WA). Capacity metrics Two capacity metrics are used for optical communications cables. The potential capacity is the theoretical maximum capacity of a cable and is what is usually cited in promotions. There is also lit capacity, the capacity for which terminal equipment is installed at either end. When a cable is first put into service, the full capacity is not usually utilised as demand does not yet exist. Cable owners only install the amount of expensive transmission equipment needed at a given time. More is added as demand increases until the potential capacity is reached. Espionage In December 2016 (siliconchip.au/ Article/10459), we mentioned Operation Ivy Bells, a US operation to tap into a Soviet copper communications cable during the Cold War. There were undoubtedly many other such instances from all parties. Modern optical fibres are much harder to tap into, and end-to-end encryption makes intercepting and decoding communications very difficult. Australia's electronics magazine Fig.21: various possible configurations of optical subsea communications cables. Original source: ICPC – siliconchip.au/link/abze December 2024  21 Australia’s connections to the world Many cables connect Australia to the world (and other parts of Australia). We compiled the following list showing the name of each cable, its length and the year it was or will be put into service: 1995 Bass Strait-1 241km 1999 SeaMeWe-3 39,000km 2000 Southern Cross Cable Network (SCCN) 30,500km 2001 Australia-Japan Cable (AJC) 12,700km 2003 Bass Strait-2 239km 2005 Basslink 298km 2008 Gondwana-1 2151km 2008 Telstra Endeavour 9125km 2009 PIPE Pacific Cable-1 (PPC-1) 6900km 2016 North-West Cable System 2100km 2017 Tasman Global Access (TGA) Cable 2288km 2018 Australia-Singapore Cable (ASC) 4600km 2018 Hawaiki 14,000km 2019 INDIGO-Central 4850km 2019 INDIGO-West 4600km 2020 Coral Sea Cable System (CS2) 4700km 2020 Japan-Guam-Australia South (JGA-S) 7081km 2022 Oman Australia Cable (OAC) 11,000km 2022 Southern Cross NEXT 13,700km 2023 Darwin-Jakarta-Singapore Cable (DJSC) 1000km 2026 Honomoana unknown length 2026 Tabua unknown length 2026 Sydney-MelbourneAdelaide-Perth (SMAP) 5000km 2027 Asia Connect Cable-1 (ACC-1) 19,000km 2027 Hawaiki Nui 1 10,000km 2027 Te Waipounamu 3000km TBD Umoja unknown length 22 Silicon Chip It is possible to tap into optical fibres by bending them and then examining the light leakage at the bend. Depending on the cable, this may result in a detectable reduction in light levels. While encryption makes this less of a concern, protections have been proposed to prevent it, such as using ‘bend-insensitive cable’ or a ‘quantum alarm’ to detect it. Deliberate damage – a major vulnerability With 99% of internet traffic travelling through undersea communications cables, and significant amounts of electrical power for certain communities, nations are vulnerable to being ‘shut down’ very quickly by terrorist or enemy military action. There is no obvious practical way to adequately protect such infrastructure; damage to one cable can take weeks to repair under the best conditions. It would be virtually impossible to repair multiple points of damage on one or multiple cables in any reasonable time. Hazardous areas might include volcanic locations, hot water seeps, areas prone to landslides and ecologically sensitive areas with deepwater coral etc. The location of where cables come ashore is also carefully considered. Cables are carried by special ships on giant spools. One example is the Isaac Newton, shown in Fig.22. It can carry a total of 11,900 tonnes of cable on two spools, and can perform a variety of other functions. A sea plough is used to bury the cable to prevent damage in areas close to shore – see Fig.23. There are about 60 cable installation and repair ships in service worldwide. Damage or faults Undersea cables are periodically damaged. Causes include underwater landslides, earthquakes, volcanoes, marine life, fishing trawlers (38%), anchors (25%) and, closer to shore, extreme storms, strong currents and tsunamis. Around 70% of optical cable damage occurs at depths under 200m. Communications cable life Cable faults were only responsible Most cables have a design life of for about 6% of failures from 1959 to about 25 years. However, many are 2006. Worldwide, about 100 incidents retired early because their bandwidth of cable damage or faults are recorded becomes inadequate and higher-­ per year. capacity cables are more profitable to Sharks have been known to attack install. On occasion, unused cables unburied cables for unknown reasons, might be raised and relocated to as shown in Fig.24. Because of this, another location. This might be worth- cables have been provided with extra while for countries or companies with armour. However, the International limited budgets. Cable Protection Committee stated Sometimes cables are recovered for there was no damage from the incithe valuable materials in them such dent shown in Fig.24. as copper, aluminium, lead and steel. They also wrote that sharks and Collectors may go on diving expe- other fish were responsible for only ditions to retrieve samples of cables 1% of cable faults until 2006 and none of historic interest; for example, see since then (siliconchip.au/link/abz7). http://w1tp.com/mcable.htm In 1929, transatlantic telegraph cables were cut within 100km of an Cable costs & laying the cable earthquake epicentre due to landCables cost upwards of US$25,000 slides. ($38,000) per kilometre, and recent On the 30th of March 2016, 10 Africables have been in the price range of can countries were entirely off the US$250-$300 million ($380-450 mil- internet for two days when a fishing lion) for transatlantic and US$300- trawler inadvertently cut one cable. $400 million ($450-600 million) for In 2019, Tonga’s cable was cut by a trans-Pacific cables. ship’s anchor. During the planned routing of the Then, in 2022, the cable connecting cable, hazardous zones and ecologi- Tonga was cut for over a month due to cally sensitive zones are avoided using the Hunga Tonga-Hunga Ha’apai volseabed mapping systems, such as mul- canic eruption. An earthquake on the tibeam side-scan sonar (we covered 29th of June 2024 damaged it again. sonar in June 2019; siliconchip.au/ Tonga has only limited satellite conArticle/11664). nectivity and no backup cable. Australia's electronics magazine siliconchip.com.au Fig.22: a cutaway model of the cablelaying ship Isaac Newton. Source: https://w.wiki/AoEd Sometimes, ‘accidental’ cable damage is deliberate. In 1959, a Soviet fishing trawler cut five US cables in 12 locations. And in 2021 a research cable was severed off the coast of Norway by a fishing vessel, see https://youtu.be/ pw2lO4sxZn8 Repairing faults The location of cable breaks can be determined by time-domain reflectometry (TDR). With TDR, pulses are sent down the cable and reflections from a cable break are timed. The location of the break is determined by the time taken as a fraction of the speed of light in the cable. We published a DIY TDR design in December 2014 (siliconchip. au/Article/8121). Once a fault is located, a cable repair ship is dispatched to that location and the cable is retrieved with a grapnel (Fig.26) that hooks and locks onto it, a process that sounds much easier than it really is. A damaged cable is normally cut on the sea floor (if it already isn’t cut), both ends retrieved, and a new section added. Rejoining a broken cable is a delicate process, as shown in Fig.25. What about Starlink? Figures are hard to come by, but one estimate by the US FCC suggests that only 0.37% of their international internet traffic goes via satellite. The rest is by cable. Starlink is a wonderful technology that gives internet access to users and devices anywhere in the world, but it is unlikely to significantly relieve the demand for undersea cable bandwidth. The cost for a 60,000Gbps 9000km-long undersea cable with a service life of 25 years is around US$300 million (~$450 million) or US$12 million (~$18 million) per year. That gives a cost per Gbps per year of around US$200 (~$300). The cost for 10 Starlink v3 satellites to cover roughly the same distance is US$17 million (~$25 million), with approximately 50Gbps bandwidth and a service life of five years. That gives a cost per year of just US$1.7 million (~$2.6 million) but a cost per Gbps per year of US$34,000 (~$52,000)! So Starlink cannot compete with undersea cables in terms of cost, but that is not its purpose. Its purpose is to offer internet service everywhere, provide an alternative to land-based ISPs, siliconchip.com.au Fig.23: a Soil Machine Dynamics sea cable plough on Normandy Beach, used to bury cable. Source: https://x.com/MachinePix/status/623603135404187648 Fig.24: a shark attacking an undersea cable as seen from a remotely operated vehicle (ROV) – a “megabite”? Source: https://youtu. be/1ex7uTQf4bQ Fig.25: the delicate process of repairing a cable break (or making a new join). Source: KIS-ORCA – siliconchip.au/link/abzg Fig.26: an ETA-brand ‘cut and hold’ grapnel to cut and retrieve cables from the deepest parts of the ocean. There are many different designs of this type of device. Source: https://eta-ltd.com/cut-hold-grapnel Australia's electronics magazine December 2024  23 How much does internet infrastructure weigh? On the 21st of July 2024, ABC RN (Australia) rebroadcasted a BBC program in which they tried to estimate the weight of all internet infrastructure, including cables (siliconchip.au/link/abza). They concluded that subsea cables weighed two million tonnes, while the total weight of all infrastructure was 92.5 million tonnes. Naturally, that is a rough estimate. and provide internet access in places where free speech is compromised. Other uses of optical fibre cables Active fibre optic cables can be used for seismic measurements, as vibrations in the cable alter the scattering of light in the fibre. Such measurements generate 1Gb of data per minute (see siliconchip.au/link/abz8). The future As more devices and consumers (especially in developing countries) are connected to the internet and existing consumers demand more bandwidth, it is expected that more and more cable capacity will be required. The demand for cable capacity will only be slightly offset by increased satellite capacity, so demand for undersea cables will be strong. Undersea power cables While undersea communications cables are the most prevalent, there are also numerous undersea power cables (see Fig.27). They typically traverse much shorter distances than data cables. Numerous references mention the installation of the first underwater power cable in 1811 across the Isar River in Bavaria. However, we could not find an original source for this. We did find evidence that in 1811, Baron Pavel Lvovitch Schilling devised a water-resistant electrical wire that could be laid in wet earth or rivers for the remote control of mines or for telegraphy. It was coated with natural rubber and varnish. His first use of the wire in a river was for “operations with a subaqueous galvanic conducting cord through the river Neva, at St Petersburg, in the year 1812” – see https://w.wiki/Akii and https://w.wiki/Akij AC/DC Undersea power cables carry either alternating or direct current. AC is simpler because a transformer can easily change voltages at either end of the cable. DC transmission generally requires rectification at one end to convert AC to DC to send through the cable, then an inverter at the other end to convert the DC to AC. If the cable is used bi-directionally, then inverter and rectifier equipment is required at each end. DC transmission is considerably more complicated and expensive than simply having a transformer because it requires high-power, high-voltage rectifiers and inverters. However, DC transmission has the advantage of lower energy losses for longer cable runs. That is because DC has no losses from capacitance between conductors; with AC, this capacitance must be charged and discharged twice per cycle. For DC, that means less energy is wasted as heat, and less conductor material is needed. Also, there is no skin effect with DC transmission, so all of the conductor material is used to carry current, not just the outer layer. There is a maximum theoretical length for AC power transmission because, at some point, the entire current capacity of the cable is used to charge the remaining capacitance. Of course, there are other cable length limitations for both AC and DC cables. For both AC and DC undersea cables, there are greater losses and usually greater expense than for overhead power lines. So undersea cables are only used if there is no good alternative. AC transmission is generally used for shorter cable runs, while DC is used for longer runs where the extra cost is worthwhile due to reduced power losses. However, DC systems are considered less reliable due to the complicated (and therefore failure-prone) conversion equipment at either end. Other sources of energy loss in cables include: Fig.27: a cross-section view of a 150kV 3-phase undersea power (submarine) cable. Source: https://w.wiki/ ApBx 24 Silicon Chip Australia's electronics magazine siliconchip.com.au ● Ohmic power losses due to the resistance of the conductor material, which are proportional to the square of the current and can be reduced by using higher voltages (and thus lower currents for the same power). ● Reactive power losses due to capacitance between the conductors. ● Skin effect losses due to the concentration of alternating current near the surface of a conductor, which can be reduced with separately insulated, stranded conductors. ● Power losses due to proximity with other cables, avoided by spacing cables widely apart. ● Sheath losses due to the generation of eddy currents in the protective metal sheath (armour) around conductors within a cable. ● Leakage losses due to current flowing through the dielectric (insulation) material. DC cables can be configured as monopolar or bipolar, as shown in Fig.28, or another configuration, such as series-connected. Monopolar configurations, with just one conductor (either positive or negative) at a high voltage, are the simplest and cheapest, but bipolar configurations provide more flexibility and reliability. For monopolar configurations, return circuits can be through the Earth, sea or a metallic return cable. For bipolar configurations, one cable is positive and the other negative, both at high potential, with negligible return current under normal circumstances. If a fault occurs in one cable of a Fig.28: two possible configurations for HVDC cable systems, (a) monopolar and (b) bipolar. siliconchip.com.au Fig.30: a simplified electrical model of HVAC undersea power cables. bipolar system, the other cable can still be used but at 50% of the normal current, with a return path through the Earth, sea or a metallic return cable. Electrically, an AC undersea power cable can be considered as consisting of resistance, capacitance and an inductive load, as shown in Fig.30. Terminal stations provide additional resistive and inductive loads. The first high-capacity submarine electrical cable, Gotland 1, was laid in 1954. It was 98km long and went from Gotland Island off Sweden to the mainland, with a capacity of 20MW. It carried 100kV DC and used mercury arc rectifiers to turn AC to DC, then an inverter to convert the DC into AC again. In 1970, the service was upgraded to 150kV and 30MW using thyristors for rectification. The longest undersea power cables in the world are North Sea Link (720km, 515kV DC, 1.4GW), NorNed (580km, 450kV DC, 700MW) and SAPEI (420km, 500kV DC, 1000MW), all in Europe, with Australia’s Basslink the fourth-longest. Basslink was featured in the September 2008 issue (siliconchip. au/Article/1943). It is a 290km (undersea section) 400kV DC 500MW cable between Victoria and Tasmania. The cable weighs 60kg/m. It is of monopole configuration; Fig.29 shows a cross section. It actually consists of three separate cables bundled together with polypropylene rope. The bundle comprises the HVDC cable, a return cable and a 12-core fibre-optic cable for communications. Since the return cable is at low potential, it has much less insulation (and cost) than the power cable. The proposed SingaporeLink cable is 4300km long, has a 1.75GW power rating at 525-640kV DC between Darwin and Singapore to connect intermittent solar and wind electricity generation in Australia with Singapore (siliconchip.au/link/abz9). If it goes ahead, it will be by far the world’s longest undersea electricity cable. The cable would be made in 20km lengths spliced into 200km lengths. Some questions have been raised over its technical and economic SC feasibility. Fig.29: the configuration of the Basslink cable between Victoria and Tasmania. Original source: https://tasmaniantimes.com/2016/11/what-is-your-view-onwhat-caused-the-basslink-failure Australia's electronics magazine December 2024  25 CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. Simple regulated negative supply using a 555 IC I need a negative supply to power op amp based comparators and wanted to derive it from the positive (eg, +12V) supply. Buck regulator chips using an inductor proved very effective, but the inductor consumed PCB space. Buck chips switching into a load resistor also wasted too much power. Lots of capacitor-based charge pump chips exist that would do the job, but they would require a special parts order. This circuit uses an unusual method to force PWM on the humble 555 timer. Its pin 3 output can sink and source 200mA, and it is used to drive a charge pump. When pin 3 is high, the 10μF capacitor connected to it charges close to the input supply voltage via D1. When pin 3 goes low, the negative end of that capacitor goes below 0V, and D2 is forward-biased, charging the 10μF capacitor across CON2 to a negative voltage. The process repeats on each oscillator cycle. The oscillation is timed by the 4.7kW and 680kW resistors plus the 1nF capacitor, which sets it to a suitably high frequency. So far, this is pretty standard. IC1’s pin 4 reset input is used for negative regulation. When pin 4 goes low, it forces the internal flip-flop to reset, stopping the oscillator. The 555 data sheet indicates the threshold for this pin is around 1V. In this circuit, a 10kW resistor pulls pin 4 high to start the oscillator. As the output voltage becomes more negative, feedback via the two parallel 10kW resistors (forming a 5kW resistance) pulls pin 4 down until it reaches the 1V threshold. Output pin 3 goes low and the charge pump stops until the output filter capacitor discharges slightly; the oscillation then resumes, maintaining approximately -4V at the output. Diode D3 protects IC1’s pin 4 from going too far negative when the power is switched off. Michael Harvey, Albury, NSW ($80). Songbird An easy-to-build project that is perfect as a gift. SC6633 ($30 plus postage): Songbird Kit Choose from one of four colours for the PCB (purple, green, yellow or red). The kit includes nearly all parts, plus the piezo buzzer, 3D-printed piezo mount and switched battery box (base/stand not included). See the May 2023 issue for details: siliconchip.au/Article/15785 26 Silicon Chip Australia's electronics magazine siliconchip.com.au Micromite-based four-cell voltage monitor I liked the article about the ADS1115 analog-to-digital converter (ADC) in the November 2023 issue (siliconchip. au/Article/16012), but I wanted to apply it to the Micromite. I had a project that I had been putting off to make a battery monitor for a four-cell (~12V) LiFePO4 battery with 200Ah cells. Once you have a battery monitor and a Micromite, it could be expanded to a battery controller for such things as switching off a charger or a load. However, I decided to keep it simple. The design largely follows the ADS1115 data sheet. I decided to use the ADS1115 in single-ended mode to accommodate four cells, but the design works fine for both single-ended and differential measurements. In fact, the software uses both modes. The circuit comprises two parts, one being the Micromite LCD BackPack. I chose to use the original version because of its simplicity, but other versions, including the PicoMite or siliconchip.com.au WebMite, could be used. The remainder of the circuit includes the ADS1115 module and interfacing parts. It also uses two 5V power supplies derived from the 12V battery. One is for the ADS1115, and the other is for the Micromite. I decided to use separate supplies so that the current drawn by the Micromite (much greater than for the ADS1115) would not affect the readings. I built it on a 24×36-hole stripboard using through-hole components. I paid careful attention to the ground paths to avoid voltage drops on the sensing side of the circuit. The ADS1115 is set up with a range of ±4.096V, so with the 100kW/27kW voltage divider, the maximum voltage handled is 19.2V – well above the maximum voltage of 16V that my battery can produce. Apart from the voltage divider on each channel, there is a noise-rejecting RC low-pass filter formed by a 100W resistor and a 10μF tantalum capacitor. Australia's electronics magazine The power supplies are conventional, each using a 7805 linear regulator, with 47μF capacitors on the inputs and outputs as well as 100nF capacitors in parallel with those for better high-frequency performance. The software sets up each ADC conversion and then waits for 600ms before each reading. I found that to give maximum accuracy. Each channel is separately calibrated with a bias. The bias values ranged from -3mV to +70mV and were found by comparing my Fluke multimeter (calibrated) with the readings. The 70mV offset was surprising but only occurred on that particular channel. It determines cell voltages by subtracting the readings at the top and bottom of the cell. The BASIC software is written to be easy to understand and modify. You can download it from: siliconchip.au/Shop/6/354 Grant Muir, Sockburn, New Zealand. ($80) Simple over-temperature alarm I built this circuit to monitor the temperature of charging batteries; in my case, NiCd, NiMH and SLA types. It could also be an alarm for egg incubators. I set it to trigger at about 40°C, with trimpot VR1 (which acts like a rheostat) set to 82kW. I used a 40106 hex Schmitt trigger inverter IC for IC1. A 74HC14 will work, but its supply voltage cannot exceed 5V. If an under-temperature alarm is required, you can transpose TH1 and VR1 and replace VR1 with a 500kW type. In my configuration, when 40°C is reached at TH1, the voltage at pin 1 of IC1 (about half the supply voltage) enables the 2Hz oscillator built around IC1b, which then modulates the 800Hz oscillator built around IC1c. This causes a pair of inverters (IC1d & IC1e) to drive the piezo transducer differentially, resulting in a piercing pulsing tone and a flashing LED. The unused inverter (IC1f) has its input tied to 0V, as CMOS chips do not like floating inputs. The idle current is under 0.5mA, increasing to 8mA when the alarm sounds. Because of the hysteresis built into IC1, a slight reduction in the temperature will not reset the alarm; the power needs to be disconnected. It will self-reset if the temperature drops sufficiently. In use, the thermistor is temporarily taped to the battery. Note that 40106 IC thresholds and hysteresis vary quite widely between individual devices, so the frequencies of the two oscillators may vary. If required, they can be adjusted by tweaking the feedback resistor values. Warwick Talbot, Toowoomba, Qld. ($60) Very simple HT generator I have used this simple circuit over the years to get an HT supply for a valve radio from 12V DC. It uses readily available parts and just works, despite its simplicity. Be aware that you can get a tingle if you touch the 2N3055s at the same time due to back-EMF from the power transformer. The transistors must be attached to a small heatsink but electrically insulated from it. Almost any transformer with at least two low-voltage secondaries, 28 Silicon Chip rated around 6V each, can be used. I used an 80-year-old valve radio power transformer, driving the 5V (rectifier) and 6.3V (filament) secondary windings. I took the HT from the 240V winding, ignoring the 375V A-side original HV secondary. The 2N3055s were insulated from a small heatsink with mica washers. If the transistors can be touched from outside the case, I suggest using a couple of clip-on plastic TO-3 covers (from element14 or similar). Australia's electronics magazine I have also used a Jaycar power transformer with several low-voltage secondaries, again pulling the high tension from the 230/240V winding. Keep in mind that the HT output is not regulated, so its voltage will depend on the load and the properties of the transformer used. The circuit can deliver enough power to operate an AR7 or similar communications receiver. Peter Laughton, Tabulam, NSW. ($60) siliconchip.com.au altronics.com.au Tech Savers Finish up 2024 with sleigh loads of tech gift ideas for the whole family. Prices end December 31st. SAVE $30 149 M 8196 $ 279 $ C 5162 189 $ Why pay $300 or more? C 5161 D 0507E 10000mAh SAVE 17% 29 $ Boom Box & Wireless PA Systems Need instant sound for your next big get together? 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Project by Andrew Levido f you have ever worked on high-power audio amplifiers, vintage radios or switch-mode power supplies, you have probably been ‘bitten’ by a capacitor that remained charged after the circuit was disconnected from the power source. Even if you have been careful to keep your fingers out of the way, it is all too easy to accidentally discharge such a capacitor with a soldering iron or screwdriver, with startling and perhaps damaging consequences. It is therefore always good practice to safely discharge such capacitors before working on a device. You should definitely not do this by shorting the capacitor with a test lead (or worse, a screwdriver), since the amount of energy stored can be significant and the peak currents could be huge. Doing so is not good for the capacitors, the printed circuit board (PCB), the shorting device or the nerves of anyone nearby. It’s much better to use a controlled discharge device that limits the current to an acceptable level. An obvious and common choice is to discharge the capacitors via a power resistor. That is where my thinking started when I set out to build a simple discharger for myself. I envisaged a power resistor mounted in a small case with a couple of banana jacks so I could use standard test leads siliconchip.com.au to discharge the capacitors in question. I wanted a discharger good for voltages up to about 400V DC, making it suitable for off-line switch-mode power supplies and vintage valve gear. I wanted it to be able to handle this voltage indefinitely, so the discharger would not be destroyed if it was accidentally left connected or was connected when power was applied. For example, a 10W resistor would need a value of 16kW or more to be permanently connected across a 400V supply – any lower, and the 10W rating would be exceeded. However, a 10W resistor running at its rated power can get hot enough to burn skin (or even boil water!). So you might instead use a value of about 33kW to keep temperatures reasonable, giving a maximum dissipation of 4.85W (400V2 ÷ 33kW). The problem with a resistive discharge circuit is that the capacitor voltage will fall exponentially, as shown in Fig.1. The figure shows the normalised capacitor voltage on the vertical axis and time on the horizontal axis. The decay time depends on the circuit time constant τ, given by the Normalised RC Decay 1.0 0.9 Normalised Capacitor Voltage I 0.8 0.7 0.6 0.61 0.61 0.5 0.4 0 0.37 .37 0.3 0.22 .22 0 0.2 0.15 .15 0 0 0.08 .08 0.1 0.0 0.0τ 0.5τ 1.0τ 1.5τ 2.0τ 2.5τ 0.05 0 .05 3.0τ 0 0.03 .03 3.5τ 0.02 0 .02 4.0τ 0.01 . 0 4.5τ 5.0τ Time constants: τ=RC (seconds) Fig.1: when discharging via a resistor, the voltage across a capacitor decays exponentially at a rate determined by the time constant τ, which is the product of the resistance and capacitance. Australia's electronics magazine December 2024  33 product of resistance and capacitance. The numbers adjacent to the curve indicate the level of discharge achieved after a given number of time constants. The graph shows that discharging a capacitor from 400V down to a safe level (less than say 10V) will take about four RC time constants. With a 1000µF capacitance and 16kW or 33kW resistance, the discharge would take 64 or 132 seconds (about one/two minutes) – way too long in my book. We can calculate the average power dissipated in the resistor during this process by dividing the energy stored in the capacitor by the time taken to discharge it. The energy stored in a capacitor is ½CV2, which works out to 80J in our example. We know the discharge time is 64/132 seconds, giving us an average power dissipation of 1.25W/0.6W. Neither seems like an efficient use of the resistor’s power rating. At the start of discharge, the resistor draws 25mA/12mA from the capacitors, with an instantaneous power dissipation of 10W/5W, but it decreases rapidly as the capacitor voltage falls. What if we could draw a constant 25mA and discharge the capacitor this way? We know that the relationship between the current in a capacitor and the voltage across it is I = C × ΔV/ Δt. This means the capacitor voltage will fall linearly at a rate of -I/C with a constant discharge current. In our example, this will be -25V per second, discharging to 10V in just under 16 seconds, four to eight times faster than using a resistor. The peak power dissipation will be 10W, but the average will now be 5W – much better. That is all good, but I still had to develop a simple circuit that would sink a relatively constant 25mA over a wide voltage range. It should also be polarity independent, since I wanted to be able to use the discharger without worrying about which lead goes where (one of the benefits of simple resistors...). Circuit details Fig.3: the measured current of the prototype ranges from a little over 26mA at 400V down to about 16mA at 8V. That’s enough to discharge all but the largest capacitors reasonably quickly. The resulting circuit is shown in Fig.2. The capacitor to be discharged connects via banana jacks CON3 and CON4, and a normally-closed thermal switch, to the diode bridge formed by diodes D1 to D4. The diode bridge means that it does not matter which way the capacitor is connected; either way, the positive voltage gets applied to the drain of Mosfet Q1 and the negative voltage to its source. The discharge current flows through LED1, giving a handy visual indication that the capacitor is discharging. The remaining part of the circuit is the current sink proper. The Mosfet is biased on via the string of three 47kW resistors. Three resistors are used to get sufficient voltage and power ratings, as almost all of the input voltage appears across them (a 1W resistor is generally capable of handling 400V DC, but it’s better to be safe than sorry!). As the Mosfet begins to conduct, the voltage across the 27W resistor rises until it reaches around 650mV, at which point transistor Q2 begins to switch on, pulling the Mosfet gate down and restricting the current through the Mosfet’s channel to approximately 25mA. The zener diode is required to ensure the Mosfet gate-source voltage never exceeds a safe level, particularly during start-up. You may be wondering why I used a 600V, 13A TO-220 Mosfet for an application with a maximum current of 25mA. The reason for the voltage rating should be obvious, but since we are operating this Mosfet in the linear mode, it is the power dissipation rather than the current rating that is critical. This Mosfet needs to dissipate up to 10W, so I used a TO-220 package device mounted on a heatsink. Most of the parts in the circuit are fitted to a PCB housed in a small plastic enclosure. The Mosfet and the Australia's electronics magazine siliconchip.com.au Fig.2: this shows the complete capacitor discharger circuit. It sinks a relatively constant 25mA from 10V to over 400V. Current vs Applied Voltage 30 25 Current (mA) 20 15 10 5 0 34 0 50 Silicon Chip 100 150 200 Voltage (V) 250 300 350 400 thermal switch are both mounted on a heatsink formed from a piece of aluminium angle. The thermal switch is a fail-safe device that disconnects the circuit if the heatsink temperature reaches 90°C. That should never happen under regular use, but it prevents overheating if the discharger is left connected for extended periods while power is applied. In practice, the discharge current is not perfectly regulated, as shown in Fig.3. The measured current for my unit was 26.6mA at 400V, dropping to around 20mA at 10V and 16mA at 8V. Below this, there is insufficient voltage to bias the Mosfet on, so the current drops almost to zero. The LED lights when a charged capacitor is connected; it goes out when the capacitor voltage drops to less than 10V, giving a useful indication that discharging is complete and the circuit is safe. Keep in mind that the LED will also go out if the thermal breaker trips, but that’s pretty unlikely in normal use, and you would hear it if it did (assuming you do not have severe hearing loss). The LED colour is not critical but if you use one with a higher forward voltage (like green, blue or white), it will stop discharging at a slightly higher voltage. If you want to be sure (to be sure), you can always check the capacitor’s final voltage with a DVM before proceeding to work on the circuit. If you see the voltage increasing, don’t freak out! That is a phenomenon called dielectric charge absorption. It is very common in large electrolytic capacitors; unloaded, they can recover quite a bit of their initial charge over time. Because of that, you may want to leave the discharger connected to the capacitor for a while, to make very sure it’s drained before working on the device! The PCB is a neat fit in the handheld case, with the banana sockets mounting each on one end panel. Fig.4: the PCB is quite simple, so assembly is straightforward. Ensure the diodes, LED and transistors are orientated correctly and avoid dry joints; it should work first time. Construction Construction is very straightforward. The Capacitor Discharger is built on a double-sided board coded 9047-01 that measures 90 × 50mm. Refer to the PCB overlay diagram, Fig.4, to see which parts go where. Fit the diodes first, ensuring they are in the correct positions and have all the cathode stripes facing the top of the board. Then mount the resistors, followed siliconchip.com.au Fig.5: drill the heatsink (aluminium angle) according to this diagram. The shape of the semi-circular cutout is not critical as long as there is room for the LED leads to clear the heatsink. Australia's electronics magazine December 2024  35 Parts List – Capacitor Discharger 1 double-sided PCB coded 9047-01, 90 × 50mm 1 dark grey 120 × 60 × 30mm ABS plastic moulded enclosure [Jaycar HB6032, Altronics H0216] 1 90°C normally-closed (NC) thermal switch (S1) [Jaycar ST3825, Altronics S5612] 2 panel-mounting banana jack sockets (CON3, CON4) [Jaycar PS0421, Altronics P9267] 1 pair of mains-rated probes with banana plugs 1 90mm length of 25 × 12 × 1.6mm aluminium angle [Bunnings I/N 1138107 or 0427711] 3 M3 × 10mm panhead machine screws, flat & shakeproof washers & nuts 4 No.4 × 6mm self-tapping screws 1 small tube of thermal paste 1 150mm length of mains-rated hookup wire Semiconductors 1 STP18N60M2 or AOT10N60 600V 10A Mosfet or equivalent, TO-220 (Q1) [Silicon Chip SC4571, element14 2807284, DigiKey 497-13971-5-ND] 1 BC547 45V 100mA NPN transistor, TO-92 (Q2) [Jaycar ZT2152, Altronics Z1040] 1 red 5mm 30mA LED (LED1) [element14 2322131] 1 7.5V 0.4W or 1W zener diode, DO-41 (ZD1) [Jaycar ZR1407, Altronics Z0332] 4 1N4007 1kV 1A diodes, DO-41 (D1-D4) [Jaycar ZR1007, Altronics Z0112] Resistors 3 47kW 5% 1W axial [Jaycar RR2814, Altronics R7257] 1 27W 5% ¼W axial [Jaycar RR0534, Altronics R7520] This photo and Fig.7 show the simple wiring required. Capacitor Discharger Kit (SC7404, $30 + P&P): includes the PCB, resistors, semiconductors, mounting hardware (no heatsink) and banana sockets. by the small transistor, with its flat face orientated as shown. Leave the Mosfet and LED off the board for now. The heatsink bracket is made by cutting 90mm from a piece of standard 25 × 12 × 1.6mm aluminium ‘unequal angle’, drilled as shown in Fig.5. The semi-circular cutout at the bottom centre of the heatsink is to clear the LED leads. Its exact shape is not critical; it can be formed by hand with a round file. Once drilled and deburred, the bracket can be attached to the PCB by mounting the thermal switch with two M3 × 10mm machine screws with washers and nuts. The screws should be installed from the bottom of the board to ensure they don’t interfere with the case. Use a dab of heatsink compound under the thermal switch. Make sure to line up all the holes in this step. You may need to carefully bend the terminals of the switch down to about 45° to allow the lid to be fitted. Bend the Mosfet leads and fit this using another M3 × 10mm screw, with a nut and washers in the same way. Again, use heatsink compound under the Mosfet. Carefully tighten the Mosfet down before soldering so you don’t put any undue strain on the leads. Now drill a 5mm hole right in the centre of the case top for the LED, plus two 12mm holes, centred in both end plates for the banana jacks, as shown in Fig.6. Test-fit the PCB into the case and clip the LED’s leads to the correct length so its lens just protrudes through the hole in the top of the case when assembled. Then you can solder it in permanently. Finally, fit a couple of wires to the CON1 and CON2 pads on the PCB. After that you can wire up the Fig.6: one 5mm hole is required in the top of the case for the LED, plus one 12mm hole in each end plate for the banana jacks. 36 Silicon Chip Australia's electronics magazine siliconchip.com.au connectors and thermal switch as shown in Fig.7. The wire doesn’t need to be thick but it should have mains-rated insulation to ensure it will withstand up to 400V. Finally, you can screw the board down using 6mm self-tapping screws and close up the box. Silicon Chip PDFs on USB Testing & operation To check that the Capacitor Discharger is working, you can connect it (either way) across a power supply and adjust the voltage. You should see the LED light and a current draw in the region of 18-25mA at any voltage above about 10V. Using it is as simple as connecting a pair of test probes to each side of any potentially charged capacitors – I use a cheap pair I picked up online. Remember that high voltages might be applied to those test probes; don’t use really cheap ones if you will be applying 400V DC! Still, in our experience, you don’t need to spend much money to get clips with decent insulation. If the LED lights when the clips are attached, the capacitor is charged, so hold the probes in place until it goes out. It should only take a matter of seconds if it's a single capacitor, although a large capacitor bank like in a power amplifier could take longer to fully discharge. In the case of an amplifier with two capacitor banks (positive and negative), you can connect it across both banks to discharge them at the same time. This simple, low-cost project is well worth building if you develop or serSC vice any high-voltage devices! ¯ A treasure trove of Silicon Chip magazines on a 32GB custom-made USB. ¯ Each USB is filled with a set of issues as PDFs – fully searchable and with a separate index – you just need a PDF viewer. ¯ Ordering the USB also provides you with download access for the relevant PDFs, once your order has been processed ¯ 10% off your order (not including postage cost) if you are currently subscribed to the magazine. ¯ Receive an extra discount If you already own digital copies of the magazine (in the block you are ordering). EACH BLOCK OF ISSUES COSTS $100 NOVEMBER 1987 – DECEMBER 1994 JANUARY 1995 – DECEMBER 1999 JANUARY 2000 – DECEMBER 2004 JANUARY 2005 – DECEMBER 2009 JANUARY 2010 – DECEMBER 2014 JANUARY 2015 – DECEMBER 2019 OR PAY $500 FOR ALL SIX (+ POST) Fig.7: wiring the capacitor discharger could not be more straightforward. Use any handy mains-rated hookup wire, as the maximum current is 25mA. siliconchip.com.au Australia's electronics magazine WWW.SILICONCHIP.COM. AU/SHOP/DIGITAL_PDFS December 2024  37 Part 2: Op Amps Precision Electronics Last month, we examined broad concepts related to precision circuit design and built a simple circuit to measure current over a wide range. We’d like to improve its precision, and to do so, we need to learn a bit more about working with op amps – this month’s topic. By Andrew Levido T he simple circuit we devised last time to measure the current in a hypothetical power supply is shown in Fig.1. We used basic parts and achieved an average result. The error budget we calculated for this circuit is reproduced in Table 1. The largest source of error was the op amp’s input offset voltage, which contributed 7% out of the total 9% worst-case error. One way to improve this circuit would be to select a ‘better’ op amp. The trick, of course, is to decide what exactly we mean by better in this case. There are many hundreds of op amps described by their manufacturers as “precision op amps” – they can’t all be just what we want! The ideal op amp At the macro level, it’s handy to consider op amps as an ideal component. The ideal op amp has infinitely high input impedance, so no current flows into or out of the input pins. It has infinite differential-mode gain and zero common-mode gain or offset error. That means that the output is exactly zero when the input pins are at the same voltage, regardless of what voltage that is. It also has zero output impedance, and the output voltage changes instantaneously when the differential input voltage changes, regardless of the output load impedance. Considering op amps to be ideal is handy when analysing op amp circuits; all the classic op amp equations we use every day make this assumption. For example, we can calculate the gain of a non-inverting amplifier such as that in Fig.1 to be (1 + R1 ÷ R2) because we assume that the op amp is ideal. Of course, real op amps are not ideal, although they come very close in many respects. We need to be aware of and understand the non-idealities when designing precision circuits. Input bias and offset currents Fig.2 shows the simplified circuits of two very common ‘jellybean’ lowcost op amps taken from their data sheets. Depending on where you get them, you can pay less than 10¢ per individual op amp for these useful devices, even in low quantities. The LM324 (the quad version of the LM358), a bipolar transistor based op amp designed for single-supply operation, is shown at the top. Below it, is the TL074H JFET-based op amp (an improved release of the TL074 and the quad version of the TL071H/TL072H). Both designs use a simple differential input transistor pair with current Table 1: error budget for the circuit in Fig.1 (repeated from last month) mirror loads, although the types of transistors used differ. Note that the LM324’s input stage is inverted compared to that of the TL074H; we’ll explain that shortly. Compound transistors (similar to Darlingtons) are used for the LM324 input pair for reasons that will also soon become apparent. Inspecting the LM324 circuit, it should become obvious that some small current must flow out of the input terminals to bias the transistors on. This “input bias current” (Ib) can cause an unwanted voltage at the op amp’s inputs by generating a voltage across the source impedance. The effect of bias current naturally becomes more important when the source impedance is high. For the LM324, Ib is specified to be less than -35nA at 25°C, up to -60nA over the operating temperature range (–40°C to +85°C). The usual convention is that positive currents flow into a pin, so these negative values imply that the bias current flows out of the pin. The bias current is why you may see a resistor connected from the non-­ inverting input to ground in inverting amplifier circuits. The value is chosen to have the same resistance as the source network connected to At Nominal 25°C Error Nominal Value Shunt Resistor: Stackpole CSR1225 (1% 100ppm/°C) 100mW Node A Voltage due to I × R shunt 100mV 1mV Op Amp: LM7301 (Vos ±6mV, 2μV/°C) 0mV 6mV Node A Voltage total (Line 2 + Line 3) 100mV 7mV Op Amp Gain Resistor R1: Yageo RC0805 (1% 100ppm/°C) 1kW 1.00% 0.25% Op Amp Gain Resistor R2: Yageo RC0805 (1% 100ppm/°C) 24kW 1.00% 0.25% Op Amp Gain (R1 + R2) ÷ R1 25 0.5 2.00% 0.125 0.50% Vout (Line 4 × Line 7) 2.5V 0.225V 9.00% 0.02V 0.80% 38 Silicon Chip Abs. Error Rel. Error 0-50°C (Nominal ±25°C) Abs. Error 1.00% Australia's electronics magazine 1.00% Rel. Error 0.25% 0.25mV 0.25% 0.05mV 7.00% 0.3mV 0.30% siliconchip.com.au the inverting input so that any voltage due to the bias current is equal on both inputs and therefore cancels out. Without that, a differential temperature drift can occur, making trimming the op amp almost impossible! However, the bias currents at each input will never be precisely equal due to manufacturing tolerances. Ib is actually defined as being the average of the two bias currents. The difference between them is the “input offset current” (Ios). For the LM324, this is specified to be no more than ±5nA over the full temperature range. You may have now figured out one of the main reasons for the LM324’s use of compound transistors – they have a much lower base current for the same collector current, so using compound transistors here helps to minimise that pesky input bias current. Even so, the input bias current of the FET op amp is much lower than that of a bipolar op amp due to the diodes at JFET gates being reverse-­biased during normal operation. For the TL074H, the maximum bias current is ±120pA at 25°C and ±5nA (±5000pA) over the full temperature range. Notice that while the input bias current for the FET op amp is lower at room temperature, it is much more sensitive to temperature. The input offset current is also proportionally higher as it’s harder to match JFETs than it is to match BJTs. CMOS op amps are available that use Mosfets for the inputs, which have an even higher gate impedance, and thus lower bias currents (in the femtoamps!), like the LMC6482. LM324 to be [V–, V+ – 2.0V] (over its operating temperature range). That means the input range extends from zero (V–) to 2V less than the positive supply voltage. Op amps designed for single-supply operation often have this ‘inverted’ PNP or P-channel input stage with Vcm extending to 0V. The TL074H input stage also has a Vcm limitation, but because it uses N-channel JFETs in a conventional differential pair, the limitation is on the negative rail side. The Vcm of the TL074H is [V– + 1.5V, V+]. Exceeding the common mode range can cause very odd behaviour in some devices, so you generally must ensure your input signals stay within the op amp’s rated Vcm range. Fig.1: our first attempt at sensing current from the last article. This circuit used simple parts and achieved very average results with untrimmed errors in the order of 2% at 25°C. We can do much better by selecting better parts. Input common-mode range The other thing that should be apparent is that the range of input voltages over which the differential pair can operate is limited. Looking at the LM324, the input transistors’ base-emitter junctions will be forwardbiased with the inputs at the negative rail (the ESD protection diodes will prevent them from going much lower). However, there must be some voltage drop across the Vbe junctions of the input transistors and the 6µA current source, so there will be an upper limit on the input voltage somewhat lower than the positive supply. Above this limit, the transistors will be biased off. This active input voltage range is known as the common-mode voltage range (Vcm) and is specified for the siliconchip.com.au Fig.2: these simplified internal circuits of the LM324 (top) and TL074 (bottom) op amps show the input differential pairs and push-pull output stages. The LM324’s input stage is inverted compared to the conventional differential pair of the TL074 because the LM324 is designed for single-supply operation. Australia's electronics magazine December 2024  39 This can become a problem when operating from low-voltage supplies, which are common these days. For example, the LM324 will work with a supply as low as 3V, but in this case, the Vcm range will be just [0V, 1V]. You should also be careful if you intend to use an op amp designed for dual-­ supply operation in a single-­supply circuit, as the Vcm may not extend to either voltage rail. Rail-to-rail input op amps Plenty of op amps claim to have ‘railto-rail’ inputs, such as the LM7301 we used in the first instalment of this series. These op amps usually have two differential pairs at the input – both NPN and PNP in the case of bipolar op amps, or an N-channel FET and a P-channel FET in the case of FET-­ input op amps. These work well in many applications, and their Vcm range includes both supply rails, but they have a few peculiarities you should be aware of. Because they effectively switch between two input stages, their input bias current and input offset voltage can show unusual behaviour. Fig.3 shows that, for the LM7301, the input bias current reverses polarity a volt or so below the positive supply rail. The graph also shows that the input offset voltage kicks up at the same point as the op amp switches from one input circuit to another. We saw in the last article that one of the keys to precision circuit design is to trim out constant errors (usually in software). The type of non-­linearities that rail-to-rail input op amps can introduce can make this trimming very difficult. By all means, use them when needed, but exercise caution. Input offset voltage (Vos) This brings us to input offset voltage, which is causing most of the problems with our test circuit. Identical input transistors with identical collector or drain currents at the same temperature should have identical base-emitter or gate-source characteristics. Unfortunately, manufacturing variances mean neither the transistors nor the mirrored currents will be perfectly identical, so there will be a difference in Vbe or Vgs(th) between the two input transistors. The impact of these differences means that even with the input pins connected together, the output of an op amp will saturate at one supply rail or the other (and you can’t predict which). If the loop is closed, the output voltage will be the difference in Vbe or Vgs(th) multiplied by the closed-loop gain. This difference can be modelled as a small voltage source in series with one of the inputs of otherwise perfectly matched input transistors. This is the definition of input offset voltage (Vos). In the case of the LM324, Vos is specified to be ±2mV (worst case) with ±7µV/°C of temperature drift, whereas for the TL074H, it is ±4mV (worst case) with ±2µV/°C drift. JFET op amps usually have a higher Vos since a JFET’s (or Mosfet’s) Vgs(th) parameter is less tightly controlled than the bipolar transistor’s Vbe. Reducing input offset voltage Op amp offset voltage is caused Fig.3: this extract from the LM7301 data sheet shows how the input bias current abruptly switches polarity, and the input offset voltage kicks up when the input common-mode voltage gets to within a volt or so of the positive rail. This results from the rail-to-rail input stage switching between the normal and inverted differential pairs. Both plots are for ±2.5V supply rails. 40 Silicon Chip Australia's electronics magazine by unavoidable manufacturing variation between the input transistors, so you might think we are stuck with it. However, op amp designers are a pretty creative group, and they have come up with some very clever circuits to minimise voltage offset and, more importantly, minimise offset voltage drift with temperature. The first technique is laser trimming, where the offset voltage of an op amp is measured after manufacturing and then a laser is used to adjust the value(s) of onboard resistor(s) to compensate for it – a little like having a tiny trimpot onboard the IC that’s set before it’s packaged. Doing this costs money, so high-­ precision op amps tend to cost more but can have very low offset voltages (and low drift), down to the sub-­ microvolt level in some cases. However, as it’s a static adjustment, it does nothing to improve temperature drift. An example of a laser-trimmed op amp is the OPA277PU, with a maximum Vos of ±20μV and a maximum Vos drift of ±0.15μV/°C. The second technique is auto-­ zeroing or auto-nulling, as shown in Fig.4. Along with the main op amp, OAa, the package includes nulling op amp OAb. During one phase of the clock (phase A), the inputs of OAb are connected together, so its output is its offset voltage, which is stored in capacitor C1. During the other phase (phase B), OAb measures OAa’s offset and stores it on capacitor C2. The voltage on capacitors C1 and C2 are used to null out the Vos of the nulling and the main amplifiers, respectively. The nice thing about this approach is that the primary signal through the main op amp, OAa, is never switched. OAb alternately nulls itself and OAa, more or less eliminating the offset regardless of how it changes over time. Another technique is the chopper approach, shown in Fig.5. Again, the amplifier is broken into sections OAa and OAb. On clock phase A, the two stages are connected such that neither stage inverts the input signal, while on phase B, they are connected such that both stages invert the signal. The result is that the output signal always has the right sense, but the offset voltage across the capacitor alternates in polarity and thus averages to zero. These circuits (and their variations) siliconchip.com.au Fig.4: auto-zero op amps have a second nulling amplifier that alternatively nulls its own Vos and that of the main amplifier. The result is extremely low Vos and, more importantly, very low Vos drift with temperature. Fig.5: a chopper op amp reduces the overall Vos by alternating the polarity of the signal through two stages. The output always has the same sense, but the offset voltage at the capacitor alternates in polarity and averages to zero. can achieve remarkable results in terms of low offset. The AD8551, for example, uses a nulling approach and has a maximum Vos of ±5µV with a ±40nV/°C tempco. The LTC2057 uses a chopper configuration and achieves even better results, with a maximum Vos of ±4µV with ±15nV/°C tempco. These figures are around 1000 times better than the jellybean op amps. The downside is that some switching artefacts will appear in the output, so they don’t have the best noise performance. They also tend to be limited in bandwidth and require a higher supply current, either of which could be a concern if you are building a high-bandwidth or an ultra-low power design. They are also more expensive, at around $5 for the LTC2057 and $6.50 for the AD8551. Input impedance We also need to consider the input impedance. Input impedance is the small-signal open loop impedance seen at the input. It is specified as a common-mode impedance (inputs tied together to ground) and a differential-­ mode impedance (between inputs). The common-mode impedance is usually the higher of the two. Differential mode impedance is not usually a concern at low frequencies, as negative feedback forces the voltage siliconchip.com.au between the inputs to zero, effectively bootstrapping the differential impedance to a very high value. Imperfect output stages You can see from Fig.2 that the output voltage of our op amps will not be able to swing all the way to either power rail due to the finite saturation voltage of the output transistors and the drop across the output current limiting circuits. In the case of the LM324, you can also see that the output swing may not be symmetrical. The output swing is generally described in terms of the voltage ‘headroom’ or how close the output voltage can approach the supply rails with some given load. With a 10kW load, the LM324 can reach within 0.15V of the negative rail but can only get to within 1.5V of the positive rail. On the other hand, the TL074H can get to within 0.25V of either rail with the same load. Some op amps offer output swings much closer to the rails than these basic parts, typically to within 50mV of the rails into 10kW. Still, no op amp will swing completely to the rail – a fact that caught us out in the first iteration of our test circuit in the previous article in this series (sometimes you can help them get closer with a resistor tied to one rail or the other, but it only works for one rail!). Australia's electronics magazine Op amp data sheets may show a figure for open-loop output impedance (125W in the case of the TL074H), but you can’t use this directly to determine the maximum output current or swing in closed-loop applications. That is because the effective output impedance is reduced by the loop gain. What may be important in your application is the maximum current that the op amp can source or sink, usually specified as a short-circuit current. This is typically in the ~20mA range (it’s ±26mA for the TL074H and ±40mA for the LM324). There are high-current op amps, some sourcing and sinking several amps, but they are rare and can be pricey. Gain, bandwidth & slew rate An op amp’s open loop voltage gain is not infinite, but it is pretty high, typically in the order of 100dB to 120dB at DC but dropping linearly to unity at a frequency ft, sometimes called the gain-bandwidth product (GBW). For stability, most op amps have internal dominant pole frequency compensation that reduces the op amp gain to 0dB at a frequency where the phase shift is well below 180°. Fig.6 shows a curve for a typical op amp. The open loop gain at DC is a little over 110dB, dropping from about 2Hz more-or-less linearly to ft, which is a little over 1MHz. In this December 2024  41 Table 2: error budget for the improved circuit in Fig.7 At Nominal 25°C Error Nominal Value Shunt Resistor: RESI PCSR2512DR100M6 (0.5% 15ppm/°C) 100mW Node A Voltage due to I × R shunt 100mV 0.5mV Op Amp: LTC2057 (Vos ±4μV, 15nV/°C) 0μV 4μV Node A Voltage total (Line 2 + Line 3) 100mV 0.504mV Op Amp Gain Resistor R1/R2: Vishay ACASA 1000S1002P1AT (0.1%, 0.05% matched, 15ppm/°C) 26W Op Amp Gain (R1 + R2) ÷ R1 26 0.013 0.05% 0.0098 0.038% Vout (Line 4 × Line 6) 2.6V 0.0144V 0.55% 0.002V 0.075% case, the phase shift at ft is -85°. The op amp would oscillate if the phase shift reached -180° and the gain was still greater than unity. The difference between the phase shift at ft and -180° is known as the phase margin; it is 95° in this case. This is the maximum phase shift your feedback circuit can safely introduce if you want the op amp to remain stable. It’s important to remember that the blue curve is the open loop gain. The orange line illustrates a typical closedloop gain, in this case, a gain of 10 (or 20dB). The closed loop gain is flat to about 100kHz, which is what you would expect with a gain-bandwidth product of 1MHz. One side effect of this dominant pole compensation is that it limits how quickly the op amp output can change in response to a change in the differential input voltage. This is known as the slew rate and it is typically measured in volts per microsecond (V/ μs). The LM324 has a GBW of 1.2MHz Abs. Error Silicon Chip Abs. Error 0.50% 0.50% and a slew rate of 0.5µV/s, while the TL074H has a GBW of 5.25MHz and a slew rate of 20V/µs. Op amps with a higher GBW usually (but not always) draw more supply current, and conversely, low-power op amps have a lower GBW. If you want an op amp with a low power draw and a high GBW, be prepared to pay extra. Choosing an op amp There is a lot to consider when choosing an op amp, and there are a vast number of options, so where do we start? I suggest you begin by narrowing down the parameters you really care about. Taking our current-­ measuring circuit as an example, we don’t care too much about the AC parameters, such as bandwidth and slew rate, since we are interested in DC measurements. With ±5V supplies and a signal ranging from 0V to around 2.5V, we also don’t have any stringent Vcm or output swing requirements, so we can set Australia's electronics magazine Rel. Error 0.038% 0.0375mV 0.038% 0.375μV 0.50% 0.0379mV 0.05% Fig.6: most op amps have an open-loop gain dominated by a low-frequency pole that ensures the gain (blue curve) falls to 0dB well before the phase shift reaches -180°. This ensures the op amp remains stable at any closed-loop gain. The frequency at which this occurs is known as the ft (the transition frequency) or gain bandwidth product (GBW). 42 Rel. Error 0-50°C (Nominal ±25°C) 0.038% 0.038% them aside. As long as the input and output voltages are within a couple of volts of the rails, we will be OK. Since our source impedance is very low due to the low-resistance current shunt, the contribution to error from input bias and offset currents will be negligible. So, our primary focus should be on Vos and, more importantly, its drift with temperature. Cost and availability are also factors that should not be ignored. It so happens that I had a few LTC2057s on hand, and we have already seen their Vos figures are impressive, a maximum of ±4µV with ±15nV/°C tempco. Other improvements While we are at it, we should look also at the rest of the components. The shunt resistor has a tolerance of ±1% and a tempco of 100ppm/°C. Lowvalue resistors with very tight tolerances (say in the 0.1% range or better) are extremely expensive, so they are not worthwhile since this kind of Fig.7: the improved version of the circuit from Fig.1. The LTC2057 has much better offset performance and the gain resistor ratios have much better temperature tracking. The resulting circuit will have better untrimmed accuracy but, more critically, less drift with temperature changes. siliconchip.com.au error can be trimmed out. However, it is possible to get a resistor with a much lower temperature coefficient at little extra cost. For example, the 100mW resistor in Table 2 has a tempco of ±15ppm (and a slightly better tolerance of 0.5%) for about $3.30 each in quantities of 10. We can also do better with the tempco of the gain-setting resistors. Again, we could splash out on expensive 0.01% resistors, but that would be wasting money. What matters most to us is the temperature coefficient. Further, what we really care about is the tempco of the ratio of the gain setting resistors, since if they drifted high or low together at precisely the same rate, the gain would not change. I like to use low-cost matched resistor arrays for this type of application. These have a small number of lasertrimmed resistors on a common substrate. They are well-matched in value and likely to be at the same temperature, thus tracking each other well. The Vishay ACASA range of resistors fits the bill perfectly. They are low in cost, have a 0.1% overall tolerance, and are matched to within 0.05%. The most readily available subset has an absolute temperature coefficient of ±25ppm and a relative temperature coefficient of ±15ppm. An array of four such resistors costs ~$1 each in lots of 10. We can’t quite get the 24:1 ratio of R1:R2 in the original circuit since the ACASA range comes in only a few values, but I can get an array consisting of two 100W and two 10kW resistors that can be arranged to create a 25:1 ratio. The result is a gain of 26 instead of 25, but that should not be a problem since we can scale and offset our readings in software. Fig.7 shows the revised circuit diagram. I have put these components into the error budget table (Table 2), which shows we can expect an untrimmed precision of ±0.55% at 25°C with a further 0.075% drift over the 0°C to 50°C temperature range. The overall untrimmed precision is about 20 times better than before, and the temperature performance is about 10 times better than the previous design. The error is dominated by the initial shunt tolerance, which will have to be trimmed out. Experimental results The test results are shown in Table siliconchip.com.au Measured Data Error Measured Data Current Vout Abs. Rel. 0.076 Current Vout Error Abs. Rel. -1.100 -1.3 -0.05% 0.0 0.2 0.0 0.00% 99.810 258.380 -1.1 -0.05% 97.9 259.2 -0.4 -0.01% 199.795 519.380 -0.1 0.00% 198.2 519.6 0.1 0.01% 299.311 779.470 1.3 0.05% 298.3 779.2 1.0 0.04% 400.073 1040.64 0.5 0.02% 398.3 1039.8 -0.4 -0.02% 500.314 1302.00 1.2 0.05% 498.3 1300.6 -0.2 -0.01% 600.575 1563.33 1.8 0.07% 598.3 1561.4 -0.1 0.00% 700.995 1825.17 2.6 0.10% 698.0 1822.7 0.1 0.00% 801.785 2087.33 2.7 0.11% 798.0 2084.3 -0.3 -0.01% 902.612 2350.11 3.3 0.13% 898.0 2346.5 -0.2 -0.01% 1003.431 2613.58 4.7 0.19% 998.0 2609.5 0.6 0.02% Table 3 – measurements from the Fig.7 prototype. Units: Current (mA), Vout (mV), Absolute (mV), Relative (%). Table 4 – readings after applying fixed offset and gain corrections. 3. To measure circuits of this precision, you need good instruments and a carefully designed measurement setup. The worst-case error is just under 0.2% at full scale, and it increases steadily, suggesting a gain error of some kind. These values are plotted in Fig.8, along with a line of best fit. This suggests we have an offset error of about -1.3mV (about 50µV on the input side of the op amp) and a gain error of about 0.2%, most likely due to the shunt resistor tolerance. Table 4 shows the results if we apply a fixed offset and gain correction to the measured values. That gives a trimmed precision better than ±0.04%. From the error budget, you will see that the tempco is of the same order (±0.075%), so we can achieve an overall precision of a little over 0.1%. That is a tenfold improvement over our initial circuit. Next time, we will look at how we could measure this current if the shunt were in the positive supply instead of being ground-referenced. That is often desirable so the load can share a common ground with the supply (which would be necessary if both were Earthed). References • AD8551 data sheet: siliconchip. au/link/ac01 • “Demystifying Auto-Zero Amplifiers Part 1”: siliconchip.au/link/ac02 • LM324B data sheet: siliconchip. au/link/ac03 • LM7301 data sheet: siliconchip. au/link/ac04 • LTC2057 data sheet: siliconchip. au/link/ac05 • TL074H data sheet: siliconchip. SC au/link/ac06 Fig.8: a plot of the data points from Table 3 with a line of best fit. This suggests an offset of -1.3mV and a gain error of about 0.2%. We can use these figures to trim the measured values and eliminate fixed errors. Australia's electronics magazine December 2024  43 Part 1: by Nicholas Vinen Compact HiFi headphone Amplifier This Headphone Amplifier is easy to build, sounds great, doesn’t cost too much to make and fits into a compact instrument case. It’s ideal for beginners or just those who want to get the best out of a set of traditional wired headphones. It’s powered by a plugpack, so no mains wiring is required. I t has been a while since we’ve published a headphone amplifier. The reason I decided to design a new one is that my last design (in the September & October 2011 issues; siliconchip.au/ Series/32) had excellent audio quality, but was a bit overkill for many people. It was fairly large, somewhat expensive to build and consumed a fair bit of power, but you can’t really fault the resulting sound quality. Before that, we published the Studio Series Headphone Amplifier (November 2005; siliconchip.au/Series/320), which was not an integrated design (it required a separate power supply board), didn’t really fit into any particular case and was a fairly basic design with modest output power and had decent but not amazing audio quality. I thought there was room for something in between: an amplifier with excellent audio quality that fit neatly into a compact case and wasn’t too difficult or expensive to build. That’s precisely what this is. It’s also beginner-­friendly and has the handy feature of two stereo inputs that are mixed with independent volume controls. Fig.1: the Amp’s distortion versus frequency for four common headphone/earphone load impedances. Distortion is lower for higher load impedances due to the lower output current required; the 600W curve is higher mainly due to the lower test power due to voltage swing limitations. 44 Silicon Chip That means you can connect two sound sources such as a TV and a computer, a CD player and a TV or something like that. With the separate volume controls, it’s easy to account for different output levels from those devices, and you can also easily mute one if both are active. If you want to save time and money, you can build it with just one stereo input. You have the choice of 3.5mm or 6.35mm jack sockets for the output (or both, optionally connected in parallel). Power is from a 9-12V AC 1-2A plugpack, a type that’s readily Fig.2: this shows how distortion varies with the output power level, at a fixed frequency. The onset of clipping is around 0.9W for an 8W load, due to current delivery limitations; a little over 1W for 16W; around 0.75W for 32W; or 90mW for a 600W load due to voltage swing limitations. Australia's electronics magazine siliconchip.com.au Complete Kit (SC6885; $70) Features & Specifications 🎼 Drives stereo headphones with impedances from 8Ω and up 🎼 Two outputs to suit 3.5mm or 6.35mm jack plugs 🎼 Two stereo RCA inputs with independent volume controls 🎼 Powered by a 9-12V AC plugpack 🎼 Power on/off switch and power indicator LED 🎼 Signal-to-noise ratio: 103dB with respect to 250mW into 8Ω 🎼 Total harmonic distortion: <0.0025% <at> 1kHz, <0.01% <at> 10kHz (see Figs.1 & 2) 🎼 Frequency response: 10Hz to 100kHz, +0,-0.2dB (16Ω load; see Fig.3) 🎼 Channel separation: >70dB <at> 1kHz (see Fig.4) 🎼 Maximum output power (9V AC supply): 0.9W into 8Ω, 1W into 16Ω, 0.75W into 32Ω, 80-140mW (12V AC) into 600Ω 🎼 Class-AB operating mode (Class-A at lower power levels) 🎼 Inexpensive and easy to build 🎼 Fits into compact 155×86×30mm ABS instrument case available from most suppliers. There is an onboard power switch and power indicator LED. The headphone amplifier section is based on common low-noise, low-­ distortion op amps with transistor buffers to boost the output current. It will drive any headphones from 8W to 600W. It won’t deliver a ton of power, but should be more than enough for any headphones, up to a watt (or maybe more) per channel. If you really wanted to, you could use it to drive a pair of high-efficiency speakers to modest sound levels (eg, for use with a computer). While it isn’t really designed for that task, it will work as long as the speakers are efficient enough and you’re close to them. This design uses all through-hole parts and it fits into a really nice little snap-together compact case that’s just 155mm wide, 30mm tall and 86mm deep. So it takes up barely any room. The modest power consumption means it only gets a little warm during typical use, despite being unvented. There’s really nothing tricky to the construction. The only slightly fiddly Fig.3: the Amp’s frequency response is very flat for all load impedances within the audible range (20Hz–20kHz). The deviation above 20kHz is due to the output filter. The vertical shifts are due to the Amp’s output impedance (the level reduces slightly for lower load impedances). siliconchip.com.au Includes the case but not a power supply bits are winding the inductors for the output filter (which only takes a few minutes) and mounting the output transistors and heatsinks, which is only difficult because the thermal paste can get on your fingers. There is one adjustment per channel for quiescent current. It’s easy to make by monitoring the voltage between pairs of test points with a DMM while twiddling a trimpot. With a circuit that isn’t too difficult to understand and straightforward construction, this should be a good project for relative beginners. Performance At low signal levels, up to around 5mW (8W), 10mW (16W) or 20mW (32W/600W), the Headphone Amplifier operates in Class-A mode. Many headphones and earphones will produce reasonable volume levels at such powers. If your headphones require more power, or there are loud transients (like drum hits), the amplifier will automatically switch to Class-B (this is known as Class-AB operation). The resulting performance is pretty good – not as good as our very best amplifiers, but certainly well above average. It’s better than ‘CD quality’ under most conditions (which equates to about 0.0018% distortion at 1kHz with a 96dB signal-to-noise ratio). Fig.4: there’s a small amount of signal bleed between channels but it’s attenuated by more than 70dB at 1kHz and below, so it is unlikely to be noticeable. Most stereo content has less separation than this anyway. Australia's electronics magazine December 2024  45 The power supply section is on the left, signal input/ mixing in the middle and power output on the right. The performance was measured with a 9V AC plugpack; using a 12V plugpack will give the same or better performance. Fig.1 shows how the total harmonic distortion plus noise (THD+N) level varies with frequency at 250mW (a high level for headphones!) into four common headphone load impedances. The performance is excellent for 32W headphones, well below 0.001% even up to several kilohertz. It’s almost as good for 16W, reaching only around 0.0015% at 1kHz for 16W & 600W loads. Even for the relatively low impedance of 8W, more typical for loudspeakers, the THD+N is just 0.0025% at 1kHz for a fairly high output level (250mW) and remains below 0.01% up to 10kHz. Fig.2 shows how THD+N varies with power level. As the performance is essentially limited by noise, it is a steadily descending line until the point where it goes into clipping. That figure will give you a pretty good idea of how much power can be delivered with the 9V AC supply. Fig.3 shows the frequency response, which is basically flat across the audible spectrum. Fig.4 shows the channel 46 Silicon Chip separation, which we think is pretty reasonable. You’re unlikely to notice any signal bleeding between the channels. Note that the maximum power delivery into high-impedance loads will depend on the supply voltage. Testing with a 9V AC plugpack, we got around 90mW into a 600W load before clipping, but we’d expect closer to 150mW with a 12V AC plugpack. Most headphones and earphones are well below 600W, so they are unlikely to run into voltage swing limitations even with a 9V AC supply. more than annoyance. It didn’t always happen, but it’s still a good idea to take the headphones off before switching the amplifier off. We also tested it by plugging in the Exteek C28 Bluetooth adaptor (reviewed in the September 2024 issue; siliconchip.au/Article/16569). We connected it to one input using a 3.5mm jack to twin RCA plug lead. That worked fine, and the Amp’s gain was more than enough to drive the headphones to deafening levels from its relatively low-level output. Subjective testing The full circuit diagram is shown in Fig.5. We’ll start by describing the input section and volume control, then the power amplification section, then the power supply. This description is for the full version of the circuit; later, we’ll explain two ways it can be cut down. The stereo input signals are applied to either of dual RCA sockets CON2 & CON3. They pass through an RF rejecting filter comprising ferrite beads, 100W series resistors and 470pF ceramic capacitors to ground. This should help eliminate any RF (eg, AM radio or switch-mode hash) picked up by the signal leads that I tested the Amp with a pair of Philips SHP9000 32W headphones (which, in my opinion, are excellent). As expected based on the flat frequency response and low distortion, the sound quality was topnotch, with lots of punchy bass, plenty of treble and no audible noise or artefacts. There was no noticeable noise at switch-on with the headphones plugged in, although more sensitive headphones may make a noise. There was sometimes a modest crack or thump sound at switch-off, although it was not loud enough to cause anything Australia's electronics magazine Circuit details siliconchip.com.au could otherwise be demodulated by the following circuitry. The signals are then AC-coupled using back-to-back polarised electrolytic capacitors. This is a cheaper and generally more compact configuration than non-polarised electrolytic capacitors, and has no real disadvantages. We use high-value coupling capacitors to retain good bass response, it also keeps the source impedance low for the following stages, to avoid noise creeping in. The capacitor voltage ratings here are pretty high, so that if a faulty signal source delivering +18V or -18V DC (or more) is connected to one of the inputs, it won’t damage anything. It’s important to AC-couple signals to potentiometers to avoid crackle when they are rotated. The signal is applied to the top of the potentiometers, which act as variable voltage dividers, the attenuated signal appearing at the wiper. The potentiometers have a ‘logarithmic taper’, which is suitable for volume control since it better matches the way we hear loudness. Linear potentiometers tend to give poor control at the lower end of the volume range. From the potentiometer wipers, the signals are again AC-coupled to the following op amp buffer stages, so that the op amp bias currents don’t cause a DC voltage across the pots. Otherwise, that can also cause crackle when the pots are rotated. Here we only need a polarised capacitor because we know the op amp input will be slightly positive due to the bias current flowing out of it. That is true for either of the op amp alternatives specified (NE5532 or LM833, which should both perform well). 100kW resistors to ground both DC-bias their input signal to 0V and provide a path for that bias current to flow. The signals from the two pairs of buffers are then mixed using 10kW resistors and the mixed audio is fed to the power amplifier, on the right-hand side of the diagram. The 1MW resistors to ground provide a path for IC1’s input bias currents to flow without IC2 and IC3 having to sink it, although the circuit would still work if those resistors were left out. Parts List – Compact Headphone Amplifier This section is based on dual lownoise op amp IC1 and medium-power 1 double-sided blue PCB coded 01103241, 148 × 80mm 1 155×86×30mm ABS instrument case [Altronics H0377, DigiKey 377-1700-ND, Mouser 563-PC-11477] 1 9-12V 1-2A AC plugpack 1 PCB-mount right-angle miniature SPDT toggle switch (S1) [Altronics S1320] 1 PCB-mount barrel socket to suit plugpack (CON1) 2(1) dual horizontal white/red RCA sockets (CON2, CON3) [RCA-210; Silicon Chip SC4850] 1 PCB-mounting DPST 3.5mm stereo jack socket (CON4) [Altronics P0092, Jaycar PS0133] AND/OR 1 PCB-mounting DPST or DPDT 6.35mm stereo jack socket (CON5) [Altronics P0073 or P0076/P0076A] – not the taller version 4(2) small ferrite beads (FB1-FB4) 1 2-pin header with jumper shunt (JP1) 2(1) 10kW dual-gang logarithmic taper 9mm right-angle PCB-mount potentiometers (VR1, VR2) 2 2kW top-adjust mini trimpots (VR3, VR4) 3(1) 8-pin DIL IC sockets (optional, for IC1-IC3) Wire & hardware 1 2m length of 0.25-0.4mm diameter enamelled copper wire (for L1 & L2) 2 M3 × 16mm panhead machine screws 4 M3 × 10mm panhead machine screws 6 M3 flat washers 6 M3 hex nuts 4 No.4 × 5-6mm panhead self-tapping screws 2 TO-220 micro-U flag heatsinks (15 × 10 × 20mm) 2(1) small knobs to suit VR1 & VR2 4 small self-adhesive rubber feet Semiconductors 3 NE5532 or LM833 low-noise, low-distortion op amps (IC1-IC3) ♦ 5 TTC004B 160V 1.5A NPN transistors, TO-126 (Q1, Q3, Q5, Q7, Q8) 3 TTA004B 160V 1.5A PNP transistors, TO-126 (Q2, Q4, Q6) 1 3mm blue LED with diffused lens (LED1) 2 1N5819 40V 1A schottky diodes (D1, D2) ♦ only one is required for cut-down version (unbuffered or single-channel) Capacitors (maximum 20mm height) 4 1000μF 25V low-ESR electrolytic (5mm pitch, maximum diameter 13mm) 2 470μF 10V electrolytic (5mm pitch, maximum diameter 10mm) 8(4) 100μF 50V electrolytic (5mm pitch, maximum diameter 8mm) 4 100μF 25V low-ESR electrolytic (5mm pitch, maximum diameter 8mm) 4(2) 100μF 16V electrolytic (5mm pitch, maximum diameter 8mm) 2 10μF 50V electrolytic (2.5mm pitch, maximum diameter 6.3mm) 2 100nF 63V MKT 3(1) 100nF 50V MKT, ceramic or multi-layer ceramic 4(2) 470pF 50V NP0/C0G ceramic 2 100pF 50V NP0/C0G ceramic Resistors (all ¼W 1% unless noted) 2(0) 1MW 4(2) 100kW 7(3) 10kW 4 4.7kW 2 3kW 4 1kW 2 220W 4(2) 100W 2 10W 1W 5% 4 1W ½W (5% OK) n number in bracket refers to quantities for the single-channel version siliconchip.com.au Australia's electronics magazine Power amplifier December 2024  47 Fig.5: the full Headphone Amplifier circuit; the two stereo inputs are at upper left, the buffer and mixer left of centre, the output section at upper right and the power supply at lower right. It’s all pretty conventional, but note the use of capacitance multipliers rather than regulators to provide reasonably steady V+ and V− rails without requiring a specific AC supply voltage. transistors Q3-Q8. As the left and right channels are essentially identical, we’ll stick to describing the right channel, with the corresponding left-­ channel designators being given in brackets (parentheses). The incoming signal is fed into the non-inverting input, pin 3, of IC1a. IC1a is configured as a non-inverting amplifier with a default gain of four times (12dB), although that can be changed by varying the 3kW and 1kW resistor values between the output and the feedback point, the pin 2 inverting input of IC1a. The bottom end of the divider is connected to signal ground via a 470μF capacitor rather than directly, reducing the amplifier DC gain to unity. That way, the circuit doesn’t amplify the op 48 Silicon Chip amp’s inherent offset voltage (or any other offsets in the circuit). Most of the current to drive the headphones is supplied by NPN transistor Q3 (Q5) and PNP transistor Q4 (Q6), which are complementary emitter-­followers. As the base voltage of Q3 rises, it sources more current into the output via its 1W emitter resistor, reducing its base-emitter voltage until it stabilises. Similarly, when Q4’s base is pulled down, its emitter pulls the output down and it too stabilises at a more-or-less fixed base-emitter voltage differential. As Q3 and Q4 both have base-emitter voltage drops of around 0.7V when conducting a few milliamps, if we arrange for a difference of around 1.5V between the two bases (with Q3’s base Australia's electronics magazine voltage being higher than Q4’s), a small amount of current will constantly flow from the V+ rail, through Q3, the two 1W emitter resistors, then Q4 and back to the V- rail. This is called the quiescent current. By having a small quiescent current, we keep Q3 and Q4 in conduction all the time, and we only have to vary the amount of conduction to smoothly control the output signal, rather than switching Q3 or Q4 on when needed. This is called Class-AB (sometimes Class-B) and it has the benefit of minimising (and ideally, virtually eliminating) crossover distortion. Crossover distortion is an undesirable step in the output voltage as it passes through 0V, which an AC audio signal does frequently. siliconchip.com.au To achieve the required ~1.5V between the bases, we have NPN transistor Q7 (Q8), which acts as a ‘Vbe multiplier’. There are 4.7kW resistors from the V+ and V- rails connected to its collector and emitter, which provide a small bias current of about 3mA through it. Trimpot VR3 (VR4) is connected across the transistor such that we can vary the collector-base and emitter-­ base resistances. The ratio of those siliconchip.com.au resistances causes a multiple of its mostly fixed base-emitter voltage (again, about 0.7V) to appear between its collector and emitter. By adjusting the trimpot for a gain of a little over two times, we get the required 1.5V. You will note that its collector and emitter connect to the bases of Q3 & Q4, so that voltage appears across them. It is stabilised by a 10μF capacitor as the output swings up and down (and thus the bias in Q7 varies slightly). Australia's electronics magazine The 10kW resistor across the trimpot prevents Q7 from switching off fully if the trimpot is intermittent, which would cause a high current to be conducted by Q3 & Q4, possibly damaging them. Another thing you might notice is that Q7 is the same type of transistor as Q3, even though it only needs to handle a tiny current and power. That is because Q3’s base-emitter voltage will vary as it changes in temperature. By December 2024  49 mounting Q7 in contact with Q3, the bias voltage changes proportionally, so Q3 always receives the correct bias voltage. Q4 is the complementary type to Q3; while we are not tracking its temperature directly, its dissipation will very closely match that of Q3, so its temperature should as well, and thus its base-emitter voltage will be very similar to Q4’s. So the thermal tracking by Q7 will compensate for temperature changes in both output transistors and their required bias voltages. The 1W emitter resistors provide a little local negative feedback for Q3 & Q4 and also help to stabilise the quiescent current, by making the exact bias voltage across their bases less critical. The junction of these resistors is the amplifier output, which is fed to the headphone socket(s) via an RLC filter comprising a 10W resistor in parallel with a 4.7μH inductor and then a 100nF capacitor to ground. This filter is there to isolate the amplifier output from the headphones, so that any reactance at the headphone socket (eg, from cable capacitance or driver properties) cannot destabilise the amplifier and cause it to oscillator. The values have been chosen so the filter doesn’t change the overall frequency response when combined with typical headphone impedances. Finally, there is a 1kW resistor between the output of op amp IC1a and the junction of the 1W emitter resistors. That means the op amp’s output contributes a tiny bit of current to the amp output, helping to cancel out any small amounts of distortion caused by the output stage that the feedback loop is too slow to handle. CON4 gives you the option to use the smaller type of headphone jack, while CON5 is the larger and more robust type. If both are fitted, inserting a plug into CON5 will disconnect the ground path for CON4, unless there is a shorting block on jumper JP1. If there is, both headphones will be driven in parallel. JP1 must also be shorted if CON5 is omitted so that CON4 can be used. Output transistors We chose the TTA004B (PNP) and complementary TTC004B (NPN) because they are inexpensive, compact and designed for audio use. They have a high maximum collector voltage of 160V (not that useful in this application), a high transition frequency of 100MHz, low output capacitance and a reasonably high continuous current limit of 1.5A each. While they don’t have a super high current gain, it is pretty good at 140280 at 100mA (typically >200). All these properties combine to make them good as part of a feedback loop to deliver a reasonable amount of current while minimising distortion. The current gain (beta [β] or hfe) is still usefully high at 1A (around 100). They are also very linear, having a very flat hfe curve from 1mA to over 100mA. So overall, they are excellent medium-power audio transistors. Power supply Fig.6: we can omit IC1 & IC2 by coupling the signals from the wipers of VR1 & VR2 directly to the non-inverting inputs of IC1 & IC2 and removing the redundant pair of DC-biasing resistors. This will still work and save a bit of money, but the volume controls will have some interaction. Rather than an unregulated or a regulated supply, we have opted for a capacitance-multiplier type supply. This has the advantage of delivering much smoother rails to the op amps and output stage than an unregulated supply, without the power loss of a regulated supply or pinning us to a particular regulated supply voltage. The incoming low-voltage AC from the plugpack is converted to pulsating DC by the full-wave voltage doubler formed by schottky diodes D1 and D2. Schottky diodes are used here to minimise the voltage loss, so we can get decent output power from just 9V AC, and to improve efficiency. They achieve that by having a low forward voltage drop when in conduction. The result is about 12V DC across the two 1000μF capacitors (assuming a 9V plugpack), giving an unregulated ±12V supply. This will have Australia's electronics magazine siliconchip.com.au 50 Silicon Chip an increasing amount of AC ripple as the load on the supply goes up due to those capacitors discharging between peaks in the mains cycle. The ripple will be 50Hz, not 100Hz, due to the diode configuration. We measured over 300mV of ripple on our prototype with no signal, and obviously that increases as we load the output more. We could add two regulators to the output but they would need to be matched to the plugpack; for example, ±12V regulators might work well if the plugpack is 12V AC and thus develops sufficient input voltage for them to regulate, but they would be useless with a 9V AC plugpack. There’s also the problem that under load, the ripple could cause the regulators to enter dropout. Instead, we use capacitance multipliers formed by transistors Q1 & Q2, operating as complementary emitter-followers, with another set of 1000μF capacitors between their bases and ground. They are biased on by 220W resistors from each collector to the associated base. You can think of these as ‘variable regulators’ that produce a smoothed output but with the output voltage being related to the input voltage. That’s because the base capacitors charge to just below the average of the input voltage due to the RC low-pass filters formed by them and the 220W resistors. Keep in mind that, as they operate as emitter followers, the emitter voltage for a fixed load current is essentially a fixed amount below the base voltage (around 0.7V). So if the base voltage is steady, thanks to that lowpass filter action, as long as the collector voltages don’t drop too low due to excessive ripple, the voltage at the emitters will be essentially constant. As a result, with say ±12V DC at the collectors overlaid with several hundred millivolts of ripple (we measured around 350mV in our prototype), the outputs at their emitters will be close to ±10.5V DC with much lower ripple (10mV in our prototype). That’s a reduction of 35 times or 31dB. While the amplifier section has good ripple rejection, some may still be audible in the output with 350mV+ on the supply rails. We doubt any will be detectable with just 10mV of ripple on the supply rails, and the performance figures support that. siliconchip.com.au Fig.7: if you only need one stereo input, the circuit can be further simplified as shown here. Only one op amp, IC1, is required as there is no longer any signal mixing. There are four 100μF supply rail bypass/filter capacitors after Q1/Q2 although, two of which are physically located close to the output stages. Thus, they are shown on the circuit diagram at upper right. Putting them closer to the output transistors means less voltage drop during high-current transients. The power LED is connected between the two rails so it doesn’t ruin the symmetry of the device. Its current is limited to around 2-3mA by its 10kW series resistor. Variations There are two variations to this circuit that can be built on the same board. The first is the same as the full circuit shown in Fig.5 but without buffer op amps IC2 & IC3. The differences are only in that section, and they are shown in Fig.6. The signal path is the same as before up to the wipers of the volume control potentiometers. Subsequently, rather than being coupled to buffer op amps, the signals are coupled directly to the mixer resistors. This means that the signal sources are driving a lower impedance. Now the 1MW resistors to ground are required, as otherwise there would be no DC bias for the signals going to IC1a. The relatively high value of the 1MW DC bias resistors was chosen to avoid Australia's electronics magazine too much attenuation when combined with the higher source impedances due to the mixer resistors. This version has the advantage of retaining the two separate inputs but with fewer components and lower power consumption. However, due to the way the signals are mixed, there will be interactions between the two volume controls. That means that if you adjust the level of one source up or down, the level of the other source may also change a little. If that’s likely to bother you, or you bought a kit that came with all the op amps, you might as well just build the full version. But we thought we’d present this cut-down version as it doesn’t require any modifications to the PCB, just a few wire links need to be added to bypass the missing op amps. The other version is the simplest configuration, with just a single stereo input. It is shown in Fig.7. In this case, we don’t need the buffer op amps since there is no longer any mixing going on; the signal from the sole volume control can simply be coupled straight to IC1. Next month The second and final article on this Headphone Amplifier next month will have the PCB assembly instructions, case preparation, testing, adjustment details and some usage tips. SC December 2024  51 SILICON CHIP Mini Projects #017 – by Tim Blythman Automatic Night Light While Arduino modules make it easy to add sensors to microcontrollers, the same modules can often be used without a micro. For example, Jaycar’s XC4444 PIR sensor modules can be easily turned into a night light controller with just a few extra parts. M any night lights are programmed to only turn on when they detect motion in the dark, using a PIR (passive infrared) motion sensor and an LDR (light dependent resistor) to detect the ambient light level. It’s certainly a handy thing to have around the house. Recently, we came across an application note explaining how to add an LDR to the BISS0001 PIR sensor controller chips, as used in Jaycar’s XC4444 PIR sensor module. By adding an LDR to the PIR module, the PIR sensor will only operate when the LDR has a high resistance; that is, when it is dark. We just need a way for the PIR sensor module to switch on a light, and we have a useful circuit. One bonus of this arrangement is that these PIR sensor controllers are designed to have a very low quiescent (idle) current, so our circuit is wellsuited to being powered by batteries. It will only draw any significant current when the light is actually on. We’ll mention some other features of the PIR sensor module a bit later. It is quite configurable and has some adjustments that can be changed to tweak its behaviour. The XC4444 PIR module This module has a PIR sensor with two sensing elements. The elements detect the IR radiation that is passively emitted from people and animals. The elements are arranged so the output is their difference. When a passive IR emitter walks past, the signal changes as the elements report differing amounts of IR radiation. The BISS0001 PIR sensor controller chip mentioned earlier turns this into a digital output signal that is usually low but goes high when a moving object is detected. The module has two trimpots to set the sensitivity and the duration of the output pulse. Figs.1 & 2 show a block diagram and their corresponding parts Fig.1 (left): this block diagram shows what is built into the PIR sensor module’s small PCB. The output has a series 1kW resistor, so we can directly connect it to a bipolar transistor’s base. DELAY TRIMPOT SENSITIVITY TRIMPOT VOLTAGE REGULATOR LDR (NOT FITTED) After removing the plastic lens, the LDR module is soldered to the pads labelled “RL” on the PIR module as shown. The LDR’s light-sensitive side must be facing the same direction as the IR sensor. 52 Silicon Chip PIR SENSOR ON REVERSE OF PCB POWER AND IO HEADER CONTROLLER IC Fig.2 (above): the locations of the parts on the PIR PCB. The two pads near the header are for the optional LDR, while two others can accept a thermistor to provide temperature compensation. Australia's electronics magazine siliconchip.com.au ADVANCED TEST T EEZERS The Advanced Test Tweezers have 10 different modes, so you can measure ☑ Resistance: 1Ω to 40MΩ, ±1% ☑ Capacitance: 10pF to 150μF, ±5% ☑ Diode forward voltage: 0-2.4V, ±2% ☑ Combined resistance/ capacitance/diode display ☑ Voltmeter: 0 to ±30V ±2% ☑ Oscilloscope: ranges ±30V at up to 25kSa/s ☑ Serial UART decoder ☑ I/V curve plotter ☑ Logic probe ☑ Audio tone/square wave on the module. Note that the module does not come with the LDR fitted, so it needs to be added, but that is quite easy. emitter (E) pin. As a result, all three elements of the RGB LED light up, providing white illumination. The human eye is surprisingly sensitive, so even a small LED module like this will provide adequate light in a dark room. Circuit details Fig.3 is the circuit for our Night Light. Power comes from a 3×AA battery holder, providing a nominal 4.5V. The PIR sensor module contains a 3.3V regulator and will work with any supply voltage from 4V to 12V. We have arranged for the output of the sensor to drive an NPN transistor. A bipolar junction transistor like this should have a base resistor to limit the base current, but there is actually a 1kW resistor in series with the module’s output, so an external resistor is not needed. When the output goes high, it biases the transistor on and current can flow from the transistor’s collector (C) to its generator Assembly Because soldering is needed to add the LDR to the PIR module, we decided to build the Night Light on a small prototyping board. The wiring is elementary and could easily be done without a prototyping board, but it makes a nice, stable base for the device. The first step is to fit the LDR to the PIR module. The large plastic lens is just a friction fit to the PIR module’s PCB, so it can be pulled or prised off with a flat-tipped screwdriver. Having done that, solder the LDR to the pads labelled RL on the PCB, with Complete Kit (Cat SC6631) siliconchip.com.au/Shop/20/6631 The kit includes everything pictured, except the lithium coin cell and optional programming header. See the series of articles in the February & March 2023 issues for more details (siliconchip.com.au/Series/396). the LDR’s light sensitive side facing in the same direction as the existing IR sensor. Make sure the LDR doesn’t block the IR sensor; refer to our photos to see how we arranged it. After that, pop the lens back on. Next, carefully bend the three-pin header to allow the PIR module to be mounted facing outwards on the prototyping board. Again, refer to our photos if you aren’t sure about this. The prototyping board has two copper tracks that snake around it, which we used for the Vcc (4.5V) and GND (0V) rails. Fig.4 shows how we laid out the parts and wiring. Note that this assumes you have a −BRG marked RGB LED module, as we got from our local Jaycar. Other versions of the module may be wired differently and may not even need the three external resistors. Fig.3: in our Night Light circuit, a signal from the PIR module drives the transistor which then switches the LED module on. It will run for a long time from three AA cells, using negligible power until it is activated. siliconchip.com.au Australia's electronics magazine December 2024  53 Parts List – Automatic Night Light (JMP017) 1 PIR sensor module [Jaycar XC4444] 1 RGB LED module [Jaycar XC4428] 1 light-dependent resistor (LDR) [Jaycar RD3480] 1 BC546, BC547, BC548, BC549 or similar NPN transistor [Jaycar ZT2154] 3 150W ½W axial resistors [Jaycar RR0552] 1 mini prototyping board [Jaycar HP9556] 1 3×AA cell holder [Jaycar PH9274] 3 AA cells 1 10cm length of insulated wire (cut from excess length on battery holder) 4 self-adhesive feet (optional) [Jaycar HP0815] If you have a part marked +BRG or similar (meaning it is a common anode type, instead of common cathode), the + or anode should go to the red 4.5V supply and the common end of the three resistors should connect to the transistor’s collector. If you want to use a different LED, make sure that it has the appropriate series resistor for your chosen voltage and connect the anode to 4.5V and the cathode to the transistor collector. We recommend fitting the lower-­ profile parts (like resistors) and wires before the modules, as they are difficult to get to otherwise. Fortunately, two of the three transistor leads line up with two of the PIR module’s leads, simplifying the layout. Make sure you don’t get the transistor backwards. Now add the LED module, battery holder and insert the cells. You can test that the LED is wired correctly by shorting the outer two leads of the transistor (emitter and collector), which could cause it to light, as long as the cells are inserted in the battery holder. If all is well, remove the cells, solder the PIR module in place and then refit the cells. As we’ve added the LDR to the PIR module, you’ll need a dark room to test the Night Light. Walk in front of it and check that the LED lights up. It might only be for a second or so, but that is enough to know it is working. Conclusion Using the two trimpots shown in Fig.2, you can adjust the sensitivity and delay time. Both increase when the trimpots are rotated clockwise. The delay refers to the time that the output is high and thus the time the LED is on after each trigger event. We suggest setting the delay to its minimum (fully anti-clockwise) and sensitivity near the middle, then adjust the sensitivity until you are happy with how close you have to get before it’s triggered. That’s easier to do when the LED only stays on for about a second at a time. After that, adjust the delay to your liking. The working range is about one second to three minutes. Fit rubber feet if you wish, and set up the Night Light where it is needed. We measured the current draw of our prototype at 50μA when idle and 35mA when the LED was on, so the battery life will mostly depend on how much the light is activated. If the Night Light is used infrequently, the AA cells should last for several years. You will see the light getting dimmer as the battery SC goes flat. Fig.4 (left): a top view of how we laid out our prototype. The copper pattern is not visible from this angle; we have shown it here so you can see how the 4.5V and ground rails are connected on the other side of the PCB. The photo above shows how we have laid out the wiring on our prototyping board. The adjacent close-up photo shows the two points under the board where we used blobs of solder to connect to the 4.5V supply and ground rails. 54 Silicon Chip Australia's electronics magazine siliconchip.com.au 'TIS THE SEASON FOR TECHY GIFTS? MARK YOUR ADVENT CALENDAR: Wed 4th to Tue 24th of Dec, 2024 NOW ONLY $ 99 FILAMENT AUTO-FEED SAVE $30 1080P HD CAMERA WITH FIRST PERSON VIEW $ R/C FPV Foldable Drone with 1080p Camera Easy & fun to fly. Auto take-off and landing. Headless mode and auto return. 360° flip & rotation. Ages 14+. 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Rechargeable via USB. LA9025 DUAL USB PORTS NOW ONLY 1495 HALF PRICE SIMPLY PLACE YOUR DEVICE DOWN FOR FAST WIRELESS CHARGING 15W Wireless Qi Fast Charger Fast charges the latest devices. MB3671 SALE ENDS TUESDAY 24.12.2024 SCAN QR CODE ADD GPS ACCURACY TO ANY VEHICLE . . SAVE $10 SAVE $40 $ NOW ONLY 99 HEAD OFFICE Rhodes Corporate Park, Building F, Suite 1.01 1 Homebush Bay Drive, Rhodes NSW 2138 Ph: (02) 8832 3100 ONLINE ORDERS www.jaycar.com.au techstore<at>jaycar.com.au In-Car Handsfree Kit FM transmitter. Make calls or stream music from a Smartphone via Bluetooth ®. AR3140 SILICON CHIP Mini Projects #016 – by Tim Blythman WiFi Weather Logger This simple but incredibly useful project can put the weather at your fingertips. It monitors and logs temperature and humidity. You can download the logs and see all the important statistics from a web browser on any device connected to your WiFi network. Y ou can buy all manner of wireless weather stations but this one is simple and easy to build. Because it’s programmed using the Arduino IDE, you can customise it as you wish. The hardware assembly is simple as it consists of just three pluggable boards. You might need to do some soldering if the headers are not already attached when you buy them, but that’s about it. We’re using a WiFi Mini board, which sports an ESP8266 processor. A module based on the DHT11 sensor measures the temperature and humidity. Finally, a microSD card module is used to save the data. Fig.1 shows the circuit for the Logger, with the three boxes showing the contents of each of the peripheral modules. The WiFi Mini board has a few niceties that aren’t shown, such as a USB-serial converter and voltage regulator, while the other modules are quite minimal. The processor on the WiFi Mini board connects to the DHT11 sensor using its D4 digital input/output pin. It uses a simple bidirectional one-wire protocol, so only one pin is needed, along with the included 5.1kW pullup resistor. You’ll note that the onboard LED of the WiFi Mini also uses the D4 pin. It doesn’t interfere, and we know communication is occurring when the LED flashes. The SPI pins of the WiFi Mini board connect to their respective pins on the microSD card socket. siliconchip.com.au If you don’t have the exact DHT11 module or microSD card shield, then you should be able to work out the connections using Fig.1. The software library we are using also supports the similar DHT22 sensors without requiring any changes. However, note that we have not tested this. Construction Ensure that the three boards are fitted with suitable headers. We used stackable headers on all of them for flexibility, but you could use male or female headers for some to make the stack more compact. Simply plug the boards together, being sure to match their orientation to our photos! We placed the DHT module at the bottom so it wouldn’t be affected by any heat rising from boards below it. We put the WiFi Mini board at the top to keep it free from radio interference, meaning the microSD card module ended up in the middle. If you don’t have the shields, a breadboard or prototyping board might help to make the connections, although you might need a few jumper wires to complete the circuitry. Fit a microSD card to the socket, Fig.1: the circuit consists of three shield boards that we plugged together, although you could use different modules/components and jumper wires if you already have them. The coloured boxes show the contents of the two peripheral boards and how they connect to the processor board. Australia's electronics magazine December 2024  59 freshly FAT-formatted if possible. FAT16 and FAT32 are supported; usually, FAT16 is used for cards up to 2GB. The fewer files on the card, the less processing the WiFi Mini will have to do to read it. The Logger generates less than 1MB of data per year, so even a low-capacity card should be sufficient. Parts List – Weather Logger (JMP016) 1 WiFi Mini ESP8266 Main Board (MOD1) [Jaycar XC3802] 1 microSD card shield (MOD2) [Jaycar XC3852] 1 DHT11 temperature and humidity shield (MOD3) [Jaycar XC3856] 1 FAT-formatted microSD card 1 micro USB cable for programming and power The WiFi Weather Logger uses just these three modules, a microSD card and a micro USB cable. Add http://arduino.esp8266.com/ stable/package_esp8266com_index. json to the Additional Board Manager URLs list, then search for and install “esp8266” from the Board Manager. You’ll also need the DHTNEW library. We’ve included a copy of version 4.3.1 (the one we used) in the software download package at siliconchip. au/Shop/6/512 You can also search for DHTNEW in the Library Manager to install the library named DHTNEW, or download it from https://github.com/RobTillaart/ DHTNEW If you haven’t used the WiFi Mini before, your computer may need drivers. These can be found in the download section of the Jaycar WiFi Mini product page at www.jaycar.com.au/p/ XC3802 Open the WIFI_WEATHER_LOGGER sketch from the software download package. You will have to edit the sketch to include your WiFi name (SSID) and password, which are set by #defines at the very start of the sketch. The NORMAL_OFFSET #define can be altered to set your local timezone offset in minutes. The default of 600 minutes (+10 hours) is correct for Sydney, Melbourne, Hobart and Canberra. You can see these in Screen 1. Select the serial port of the WiFi Mini and choose “D1 R2 & Mini” for the board type, then upload the sketch to the board. When this completes, open the serial monitor at 115,200 baud and check that everything is working as expected, as shown in Screen 2. The WiFi Mini will reboot after 30 seconds if it does not successfully connect to WiFi. Otherwise, it will report its progress in initialising the hardware. Typing ‘~’ followed by Enter in the Serial Monitor will run the card contents listing seen at the bottom of Screen 2. If you have a problem, trying resetting the WiFi Mini with its RESET button. Check that the card is correctly inserted if it is giving an error. Typing ‘s’ followed by Enter in the Serial Monitor will show its status, while Australia's electronics magazine siliconchip.com.au Software The Logger’s software will connect to the internet and fetch the time using NTP (network time protocol). It will then get temperature and humidity data from the DHT11 sensor hourly and log them to the microSD card. It also presents an HTTP server that can be used to check the status and download data using a web browser. You’ll need to install the Arduino IDE (if you don’t already have it, you can download it from siliconchip.au/ link/aatq) and the ESP8266 processor board add-on. The add-on is installed from the Preferences menu of the IDE. Screen 1: make sure to update your WiFi network settings here so that the Logger can connect to your WiFi network. Adjust the time zone offset (in minutes) relative to UTC to suit your location. Connecting to Tim. ................ Connected! IP address: 192.168.xxx.xxx Getting time from NTP sending NTP packet... 48 bytes received. NTP is 0xEA5C1453 Time is 13:17:39 on 06/08/2024 (local time) SD card initialised Root directory found STATUS: IP=192.168.xxx.xxx, Card OK, NTP OK Card listing: 0 System Volume Information [FOLDER] 1 202404.csv 29541 bytes 2 202405.csv 30525 bytes 3 202406.csv 29541 bytes 4 202407.csv 30525 bytes 5 202408.csv 5516 bytes Screen 2: The Arduino Serial Monitor should show something like this if all is working well. The serial port is set to 115,200 baud. 60 Silicon Chip typing a number followed by Enter (as per listing on Screen 2) will show the corresponding file contents. If you see the LED flashing briefly, about once every two seconds, most likely everything is working as expected. If you have the necessary experience, you can modify the Arduino software to collect and log other data. Web interface Note down the IP address from the Screen 2 status report and type that into a web browser’s address bar. This will only be accessible from your local network (eg, devices that are on the same WiFi network). This should show a web page like the one shown in Screen 3. Your values and the files may be different. Check that the time is correct and that you are getting reasonable readings for temperature and humidity. You can refresh the page to get the latest data. The Logger will record data to the microSD card once per hour, on the hour, so let it run for a while to accrue some data. After that, refresh the page and see that you can download the CSV data files by clicking on the links. One file is generated for each month; each will grow to about 30kB. Screen 3: the Logger’s web page shows the current status and lists any files that can be downloaded from the microSD card. CSV files You can open the CSV files in a spreadsheet program like Excel or LibreOffice Calc. The leftmost column is a so-called ‘date serial number’. If you change this column to a date or time format, it will show as such and should match the date and time text in the second column. You can see this in Screen 4, along with the actual temperature and humidity readings, which can now be charted or converted to another data format as needed. Completion You can now install the Logger in its final location. You’ll need power, and you should make sure that the Logger and its wiring are protected from the sun and rain if it is outside. The shelter used to protect meteorological instruments is called a Stevenson screen (instrument shelter). We had no trouble finding versions online that could be 3D-printed. Otherwise, an inverted plastic container should do the trick. SC siliconchip.com.au Screen 4: CSV files from the Logger can be viewed in a spreadsheet program. You can change the format of column A to a suitable date or time format. Australia's electronics magazine December 2024  61 Raspberry Pi Pico 2 Review by Tim Blythman T he new Raspberry Pi Pico 2 microcontroller board was released in August this year. We have reviewed the new Raspberry Pi 5 single-board computer (SBC) in the July 2024 issue (siliconchip.au/Article/16323). The original Pico was released in 2021, followed by the WiFi and Bluetooth equipped Pico W in 2022. Both these boards are based on the RP2040 microcontroller, the first microcontroller designed by the Raspberry Pi Foundation. The Raspberry Pi 5 introduced the RP1 microcontroller, acting as an I/O controller. Like the Raspberry Pi SBCs, the Pico was designed to be low cost and easy to use, with a target price of US$4 (about $6). By the time we reviewed The last 12 months saw the release of the Raspberry Pi 5 single-board computer (SBC) and Raspberry Pi Ltd being listed on the London Stock Exchange. Most interesting for us was the recent release of the Raspberry Pi Pico 2 microcontroller board with the new RP2350 microcontroller. it, it could be programmed in the C language, with the Arduino IDE and MicroPython; PicoMite BASIC was released soon afterwards (December 2021; siliconchip.au/Article/15125). About a year later, the Pico W was released. It shares the same form factor and processor as the Pico but includes an Infineon CYW43439 radio module, adding WiFi and Bluetooth support. The bare RP2040 microcontroller later became available for purchase at around one dollar, from the likes of DigiKey and Mouser. That led to its incorporation into many thirdparty boards. At Silicon Chip, we created the Pico BackPack, which adds features like an LCD touchscreen, microSD card socket and audio output to a Pico or Pico W. That was detailed in the March 2022 issue (siliconchip.au/ Article/15236), with the Pico W BackPack introduced in January 2023 (siliconchip.au/ Article/15616). We have used the Pico and Pico W in various projects, including the VGA PicoMite, WebMite, Pico Audio Analyser and Pico Gamer. So we were very interested to see what the Pico 2 has to offer. There are a lot of similarities; it has the same layout and footprint as the Pico & Pico W. Apart from the silkscreen being marked as a Pico 2, you might not even know it was a different board! It appears the Pico 2 is backwards compatible with the Pico; we shall investigate that later. The Pico 2 is aimed to be available for US$5, and we purchased our test boards for about $8 (excluding delivery), which is much the same price at the time of writing. The RP2350 The new RP2350 microcontroller is actually a series of four new parts; it is the RP2350A variant that is fitted to the Pico 2. Table 1 shows a comparison between the RP2040 and the members of the RP2350 family. Like their respective microcontroller boards, there is a lot of similarity between the RP2040 and the RP2350. The two important differences are in the processor and the inbuilt RAM; these explain the differences between the part numbers. The Pico 2 (left) looks very similar to the Pico (right). The notable differences are in the silkscreen and that the Pico 2 uses smaller passives. The different core power supply is visible in the components above and to the right of the RP2350. The larger component in that area is an inductor that’s used in the switching mode of the RP2350 core supply. 62 Silicon Chip Australia's electronics magazine siliconchip.com.au The RP2350 has a dual ARM Cortex M33 processor compared to the RP2040’s dual ARM Cortex M0+ (hence the ‘3’ in RP2350), while the ‘5’ indicates that it has twice as much RAM (see Fig.1). Its data sheet can be found at siliconchip.au/link/ac1u The QSPI controller (which is used to communicate with an external flash memory chip) has been provided with a second interface. This can be used to connect a second flash chip or a PSRAM (pseudo-static random access memory), to expand the memory available to the system. 8MiB (64Mbit) PSRAM chips are available for a few dollars. That is a phenomenal amount of RAM for a microcontroller, but note that the Pico 2 board does not have provision for a PSRAM chip to be fitted. The RP2350 also has a dual Hazard3 RISC-V (pronounced ‘risk five’) processor that can be selected at boot time. RISC-V is an open RISC (reduced instruction set computer) architecture that is gaining traction as an alternative to other proprietary architectures. In theory, one core can be a RISC-V processor and the other, an ARM processor. The new M33 ARM processor has native floating-point instructions that the M0+ processor in the RP2040 lacks; floating-point support for the RP2040 is provided by software routines in ROM. That means a big uplift in performance when performing floating-point calculations. The M33 also includes Arm TrustZone and secure boot, using an OTP (one-time programmable) memory to store an encryption key. The M33 processor also performs better (at the same processor clock speed) than the M0+ in tests such as the Dhrystone benchmarks. The security features are not available when the RISC-V cores are used. The RP2350 has a nominal maximum clock speed of 150MHz, although we have already read reports that it can be overclocked (much like the RP2040). There are reports of operation up to 300MHz. Such overclocking is also subject to the limits of the flash memory chip. Two of the RP2350 variants boast a larger chip with more I/O pins; those have the ‘B’ suffix. These have 48 general-­purpose I/O pins, compared to just 30 on the RP2040 and ‘A’ variants. Then there are the RP2354 variants, siliconchip.com.au Table 1 – RP2040 and RP2350 family comparison RP2040 RP2350A RP2350B Dual ARM Cortex M0+ Dual ARM Cortex M33 and Hazard3 RISC-V External only RP2354A RP2354B 2MiB internal Processor (CPU) Flash memory 264kiB 520kiB plus external PSRAM RAM 133MHz 150MHz Clock 56 60 80 60 80 Pins 30 30 48 30 48 GPIO 2 UART 2 SPI 2 I2C 16 24 4 4 PWM 8 4 8 ADC channels Full-speed host or device USB 8 12 PIO state machines – HSTX peripheral, secure boot with OTP storage, hardware random number generator Other which bond a 2MiB (16Mbit) Winbond W25Q16JVWI QSPI NOR flash memory chip to the RP2350 processor die; this die is otherwise identical to a bare RP2350A or RP2350B chip. Thus, the four variants of the RP2350 are the 60-pin ‘A’ versions and 80-pin ‘B’ versions, either with (RP2354) or without (RP2350) an attached flash memory chip. Having only four ADC (analog-­todigital converter) channels on the RP2040 saw the Pico falling short compared to many other microcontrollers’ analog abilities. The larger RP2350B variants now have eight ADC channels, which means that the Pico 2 is still stuck with only four channels. During our development of the Pico Audio Analyser (November 2023 issue; siliconchip.au/Article/16011), we looked closely at some errors that had been identified in the ADC silicon Fig.1: the part naming of the RP2350 (and RP2040) is based on this scheme. The RP2354 parts have 2MiB (24 × 128kB) of non-volatile storage in the form of a flash memory chip bonded to the processor die. Australia's electronics magazine hardware of the RP2040. The RP2350 data sheet indicates that those have been fixed in the newer chip. The novel PIO (programmable input output) peripheral saw a lot of attention, and has been put to good use in emulating all sorts of peripheral functions. That includes SPI, USB and even the protocol that is used to control WS2812 programmable LEDs. We used the PIO to generate digital video in the Pico Digital Video Terminal (March & April 2024; siliconchip. au/Series/413). The RP2350 provides 12 PIO state machines, up from the RP2040’s eight. There are also some minor updates to the PIO peripheral itself. The RP2350 also has a new HSTX peripheral; this stands for ‘high-speed serial transmit’. It can stream data out on eight I/O pins at up to 300MHz (using double-data-rate output registers). There is example code to use the HSTX to generate DVI-­compatible video. The RP2350 data sheet notes that each processor core implements a TMDS (transition minimised differential signalling) encoding algorithm. TMDS is an encoding used with HDMI and DVI video, so clearly there is an intention for the RP2350 to be able to directly produce video output. Power management on the RP2350 has been improved by splitting the power domains and allowing some December 2024  63 1 2 39 USB BOOTSEL LED Fig.3: an easy way to tell the Pico from the Pico 2 is the drive volume label displayed by the bootloader. The RP2350 label indicates that it’s a Pico 2. A Pico or other RP2040-based board would show this as RPI-RP2. DEBUG parts to be selectively powered off, thus potentially using less power than the RP2040 in sleep mode. The Pico 2 Unsurprisingly, the biggest difference between the Pico and Pico 2 is the new processor chip. As well as doubling the RAM, the Pico 2 has double the available flash memory, with a 4MiB (32Mbit) flash memory chip onboard. The data sheet for the Pico 2 can be downloaded from siliconchip. au/link/ac1v That’s about the extent of the changes between the two boards. The same RT6150 buck/boost regulator allows the Pico 2 to operate from anywhere between 1.8V and 5.5V. Similar to the Pico, the Pico 2 also has a diode between the VBUS and VSYS pins. The Pico 2 appears to use smaller passive components, and there is some extra circuitry related to the RP2350’s core 1.1V power supply, which has a regulator that can operate in both linear and switching modes, allowing it to achieve better efficiency. The rear of the Pico 2 has test points in the same place as the Pico, with the addition of an extra test point in the area of the switching regulator’s circuitry. Otherwise, a 2024 copyright notice is the most prominent difference. From what we can see, there isn’t even a new pinout diagram for the Pico 2; the Pico diagram has simply been annotated to include the Pico 2. So it appears that there are no electrical or mechanical reasons that rule out using a Pico 2 in place of a Pico. 64 Silicon Chip Fig.2: the Pico and Pico 2 share this pinout diagram, meaning that I/O and peripheral mappings are identical. The new HSTX peripheral is not shown; it uses the GP12-GP19 pins of the Pico 2. Source: www.raspberrypi.com/ documentation/microcontrollers/ pico-series.html Fig.2 shows the pinout. It does not note the HSTX-capable pins, presumably to retain the consistency between the Pico and Pico 2 diagrams. The HSTX pins are fixed to GPIOs 12-19. A fault in the silicon While the Pico 2 may appear to be better in all ways than the Pico, there is already a severe erratum that can probably only be fixed by a revision of the RP2350 silicon. The data sheet notes this as erratum RP2350-E9, and it applies to stepping A2; this is the marking on our Pico 2. An excessive leakage current is sourced from a digital input pin if its voltage is in the undefined input voltage region, between valid high and low levels. When connected to a high impedance source, this could result in erroneous readings. It is especially a problem if the internal pull-down is active, since the weak pull-down cannot overcome the leakage and the pin remains stuck in the undefined input voltage region (around 2.2V for a 3.3V supply). Software fixes can be applied to some but not all situations. The general advice is to use an external pulldown resistor of no more than 8.2kW instead of the internal pull-down when required. The good news is that the bug that caused poor ADC performance in the RP2040 is fixed in the RP2350. Security We aren’t surprised that security was a low priority for the Raspberry Pi Foundation in creating a cheap and Australia's electronics magazine easy to use board in the Pico. The Arm TrustZone and secure boot features of the RP2350 intend to address one of the claimed weaknesses of the RP2040: a lack of security for the program flash memory. For example, reading or modifying the program in the flash chip (on the original Pico) would be as easy as accessing the flash chip and performing read or write commands. The security on the RP2350 depends on the flash memory contents being encrypted and signed. The encryption means that the data stored on the chip is meaningless until the processor decrypts it. The signing process is a way to tell if the data has been modified, and generally involves creating a hash or checksum of the data that can indicate if it has been changed. The signing is necessary as the encryption only means that the data cannot be easily read. It would still be possible, for example, to write random data to the flash chip in the hope of provoking insecure behaviour. The signing prevents any modified data from being run. The OTP (one-time-programmable) memory of the RP2350 can be used to store the keys needed to decrypt and check flash data, among other things. The OTP can be locked and hidden by programming specific bits. To test the security, the Raspberry Pi Foundation launched a competition with a $20,000 prize to see if anyone can break into the locked OTP memory. The competition is available at https://github.com/raspberrypi/ rp2350_hacking_challenge siliconchip.com.au Photos 1-4 (left-to-right): » the Seeed Technology XIAO RP2350 is one of the smaller RP2350 boards and has a USB-C socket. It appears that it will not cost much more than a Pico 2. » Pimoroni’s PGA2350 RP2350B is a compact but comprehensive breakout board for the 80-pin RP2350B. It includes 16MiB of flash memory and an 8MiB PSRAM chip. » the Pimoroni Tiny 2350 appears to be pin-compatible with their Tiny 2040. We noted the Tiny 2040 in our original review of the Pico; it was one of the early RP2040 boards. » Sparkfun’s Pro Micro RP2350 has a USB-C socket and incorporates a PSRAM chip, giving access to over 8MiB of random access memory. It also has a 16MiB flash memory chip. One of the great features of the RP2040 on the original Pico is the ROM bootloader, which makes it almost impossible to ‘brick’. The OTP provides a means to permanently modify the RP2350’s behaviour, so it’s possible that a wrong OTP operation could brick the RP2350. However, we understand that has been deliberately made difficult to do. Hands-on testing We are in the process of doing some detailed testing of the Pico 2 with our previous Pico projects, including the Pico Audio Analyser, which should hopefully improve its performance. To summarise what we’ve found, the Pico 2 works just about seamlessly in all cases where we had previously used a Pico! Of course, the differing architectures mean that code recompilation is required, but we generally have not had to make any changes to the code itself. For example, we fitted a Pico 2 to the prototype for our Pico Computer project (see page 66) and compiled the exact same Arduino sketch files (without any changes whatsoever) and the Pico 2 worked exactly as expected. Similarly, the example MicroPython program and libraries that we created for the BackPack worked without any changes on the Pico 2. The process for setting up the Pico-series C SDK (software development kit) on a Windows machine has changed substantially. Still, apart from that, we had little trouble in compiling the exact same code as we used with a Pico. siliconchip.com.au With just one click, we were able to create a separate project to use the RISC-V processor (instead of the ARM processor) and that too compiled flawlessly and worked identically. Curiously, the compiled RISC-V code is about half the size of the ARM code. We’ve also seen early versions of PicoMite BASIC for the RP2350. Downloads and a discussion can be found on TheBackShed Forum, see siliconchip.au/link/ac1w It looks like we will soon see new features in PicoMite BASIC. There is an HDMI video version, using the HSTX peripheral (in addition to VGA), and PicoMite BASIC has been bumped to version 6.0.0. For more background on setting up the Pico-series C SDK, trying out the various PicoMite BASIC RP2350 versions and porting our various projects to use the Pico 2. We plan to publish another article in the near future. What about a Pico 2 W? The launch announcement of the Pico 2 (siliconchip.au/link/ac1i) offered some hints on the availability of a WiFi version, as well as bare RP2350 chips. At this stage, it appears the Pico 2 W will feature the same Infineon CYW43439 radio module and should be available before the end of 2024. Bare RP2350 chips in all four variants are also expected to be available by the end of the year. DigiKey and Mouser already stock the RP2040 chip at just over $1, so we would not be surprised to see them carrying the RP2350 Australia's electronics magazine variants in the near future, presumably at a slightly higher price. Other RP2350 boards Other companies have already announced RP2350-based products. It appears some firms have had access to the RP2350 for some time before the launch, allowing them to develop a range of products, test out the chips and their software. It was the makers of the Bus Pirate (https://buspirate.com) who identified the erratum mentioned earlier. Bus Pirate is an open-source digital tool for working with microcontrollers and other digital ICs. Photos 1-4 show some of the new boards that have been announced. At the time of writing, we have not seen any of these boards available to purchase. Conclusion The Pico 2 looks to be just about better than the Pico in every way, as long you can avoid the leakage current problem. The extra RAM and improved ADC would definitely have been beneficial for our Pico Audio Analyser project had the Pico 2 been available when we were designing it. While it might appear that the Pico 2 could easily obsolete the Pico, there is a note on the Pico’s product page that it will be available until January 2036. The Pico 2 is similarly noted as being available until January 2040. Subject to stock levels and demand, the Pico 2 is available from Altronics, DigiKey and Mouser. SC December 2024  65 By Tim Blythman THE PICO COMPUTER A computer terminal using a Raspberry Pi Pico Turn a Raspberry Pi Pico, Pico W or Pico 2 board into a standalone computer with a USB keyboard and HDMI monitor. With the Pico Computer PCB, all the required circuitry fits in a compact and handy enclosure. I n April 2024, we presented the Digital Video Terminal (siliconchip.au/ Series/413) that can connect to a monitor via HDMI, a USB keyboard and a Raspberry Pi Pico or other device with a serial port. It provides a freestanding terminal console, ideal for working with many single-board computers. Its block diagram is shown in Fig.1. The Pico Computer Board can plug onto the Digital Video Terminal’s PCB, turning it into the Pico Computer with many features. For example, the Computer Board includes (among other features) an RTCC (real-time clock and calendar chip), a microSD card slot, an IR receiver and a 3.5mm stereo jack for audio. This turns the Terminal into a fully-fledged standalone Pico-based computer, fitting in the same compact footprint as the Terminal alone. Fig.2 shows the block diagram of the Computer Board integrated with a Digital Video Terminal. The new hardware is shown in the centre. The Computer Board replaces MOD2 of the Terminal and adds many extra features. Digital Video Terminal functions The earlier Digital Video Terminal is compact at just 105 × 80 × 25mm and Fig.1: the original Digital Video Terminal required a Pico (or similar device) to be connected externally via USB (shown at top centre) to access the keyboard and display facilities. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au Features & Specifications ► Digital display and USB keyboard ► I2C header and onboard I2C real- time clock with battery ► Audio DAC with 3.5mm socket and header for output ► Options for onboard SPI flash (4MiB) and PSRAM (8MiB) ► microSD card slot ► USB host for devices like flash drives ► Optional PWM audio module ► Infrared remote receiver ► All Pico I/Os are broken out on a handy header ► Two user-controlled LEDs ► Digital Video Terminal for input and display ► Can use a Pico, Pico W or Pico 2 ► Fits in an Altronics H0192 instrument case includes three Pico microcontroller boards (MOD1-MOD3) that provide three distinct functions. MOD1 is the serial video display interface. It accepts serial data from MOD2 and interprets that according to the VT100 standard, generating video that is delivered via the HDMI socket for display on a modern monitor or TV. MOD3 is configured as a USB host supporting a USB keyboard. It receives keystrokes and sends them as VT100 data to MOD2. MOD2 works as a USB host that expects a USB-serial device to be connected. Devices like the Pico microcontroller board are recognised, as are various others. MOD2 simply channels data to and from the connected USB device, MOD1 and MOD3. The idea was to provide a video terminal with a keyboard that could interface with just about any USB/serial device. However, it occurred to us that MOD2 could be replaced by a Pico (or Pico W) board running different firmware and communicating directly with its keyboard and display controllers, turning it into a standalone computer. For example, a Pico loaded with the PicoMite firmware would turn the Terminal into a standalone BASIC computer with its own HDMI-compatible display and USB keyboard. We provided a few ideas in this vein in the Digital Video Terminal article. While many readers might be happy tinkering with such a machine, we thought it would be nice to flesh the concept out and provide plans to build such a computer. That is the idea behind the Pico Computer. Pico Computer The Pico Computer, like many of our similar projects, combines a microcontroller with a set of useful other devices. While they typically use an LCD panel as the display, in this case, it connects to a modern HDMI display device. It uses a similarly shaped PCB to the Digital Video Terminal that accepts a Pico microcontroller board. Thus, it can stack above a Terminal PCB and fit in the Altronics H0192 enclosure. While it is designed to be used with the Digital Video Terminal, it could have other applications. We are planning a project where it is used in a standalone capacity. It’s made from a mix of modules, through-hole parts and SMDs (surface-­ mounting devices). The SMDs are in SOIC or M3216 (imperial 1206) packages or larger, so it is straightforward to build as long as you have the correct tools and reasonable soldering skills. Circuit details The circuit of the Pico Computer Board is shown in Fig.3. Since it is intended to replace MOD2 in the Digital Video Terminal, we needed to provide a means to connect the two boards. A pair of 20-way headers labelled CON11, on the underside of the PCB, connects to the Digital Video Terminal where MOD2 would normally go. You can see that CON11 only connects a small subset of the available pins. There are numerous ground pins and the VBUS 5V rail so that the whole thing can be powered by a single USB connection. The Pico/Pico W/Pico2 (MOD11) connects to all the peripherals that were shown in Fig.2. Its GP0 and GP1 pins, used for the serial console, connect to CON11 to interface with the keyboard & HDMI-compatible display. Fig.2: replacing MOD2 in the Digital Video Terminal, the Pico Computer results in a single device that has all the features shown here. siliconchip.com.au Australia's electronics magazine December 2024  67 Fig.3: nearly all the Pico’s I/O pins connect to the numerous peripheral hardware devices, but most are optional. You can choose the accessories you need and access the remaining I/O pins via CON15. The 3V3EN line (pin 37) is also connected, allowing S2 on the Digital Video Terminal to reset this Pico. While pads for all 40 pins of CON11 are present, it will be sufficient in most cases to provide the three topmost pins on each side, connecting serial data, power and ground. The 3V3EN line also connects to S11, a tactile switch, which shorts this line to ground, resetting the Pico. Also available to power the shared 5V VBUS rail is CON18, a USB-C connector with two 5.1kW resistors between its CC lines and ground, as required by the USB standards. The Pico includes an onboard 3.3V buck regulator that can provide up to 68 Silicon Chip 800mA, which provides the 3.3V rail on the Computer Board. breaks out 3.3V, ground and the I2C lines for connection to external devices if desired. The pinout I2C and I2S matches a number of I2C-based modThe remaining circuitry is self-­ ules, one of which could be mounted contained within the Computer Board. directly to the Computer Board inside MOD11 pins GP2 (SDA) and GP3 the enclosure. (SCL) are configured for the I2C bus, There are also headers to suit with the two necessary 4.7kW pullup MOD12, a PCM5102A-based stereo resistors. audio DAC module. It takes power IC1 is a DS3231 or DS3231M real- from the 5V VBUS rail via a 10W resistime clock & calendar (RTCC) chip; tor, with 10μF of bypassing capaciit connects to coin cell holder BAT1, tance. This combination helps to supwhich provides battery backup for its press any noise that might otherwise timekeeping when the circuit is pow- reach the module. ered off. A 100nF capacitor bypasses The I2S digital audio data comes its 3.3V supply provided by MOD11. from the GP4, GP5 and GP6 digital outFour-way header CON14 also puts of MOD11, which are configured Australia's electronics magazine siliconchip.com.au as DIN (data), BCK (bit clock) and LRCK (left-right clock) respectively. MOD12’s configuration pins are tied to +3.3V or GND as required, and the resulting audio signals are fed to CON16, a three-way header, and CON17, a 3.5mm stereo jack socket. Data storage The Computer Board offers four options for data storage. Two SPI memory chips can be fitted as IC2 and IC3. They each have a 100nF bypass capacitor and connect to the Pico’s SPI1 peripheral. It uses the GP11 I/O pin as MOSI (master out/slave in), GP8 as MISO (master in/slave out) and GP10 as SCK (clock). IC2’s CS (chip select) pin is driven by GP7, while IC3’s is driven by GP9. There are numerous options available for these ICs, but we have chosen a 64Mbit (8MB) PSRAM chip for IC2. PSRAM stands for pseudo-static RAM; it is actually a dynamic RAM (DRAM) that has an integrated refresh controller, meaning it can be treated like static RAM (SRAM). This provides volatile storage, which is fast, but the data is lost when power is removed. We also used a W25Q32 32MBit (4MB) flash memory chip for IC3, which provides non-volatile storage. The AT25SF321B-SSHB-T is another compatible 32MBit flash chip that could be used. The interfaces are electrically identical between the flash and RAM chips, so the amounts of volatile and non-­ volatile memory can be changed to suit different applications. The other two storage options are removable. A microSD card slot (CON13) is connected to the other (SPI0) interface, which uses GP19 for MOSI, GP16 for MISO and GP18 for SCK, with GP21 wired as chip select. It has 100nF and 10μF bypass capacitors on its 3.3V rail. Digital I/Os GP26 and GP27 are wired to USB-A socket CON12 via 22W resistors, along with 5V (VBUS) and ground connections. These pins are configured in software to provide a USB host interface so a USB flash drive can be connected here, although the software could be changed to suit other USB devices. Below MOD11 are LED11 and LED12; they are be driven by the GP15 and GP20 digital outputs via 1kW series resistors. They are intended to show the status of the microSD card siliconchip.com.au and USB flash drive, but you could use them for any purpose. of the peripheral connections; they are also printed on the PCB silkscreen. Other parts Options These devices use up most of the I/O pins of the Pico, but we still had some room to fit an IR receiver (IR1), which is powered from 5V (VBUS) via a 100W resistor and 10μF capacitor for bypassing. The demodulated output connects to the Pico’s GP22 I/O pin via a 1kW resistor that limits the current into that pin if the IR receiver output is pulled up to 5V. A 28-pin header (CON15) breaks out all the Pico’s accessible I/O pins, as well as providing ground and power connections. The voltage of the power connection on this header is set by JP11 and can be either the nominally 5V VBUS rail or the regulated 3.3V rail. Table 1 provides a concise summary The Computer Board can be used with various software platforms that we’ll discuss in detail a bit later. For now, we’ll point out some important points that might be relevant as you come to construction. Not all software platforms will support all the hardware features; in particular, there is no universal support for I2S audio (eg, MMBasic does not). To this end, we have designed a small drop-in PCB module that allows the Computer Board to use PWM signals to generate audio instead. The construction and use of that module is discussed in the panel titled “A PWM audio module”. The circuit is quite similar to the PWM audio circuit used on the Pico BackPack from March Table 1: peripheral connections for the Pico Computer Board Feature I/O pins/peripherals Comments Notes Serial console GP0/GP1 (UART0) Can connect to MOD1 and MOD3 on Digital Video Terminal Check the jumpers on the Terminal I2C GP2/GP3 (I2C2) I2C RTC chip onboard, also broken out to 4-pin header. DS3231 or DS3231M IC I2S GP4/GP5/GP6 (PIO) Connects to onboard Not supported PCM5102A module with audio by the PicoMite fed to a 3.5mm socket on front panel and an internal 3-pin header PWM audio GP4/GP5 (PWM2) Custom module converts PWM signals to audio for 3.5mm socket on front panel and internal 3-pin header SPI memory GP7-11 (SPI1 and two CS pins) Connects to onboard IC2 (eg, PSRAM) and IC3 (eg, flash) microSD card GP16/GP18/GP19/ microSD card socket on the GP21 (SPI0 and one front panel CS pin) USB Host GP26/GP27 (PIO) USB-A socket on the front panel IR receiver GP22 On the front panel User LEDs GP15/GP20 Adjacent to microSD socket and USB socket, respectively I/O breakout GP0-GP22, GP26GP28, power, ground 28-pin R/A header accessible from rear panel. A separate link allows selection of 3.3V or 5V power. Power input VBUS Can be powered via the USB-C power-only socket, the microUSB socket on the Pico or via the Digital Video Terminal. Australia's electronics magazine Option of PWM audio or I2S but not both Not supported by the PicoMite 1kW series resistors (~3mA) December 2024  69 2022 (siliconchip.au/Article/15236). You may wish to use this instead of I2S audio, even if your software platform supports I2S. We suspect some readers might even find it a useful module for other projects. PicoMite BASIC does not appear to have a means of interfacing to a PIO USB host, so the CON12 USB interface will not be usable with MMBasic. You could keep the USB socket and leave off the two 22W resistors, freeing up the I/O pins and turning CON12 into a USB power-only charging port. None of the internal features are mandatory; you might even wish to simply use the Computer Board as a way of breaking out the Pico’s I/O pins at the CON15 header. JP11 must be fitted to provide power to CON15. If some features are omitted, other components can also be left off; generally, these will be the passives that are immediately adjacent to that part. For example, leaving off IC2 or IC3 K CON13 microSD CD 1kW 12 3 4 5 6 78 15 14 28 13 32 MOD11 8 7 35 6 36 5 37 4 + 38 6 PIN USB-C POWER SOCKET L 10 9 33 34 CR–1220 11 SCK BCK DIN LRCK GND VIN 31 5.1kW 5.1kW G R G 12 RP2040 MCU 3 MICRO USB–B PORT 39 40 2 1 CON18 K IR1 1kW 3 2 1 12 3 4 5 6 78 21 20 3.3V GND SCL SDA 4.7kW 26 15 14 28 13 29 31 12 RP2040 MCU 100nF IC3 18 27 30 1kW CON17 100nF 10mF SWCLK IC1 SWDIO Silicon 100nF 4C .7hip kW 1 19 CON11 RASPBERRY 17 25 PI Pico 16 24 CON14 S11 GND 22 23 A LED11 1kW CON12 22W 22W Pico Digital Video BackPack 07112234D 70 K CON13 microSD CD 4 100W 10mF A LED12 11 10 32 9 33 8 100nF IC2 CON16 L 4.7kW 26 27 29 BAT1 18 RASPBERRY 17 PI Pico 16 30 IC1 19 G R G 3.3V GND SCL SDA SWCLK 100nF 4.7kW 1 SWDIO 25 GND 24 CON14 S11 20 22 23 3.3V 21 Pico Digital Video BackPack 07112234D 3.3V 100nF 10mF CON12 22W 22W PCM5102A MOD12 V+ VBUS JP11 GP0/1: TX/RX GP2/3: SDA/SCL GP4/5/6: DIN/BCK/LR GP7/9: CS IC2/IC3 GP8/10/11: SPI1 GP16/18/19: SPI0 GP15/20: LED11/12 GP21: SD CS GP22: IR RX GP26/27: PIO USB Australia's electronics magazine 10W SCK BCK DIN LRCK GND VIN 1 3 2 4 100W 10mF A LED12 We’ll then detail the modifications that are needed for the Digital Video Terminal to allow it to connect to the Computer Board. In simple terms, this involves leaving off one of the Raspberry Pi Picos (MOD2) and all its associated parts, plus fitting headers to suit. Finally, we’ll assemble all these parts together into the enclosure. Combining the Computer Board with the Digital Video Terminal requires the tallest enclosure of that series, the Altronics H0192. Start by fitting the Computer Board PCB (coded 07112234 and measuring Construction 68 × 98mm) with the surface-mounting We’ll start by working through the components that are needed, using the assembly of the Pico Computer Board, PCB overlay diagram (Fig.4) as a guide. since there may be readers who wish We will mention all parts; simply skip to build it as a standalone device. By any you do not require. itself, it should comfortably fit in the We recommend you have on hand larger of the two Altronics cases that flux paste, solder-wicking braid, tweewe used for the Digital Video Termi- zers and a fine-tipped soldering iron nal (Altronics H0191). (or medium if you’re more experienced and prefer it). A magnifier and good illumination will be helpful, and K A GP0/1: TX/RXproper ventilation is mandatory so you LED11 GP2/3: SDA/Sdon’t CL inhale flux fumes. GP4/5/6: DIN/BCK/LR The microSD card socket (CON13) G P 7 / 9 : C S I C 2 / I C 3 C O N 1 7 1kW GP8/10/11: SPI1 and USB-C socket (CON18) are the CON16 100nF GP16/18/19: SPI0 most challenging to solder, so start IC3 GP15/20: LED11/12 GP21: SD CSwith them. Apply flux to the PCB pads V+ GP22: IR RX and rest the components in place. The 100nF V B U S PCM5102A GP26/27: PIO UScard B IC2 JP11 socket has locating pegs on its MOD12 underside, while the USB-C socket will not and will require a bit more 10W care in its placement. Clean the iron’s tip and apply some 10mF fresh solder, then tack one lead. Check that the connectors are flat and within their marked pads, adjusting if necessary, then solder the remaining leads. Use extra flux and the braid to draw away any excess solder or solder bridges. You could also add some extra solder to the shell of both connectors to give mechanical strength. Follow by installing the three ICs. IC1 can be in the 8-pin narrow SOIC package (DS3231M) or a wider 16-pin SOIC package (DS3231). In both cases, its pin 1 marking must be in the GP0 GP1 GP2 GP3 GP4 GP5 GP6 GP7 GP8 GP9 GP10 GP11 GP12 GP13 GP14 GP15 GP16 GP17 GP18 GP19 GP20 GP21 GP22 GP26 GP28 GP27 V+ GND CON15 IR1 1kW means that the corresponding 100nF capacitor can be omitted too. The two resistors and 10μF capacitor below IR1 are only needed if it is fitted. Similarly, the 100nF and 10μF capacitor next to the CON13 microSD card socket are only needed if it is installed. The 100nF capacitor near IC1 is only needed if it is fitted, although the two 4.7kW resistors should be kept if you intend to connect anything to the I2C bus at CON14. The two passives above MOD12 are needed only if an audio module is fitted. 10mF Figs.4 & 5: we have used slightly larger M3216 (imperial 1206) SMD parts in this design since there was plenty of room. Check these overlays and the photos to confirm how the parts are fitted, especially CON11, since it needs to align with the header on the PCB below. siliconchip.com.au upper-left corner. The narrower part should be fitted to the upper eight pins. Make sure not to mix this up with the other ICs, which will also be 8-pin SOIC parts. If you cannot see a pin 1 marker, there will also be a bevel along the edge belonging to pin 1; it is best viewed from the end of the IC. Apply flux, place the part, tack one lead and check that the part is flat and square before soldering the other pins. We recommend fitting a PSRAM chip for IC2 like we did. It will be noticeably narrower than the Winbond flash memory chip, but both will fit on either sets of pads for IC2 and IC3. Solder these like the other parts, observing the correct orientations. Follow with the seven capacitors. They won’t be marked, but the 10uF parts should be quite a bit thicker than the 100nF parts. Add flux paste, tack one lead, check the position and then solder the other lead. You can also go back and touch the iron on the first lead to refresh the solder joint. There are 11 resistors of various values; solder them as per Fig.4. Coin cell holder BAT1 is the last SMD part. Rest it in place, being sure that the opening faces the edge of the PCB. Tack one lead, adjust, then solder the other. It’s also worth adding a decent fillet to each side to give it mechanical strength. Use this opportunity to thoroughly clean away any excess flux from the PCB using your solvent of choice. Your flux’s data sheet might suggest a specific solvent, but isopropyl alcohol or Chemtools Kleanium G2 work well in most cases. Dry the board and examine it thoroughly for solder bridges, dry joints or pins that are not connected to the pad below. It will be easier to fix these now, before any other parts are fitted. Through-hole parts Now we can start on the throughhole parts. Some of these show through the front panel, so you can use the front panel PCB to check that they are correctly aligned, although they should both simply snap into place. Fit the USB-A socket CON12 now, checking that it is flat against the PCB before soldering its pins. Add a good amount of solder to the shell pins to give it strength. CON17, the 3.5mm socket, sits along this edge too, so solder it in place next. siliconchip.com.au Kits available for this project To build the project as shown in our photos, you should purchase the SC6917 and SC7374 kits from us and the enclosure from a retailer such as Altronics, plus accessories like the USB keyboard, HDMI monitor and appropriate USB cables. When ordering the kits, you may want to also get the optional PSRAM chip (SC7377) and PWM Audio Module kit (SC7376) if you don’t plan to use the included DAC module. The Pico Computer Board kit can be used standalone, although you will have to purchase an enclosure and arrange your own panels. In this case, you can order the SC7378 kit ($50 + P&P), which is the same as the SC7374 kit plus an unprogrammed Pico. Pico Computer Board Kit (SC7374, $40) ___________________ This kit contains the PCB (07112234) and almost all the parts needed to fully populate it (except the PSRAM & RTC chip, available separately, and coin cell). It also includes the new front panel and the hardware needed to connect the PCB to the Digital Video Terminal (which is already available as a kit, see blow). The SC7374 kit does not include a Raspberry Pi Pico because the SC6917 kit has three. The Pico Computer Board fitted with all parts except the I/O breakout headers at bottom right. Note the silkscreen guide at top right for the GPIO pinouts. The only parts needed on the underside of the Computer Board are pin headers to connect it to the Digital Video Terminal underneath. Leave these off when using the Computer Board on its own. Pico Digital Video Terminal Kit (SC6917, $65) ______________ Includes everything to build the Digital Video Terminal, except the case. The Raspberry Pi Picos are supplied unprogrammed. For building instructions, see the original article in the March & April 2024 issues (siliconchip.au/Series/413). PWM Audio Module Kit (SC7376, $10) _____________________ The PWM Audio Module is available as a kit with all parts listed in the panel, including the PCB. Australia's electronics magazine December 2024  71 The infrared receiver and LEDs also show through the front panel, so use the mounted components and front panel as a jig to align them correctly. Bend the LED leads right behind the body by 90°, being sure to bend the correct way to align the cathode with the K marks on the PCB. As well as being short for the German word “Kathode”, the letter K also looks like the cathode end of the LED symbol. Locate each component into the front panel and tack one lead wherever is convenient. Gently bend the leads slightly to achieve your desired placement. The IR receiver might need to be kinked forward slightly to show through the hole in the panel. Once you are happy with the front panel components, solder the remaining leads and trim the excess from the underside. Slot switch S11 into place and solder it as well. You will only need a pair of fourway headers to connect the Computer Board to the Digital Video Terminal, but we decided to use six-way headers since the corresponding stackable headers are commonly available in a six-way part. If you plan to use the Computer Board as a standalone board, these headers are not needed. Otherwise, fit them to the underside of the PCB, closest to the Pico’s USB socket. You should check this carefully in the photos and make sure they are mounted squarely. Another option is to run insulated wires for the handful of lines that are needed: ground, VBUS, GP0 and GP1. These are pins 1, 2, 3 and 40 on the Pico footprint. We’ve specified low-profile headers for mounting the Raspberry Pi Pico (MOD11), as it will be too tall for the intended enclosure if it is fitted on standard 8.5mm-tall socket headers. There is also the option of soldering it directly (with header pins to clear CON11) to the PCB, although you will not have access to the CON11 headers on the underside of the PCB after doing that. So, if you plan to hard-solder MOD11, ensure that you have the CON11 headers in place first. Then rest the header pins in place and tack a few pins on both the PCB and Pico, check their alignment, then solder all pins as needed. If you plan to fit low-profile headers, 72 Silicon Chip A PWM Audio Module The PicoMite has long supported PWM audio, but there doesn’t appear to be any support for I2S audio using a higher-quality external DAC. PWM audio involves supplying a pulse-width modulated signal of varying duty cycle. An external circuit filters and buffers the signal, converting it to an analog voltage so it can be fed to the headphones, an external amplifier or other devices. Many 8-bit microcontrollers will have no trouble generating PWM audio, so this module could also be an attractive option for anywhere that a cheap and simple audio output is needed. It’s cheap for a reason, though; the audio will not be as clean as that from an I2S DAC. Still, it will be good enough for many purposes. This module is designed to fit in the same footprint as the popular PCM5102A DAC modules. Instead of expecting I2S (serial) data, it receives a PWM signal on two of the pins, one for each channel. The circuit (Fig.a) is much the same as the one we used on the Pico BackPack from March 2022 (siliconchip.au/Article/15236), with a minor alteration to allow it to run from a single 5V supply. The external connectors match the pin locations of the PCM5102A module, allowing this module to be mechanically equivalent. 5V power comes in on the Vcc and GND pins and feeds directly to dual low-­ voltage rail-to-rail op amp IC1, bypassed by a 100nF capacitor. A low-pass filter comprising a 10kW resistor and 10μF capacitor provides a biasing rail, VF; its DC level is set to around 4V by other biasing components downstream. The two PWM signals come in on pins 4 and 5 of the six-pin header (L_IN & R_IN). Each are treated identically as they pass through the filter and buffer circuitry, so we will look at one channel only. Assuming an average 1.65V level (as expected for 50% duty cycle PWM from a 3.3V microcontroller), the biasing and filter circuit consisting of the 47kW and 22kW resistors and 1nF capacitor brings the level closer to the middle of the op amp’s range. They also attenuate the higher frequency elements of the squareedge PWM artefacts. Fig.a: this simple circuit filters and buffers two PWM signals from the Pico to provide a basic stereo audio output. You could also connect it to an Arduino or other microcontroller; the output should be able to drive headphones or a small speaker. as recommended, solder the corresponding pin headers to the Pico. You can then use them as a jig to ensure that the low-profile header sockets are square and aligned as you solder them to the PCB. MOD12 should be mounted directly Australia's electronics magazine to the PCB or on low-profile headers only. We opted to solder it directly to the PCB, even though that blocks off access to one of the mounting holes. The remaining headers are optional, and you can fit socket or pin headers to suit your purposes. The demonstration siliconchip.com.au There are just a few resistors and capacitors on the top side of the PCB. Take care with the orientation of IC1. We used standard pin headers, which can be soldered directly to the other PCB or plugged into socket headers. The op amp is configured for unity gain, so it simply buffers the signal that reaches its inputs, while the 10μF capacitor, 100W resistor and 100kW resistor provide AC coupling and biasing to ground. The output is suitable for driving headphones or an external amplifier. As long as the supply voltage suits the op amp and is not less than the incoming PWM amplitude, we expect the circuit will work fine. For example, a 5V PWM signal will work with a 5V supply. Those will with some expertise might tweak the component values to suit their application. Assembly The parts are SOIC and M3216 (imperial 1206), so you will need the standard surface-mount assembly tools (see the construction section in the main article). The top of the PCB is populated with pairs of components that are mirrored across channels, so each silkscreen marking corresponds to the two adjacent passive components. The PCB overlay diagrams shown in Fig.b depict the placement of the components. Work through them on the top side, taking care with the capacitors, since they will not be marked. Flip the PCB over and carefully align and solder the solitary IC, being sure to match the edge bevel to the silkscreen marking, then fit the remaining components on this side. Clean the PCB with an appropriate solvent and dry thoroughly. Solder headers to suit the application. The PWM Audio Module can now be fitted to the Computer Board PCB for testing. We have provided sample code in the PicoMite BASIC examples to use this module. Fig.b: assembly of the module is straightforward. The main thing to watch out for is to avoid mixing up the unmarked capacitors with different values. Parts List – PWM Audio Module 1 double-sided PCB coded 07112238, 32 × 17mm 1 3-way header, 2.54mm pitch (for audio output) 1 6-way header, 2.54mm pitch (power and signal inputs) 1 MCP6002 or similar low-voltage rail-to-rail op amp, SOIC-8 (IC1) 3 10μF M3216/1206 size 10V X7R SMD ceramic capacitors 1 100nF M3216/1206 size 50V X7R SMD ceramic capacitor 2 1nF M3216/1206 size 50V X7R, C0G or NP0 SMD ceramic capacitors 2 100kW SMD M3216/1206 size ¼W 1% resistors 4 47kW SMD M3216/1206 size ¼W 1% resistors 2 22kW SMD M3216/1206 size ¼W 1% resistors 3 10kW SMD M3216/1206 size ¼W 1% resistors 2 100W SMD M3216/1206 size ¼W 1% resistors software does not need to connect to any external circuitry apart from the likes of a microSD card, USB flash drive or headphones. You can even test the Computer Board without the Digital Video Terminal; as long as you have a serial siliconchip.com.au terminal program you can use to view the output and enter commands. Building the Digital Video Terminal If you are connecting the Computer Board to a Digital Video Terminal, a Australia's electronics magazine few minor changes are needed for the Terminal. It’s helpful to refer to the original Digital Video Terminal articles (March-April 2024; siliconchip. au/Series/413), although experienced constructors should get by following the silkscreen markings. The MOD1 and MOD3 Picos will need to be mounted on low-profile headers or directly to the Digital Video Terminal (07112231) PCB, since fullheight headers will be too tall and interfere with the Computer Board. You can load the firmware onto MOD1 and MOD3 before installing them. Be sure to load 0711223A.UF2 onto MOD1 and 0711223C.UF2 onto MOD3. To load the firmware, hold the BOOTSEL button on the Pico while connecting it to your computer, then copy the firmware UF2 file to the RPI-RP2 virtual drive that the Pico provides. Apart from omitting S2, MOD2, CON2, LED2, and the adjacent 1kW and two 22W resistors, most of the assembly proceeds without changes. Refer to our photo of the assembled Terminal; note that we’ve also left off some of the other components that we are not using. We used stackable headers to allow a 15mm spacing between the PCBs. You will need to have the headers fitted to the underside of the Computer Board PCB (07112234) to complete the alignment. Slot the stackable headers onto the headers on the underside of the Computer Board PCB, then temporarily fix the two PCBs together using 15mm spacers and machine screws. This will set the right spacing and align the boards squarely. The tips of the stackable headers should protrude through the matching MOD2 holes in the Terminal PCB. You can also check that the front panel PCB aligns with all the sockets that it presents on both boards. Solder the tips of the stackable headers to the Terminal PCB, then trim them to a neat length. Remove the screws and detach the two boards. To test the Terminal, power it from the USB-C socket (or one of the microUSB sockets if you have no USB-C cables). The LEDs onboard MOD1 and MOD3 should light up any time the board is powered; this shows they are running their firmware. Connecting a USB keyboard to CON3 should cause December 2024  73 LED3 to light up, while typing on the keyboard should make LED3 flicker. Plug your HDMI monitor or display into CON1 and check that LED1 lights and that you can see a flashing cursor in the top-left corner of the connected display. Place a single jumper on LK1, connecting pins 2 and 3 and matching the INT markings on the silkscreen. TERMINAL BACKPACK PICOMITE DEMO I2C2 DEVICE SCAN x0 x1 x2 x3 x4 x5 x6 x7 x8 0x .. 1x .. .. .. .. .. .. .. .. .. 2x .. .. .. .. .. .. .. .. .. 3x .. .. .. .. .. .. .. .. .. 4x .. .. .. .. .. .. .. .. .. 5x .. .. .. .. .. .. .. .. .. 6x .. .. .. .. .. .. .. .. 68 7x .. .. .. .. .. .. .. .. .. 1 DEVICES FOUND x9 .. .. .. .. .. .. .. If all is well, power off and detach the Digital Video Terminal, then plug the Computer Board into the Terminal and reconnect the keyboard and monitor. Demonstration software You can try the two software demos quite easily thanks to the Pico’s xA .. .. .. .. .. .. .. xB .. .. .. .. .. .. .. xC .. .. .. .. .. .. .. xD .. .. .. .. .. .. .. xE .. .. .. .. .. .. .. xF .. .. .. .. .. .. .. Screen 1: the PicoMite BASIC example (seen here in the TeraTerm serial terminal IC2 ID:&H00000C0D. IC2 IS ESP PSRAM IC3 ID:&HEF401600. IC3 IS WINBOND 25Q32 program) scans for devices and READY displays what it finds. Various A:/>ir commands can be used to interact WAITING FOR IR SIGNAL with the PicoMite’s internal file PRESS ANY KEY TO EXIT system or that of a microSD card. Received device = 255 key = 162 Starting Pico Digital Video Terminal BackPack SD OK A: SD card root has 32 files totalling 171393622 bytes. USB MSC OK B: USB MSC card root has 50 files totalling 175713 bytes. RTC found RTC started OK Time is 14:19:46 on 2/9/2024 Screen 2: This display is produced IC2 ID is 0xD. by the Pico Computer’s Arduino IC3 ID is 0xEF401600. demo software on the Digital Video Audio started OK Terminal. It provides a status report I2C scan: and also provides commands to 0x68 I2C scan done. access the included hardware. A:/> ir Waiting for IR signals Press any key to exit Protocol=NEC Address=0x0 Command=0x16 Raw-Data=0xE916FF00 32 bits LSB first Unknown IR Signal Protocol=NEC Address=0x0 Command=0x5E Raw-Data=0xA15EFF00 32 bits LSB first Protocol=NEC Address=0x0 Command=0x5E Repeat gap=40000us A:/> tone Playing tone. Done. A:/> ▇ MPY: soft reboot MicroPython v1.23.0 on 2024-06-02; Raspberry Pi Pico with RP2040 Type “help()” for more information. >>> #Serial console >>> import uos >>> from machine import UART, Pin >>> repl_uart = UART(0, baudrate=115200, tx=Pin(0), rx=Pin(1)) >>> uos.dupterm(repl_uart, 0) >>> >>> #I2C >>> from machine import Pin, I2C >>> i2c = I2C(1, scl=Pin(3), sda=Pin(2), freq=100000) >>> i2c.scan() [104] >>> ▇ Screen 3: These commands can be used with MicroPython to configure it for use with an integrated Digital Video Terminal. They redirect the console to a serial terminal as well as the virtual USB serial port. It’s also straightforward to run an I2C scan, showing the RTC chip at address 104 (0x68) 74 Silicon Chip Australia's electronics magazine bootloader. Connect the Pico Computer to a computer using the microUSB socket on the Pico. If you do not see the RPI-RP2 virtual drive, hold its BOOTSEL button while pressing and releasing S11. You can then copy the UF2 file to the virtual drive. You can view the operation of the software either from a serial terminal program connected to the Pico’s virtual USB-serial port, or using the keyboard and monitor connections of the Terminal. The USB-serial port name or number might change due to the way that different operating systems handle these things. The source code (and other code such as libraries and BASIC OPTIONs) is also available in the software download package at siliconchip. au/Shop/6/528 To try out the external features, you will need to connect appropriate devices, like a USB flash drive or microSD card. These should be FAT formatted (FAT16 or FAT32, although the latter is more standard these days) and inserted before powering on the hardware. Connect some headphones to CON17 to try out the audio. We recommend not connecting an amplifier until you are sure that the audio is working properly. The sections below will detail which features are supported and what to expect. There are several firmware (UF2) files in the UF2 folder of the software downloads, including the three files for the Digital Video Terminal and “flash_ nuke.UF2”, which can be used to completely wipe a Pico’s flash memory. PicoMite BASIC The PicoMite BASIC (MMBasic) example demonstrates most of the available peripherals. There are several OPTIONs that can be configured from the BASIC prompt, plus a demonstration program that has its own interactive command prompt. The “Terminal BackPack BASIC. UF2” file is configured with PicoMite BASIC, the required OPTIONs and the BASIC program. You can load it using the RPI-RP2 bootloader and immediately try it out using a keyboard and monitor attached to the Pico Computer. Once the PicoMite firmware is loaded, it should flash the Pico’s onboard LED. Alternatively, the PicoMite BASIC UF2 file, OPTIONS.BAS and BASIC_ siliconchip.com.au DEMO.BAS files can be individually loaded and edited as needed. Note that the AUDIO option (for PWM audio) is configured; you will want to disable that if you have the I2S DAC module fitted instead. The demo starts by running an I2C scan and the RTC chip should be found at address 0x68, assuming it is fitted. The memory chips are also interrogated for their IDs. Screen 1 shows the boot sequence followed by the IR command, which displays codes received by IR1. The available commands can be listed by entering the HELP command. The TONE command will play audio (if the PWM audio module is fitted), while accessing B: drive allows you to examine the microSD card contents. The A: drive is an internal drive stored in the Pico’s flash memory. The PicoMiteV5.08.00.UF2 file is the same file we installed before applying the necessary OPTIONs and loading the program file. It is the current version available from https://geoffg. net/picomite.html at the time of writing this. Arduino demo The Arduino demo is loaded in similar fashion with the “Terminal BackPack Arduino.UF2” file. The source code for this, along with the libraries we used, can be found in the Arduino folder. There are other libraries that are included with the Pico Arduino board profile. More information about the board profile and its integrated libraries (including those for I2C, SPI and I2S) can be found at https://github.com/ earlephilhower/arduino-pico There is also a PWMAudio library, which should work with the PWM audio module. The Arduino demo is similar to that for PicoMite BASIC, but offers a different set of features. The A: drive refers to the microSD card, while the B: drive is the USB flash memory. The interface is meant to resemble a command prompt, although the commands are not comprehensive, but rather intended to be a simple demonstration of the hardware features. The HELP command lists the available commands. Screen 2 shows the output of this demo. The first 14 lines are automatically produced after it boots up, while the remaining lines show the IR and siliconchip.com.au Parts List – Pico Computer 1 modified Digital Video Terminal (see below) 1 Pico Computer Board (see below) 1 black front panel PCB coded 07112235, 37 × 99mm 2 4-way, 6-way, 8-way or 10-way stackable headers (CON11) 2 4-way, 6-way, 8-way or 10-way headers (CON11) 1 105 × 80 × 40mm Hammond RM2005LTBK or Multicomp MP004809 ABS instrument case [Altronics H0192] 1 micro-USB cable for power, communication and programming 4 15mm-long M3 untapped spacers 4 20mm-long M3 panhead machine screws Pico Computer Board parts 1 double-sided PCB coded 07112234, 68 × 98mm 1 Raspberry Pi Pico microcontroller module (MOD11) 2 20-way low-profile socket headers (for MOD11) [Adafruit 5585] 2 20-way low-profile header strips (for MOD11) 1 PCM5102A DAC module (MOD12) OR 1 PWM audio module (see panel) 1 1220 surface-mounting coin cell holder (BAT1) [BAT-HLD-012-SMT] 1 CR1220 Lithium coin cell (BAT1) 1 USB Type-A through-hole right-angle socket (CON12) 1 SMD microSD card socket (CON13) [Altronics P5717] 1 4-way 2.54mm pitch socket header or pin header (CON14; optional, for I2C breakout) 1 2×14-way 2.54mm pitch right-angle header (CON15; optional, for I/O breakout) 1 3-way 2.54mm pitch pin header (CON16; optional, for audio) 1 3.5mm stereo jack socket (CON17) [Altronics P0094] 1 USB-C power-only SMD socket (CON18) [GCT USB4135 or similar] 1 3-way 2.54mm pitch pin header and jumper shunt (JP11; optional, for I/O breakout) 1 6mm through-hole tactile pushbutton switch (S11) 4 small self-adhesive rubber feet Semiconductors (all optional) 1 DS3231 (wide SOIC-16) or DS3231M (SOIC-8) real-time clock & calendar (IC1) 1 64Mbit SPI PSRAM, SOIC-8 (IC2) [SC7377, Adafruit 4677, ESP-PSRAM64H or similar] 1 32Mbit SPI flash memory, SOIC-8 (IC3) [Winbond W25Q32JVS, AT25SF321B-SSHB-T or similar] 1 3-pin infrared receiver (IR1) 2 3mm green through-hole LEDs (LED11, LED12) Capacitors (all SMD X7R, M3216/1206 size) 3 10μF 10V 4 100nF 50V Resistors (all ¼W SMD M3216/1206 size, 1%) 2 5.1kW 2 4.7kW 3 1kW 1 100W 2 22W 1 10W Parts for modified Digital Video Terminal 1 double-sided PCB coded 07112231, 98 × 68mm 1 Raspberry Pi Pico programmed with 0711223A.UF2 (MOD1) 1 Raspberry Pi Pico programmed with 0711223C.UF2 (MOD3) 1 HDMI-compatible socket (CON1) [Stewart SS-53000-001] 1 USB-A through-hole right-angle socket (CON2) 1 USB-C power-only SMD socket (CON4) [GCT USB4135 or similar] 3 6mm through-hole tactile switches (S1-S3) 4 2-pin headers, 2.54mm pitch (JP1-JP4) 1 4-pin header, 2.54mm pitch (LK1) 5 jumper shunts (JP1-JP4 & LK1) 4 20-way pin headers, 2.54mm pitch (for MOD1 & MOD3) 4 20-way low-profile header sockets (optional; for MOD1 & MOD3) Semiconductors 2 2N7002 SMD N-channel Mosfets, SOT-23 (Q1, Q2) 2 green 3mm through-hole LEDs (LED1, LED3) Resistors (all ¼W SMD M2012/0805 size, 1%) 6 10kW 2 5.1kW 2 1kW 8 270W Australia's electronics magazine 2 22W December 2024  75 TONE commands. The TONE command assumes an I2S DAC is fitted and will not work with the PWM module. During operation, the LED on the Pico should be lit, as should that on the I2S DAC module. MicroPython and C SDK When used with the Pico Computer Board, the Digital Video Terminal only needs to be partially populated, as shown here. Set LK1 to the INT position. All of these photos have been shown enlarged for clarity. Since we do not use MicroPython much, we have not deeply investigated its usage with the Pico Computer, although we were able to work out some basics such as duplicating the USB-serial terminal to the connected Terminal hardware and running an I2C scan. There is a MicroPython folder in the software downloads with some brief notes and sample code to get you started. That includes links to recommended libraries, along with a copy of the MicroPython UF2 file we tested. Screen 3 shows the Digital Video Terminal being configured, followed by an I2C scan identifying the RTC chip at address 104 decimal (0x68 in hexadecimal). Note that you will have to run the first command to configure the terminal from the USB serial port, since that is what makes the hardware serial port available. Subsequently, the USB keyboard and HDMI monitor attached to the Terminal can be used to interact directly with MicroPython. We have not created any demonstrations using the C SDK. There are not many integrated high-level libraries for the peripherals on the Pico Computer, so we have focused our attention on the Arduino code (which is based on the C SDK anyway). Using a Pico W or Pico 2 Take your Digital Video Terminal to the next level by adding a real-time clock, multiple storage facilities and stereo audio. The Pico Computer is the perfect basis for a custom computer project. Although we have not tested them, the examples presented here should work fine with a Pico W in place of the Pico. However, note that the LED on the Pico W is driven differently, so it will probably not light up. None of the Pico Computer peripherals depend on the WiFi or Bluetooth features the Pico W provides. At the time of writing, the Pico 2 has just became available, with a much faster processor and twice as much RAM. We performed some quick tests by recompiling the Arduino code for the Pico 2 and uploading it to the Pico Computer. Everything seemed to work as expected, so if you’re after a more powerful computer, the Pico 2 may be for you! Australia's electronics magazine siliconchip.com.au 76 Silicon Chip Completion Disconnect all the cables and fit a 1220 cell in BAT1 if you have fitted an RTC chip. Slot in the front panel PCB and secure the two boards into the base of the enclosure by threading machine screws through the top PCB, spacer, then bottom PCB and into the enclosure’s lower half. If the I2S DAC module is permanently affixed and blocking that hole, a short screw can be used to affix the lower PCB directly to the enclosure. We have not designed a rear panel, since there are a few options for which sockets to use. If you don’t need access to any of the Pico USB connections or the rear USB-C socket, no rear panel holes are needed and the Pico Computer can be powered from the front panel USB-C socket. If you need access to the CON15 I/O breakout header, you might decide to leave the rear panel off completely. In that case, you should glue a small piece of plastic to the enclosure to restrict access to the coin cell. You will need to have the case fitted to ensure that the coin cell is inaccessible. The Pico Computer is not a toy, so it should be kept away from children in any case. Since the included panels are translucent, you could easily mark them by eye and then cut the required holes out. One option is to drill 3mm holes at each end of the desired cut-out, then This shows how the two PCBs are stacked. We recommend mounting the modules on low-profile headers or directly to their respective PCBs. 15mm spacers separate the two PCBs. You could use a different height but 15mm is required to match our front panel PCB. use a sharp knife or files to remove the remainder of the plastic. Now you can affix the top half of the enclosure using the included screws and reattach any necessary cables and accessories. Standalone use If you are using the Computer Board PCB without the Digital Video Terminal, it can mount directly to the base of any enclosures in the series we are using (Altronics H0190, H0191 or H0192). Which you choose depending on the height of the assembled PCB. Figs.6 and 7 are panel cutting diagrams for this scenario. We expect that readers doing this will have a specific project in mind that might create other panel requirements, so we have not created a panel PCB for this use case. Note that the heights of the LEDs and IR receiver could vary, depending on how exactly you solder them. The heights shown match the PCB panel. Conclusion Fig.6 & 7: these panel cutting diagrams are for using the Pico Computer PCB on its own without connecting it to the Digital Video Terminal PCB. If using them combined, either move all the cut-outs up by 16.6mm (15mm for the spacers and 1.6mm for the PCB thickness), or use our PCB-based front panel and save yourself the effort. siliconchip.com.au Australia's electronics magazine The Pico Computer is a great way of adding numerous features to the Pico Digital Video Terminal. It’s also a handy combination of accessories that work well with the Pico, meaning that it will have numerous applications on its own. Combined with the Digital Video Terminal, the Pico Computer has the potential to become a very interesting standalone computing device. Those skilled in programming may be interested in porting an emulator to the hardware or even writing a standalone operating system. We plan to produce another project using this hardware, and we look forward to seeing what devices other readers create. SC December 2024  77 M k 2 Variable Speed Drive For Induction Motors Part 2 by Andrew Levido Last month, we introduced the Mk2 VSD and described its features, circuit and firmware. This month, we cover construction, testing and some hints for using it. E verything, including the heatsink and fan, is mounted on a single printed circuit board (PCB) that fits into an ABS plastic enclosure measuring 220 × 165 × 60mm, as shown in the accompanying photographs. Many of the components are surface-­ mount types, but they are all relatively easy to solder by hand. There are no fine-pitch chips, and the passives are all 2 × 1.2mm or larger, except for three diodes, which are a little bit smaller but should be manageable. Anyone with a modicum of SMT soldering experience should have no trouble putting it together successfully. That said, this is a complex build, and because of the high voltages and currents involved, it is recommended only for experienced constructors. Regardless of your skill level, if you build this, you must follow the safety instructions when it comes to the testing stage. It’s also a good idea to double and triple-check your work before powering it up. We’d hate for you to put a lot of effort into building this, only for it to blow up because something was installed backwards or in the wrong spot. Assembly We recommend assembling the VSD in two stages, as described below. First, 78 Silicon Chip we will focus on the control circuitry, so we can test it safely at a low voltage and get it working. After that, we will move on to the power electronics. The VSD is built on a double-sided board coded P9048-C or 11111241 that measures 150 × 205mm. Start by fitting all the surface-mounting parts, using the overlay diagram (Fig.8) and close-up of the section near the microcontroller (Fig.9). Work methodically across the board, paying attention to the orientation of polarised components like ICs, diodes (including LEDs) and electrolytic capacitors. You can also refer to the silkscreening on the PCB. We won’t go into a great amount of detail here on how to solder SMD parts, as it is now pretty common, and many of our projects require it. However, we’ll give a quick overview and some tips. There are three main ways you could solder the SMDs: with a reflow oven, with a hot air rework station or with a soldering pencil/iron. Those with reflow ovens and hot air rework stations likely are already familiar with the required techniques, which involve adding solder paste to the board, placing the components on top and then heating the solder paste until it reflows. Manual soldering is best done with a syringe of good-quality flux paste. For Australia's electronics magazine each part, spread a thin layer of flux paste on the pads, then place the part on its pads, ensuring it is correctly orientated. One of the worst things you can do is solder an IC to the board backwards! For the microcontroller in a quad flat package, there are four possible orientations, but only one is correct (with the pin 1 dot as shown). With the part in place and a clean soldering iron, add a little solder to the tip and tack-solder one of the part’s pads. Check that all its pins are lined up with the other pads; if not, the joint can be remelted and the part gently nudged into position. Once in position, the remaining pins can be soldered and the initial one refreshed. Finally, for parts with closely spaced pins (like ICs), check for solder bridges between pins. If found, they can be cleared with the application of a little more flux paste and then solder-wicking braid. The braid can also be used to remove excess solder if there’s too much on some pins. Once all the surface-mounting parts are in place, clean the flux residue off the board, then add relay RLY2, DIP switch bank S1, trimpots VR1 & VR2, header CON17 and the input terminal blocks, CON8-CON11. Slot all four blocks together (in dovetail fashion) before soldering them in place. siliconchip.com.au Fig.8: this component overlay shows where everything goes on the PCB. Fit the surfacemounting parts first, then the DIP switch, trimpots and relay RLY2. Test the control circuitry thoroughly, as described in the text, before moving on to the power electronics. WARNING: DANGEROUS VOLTAGES This circuit is directly connected to the 230V AC mains. As such, most of the parts and wiring operate at mains potential. Contact with any part of these non-isolated circuit sections could prove fatal. Note also that the circuit can remain potentially lethal even after the 230V AC mains supply has been disconnected! To ensure safety, this circuit MUST NOT be operated unless it is fully enclosed in a plastic case. Do not connect this device to the mains with the lid of the case removed. Do not touch any part of the circuit for at least 30 second after unplugging the power cord from the mains socket. Do not attempt to build this project unless you understand what you are doing and are experienced working with high-voltage circuits. siliconchip.com.au Australia's electronics magazine December 2024  79 Fig.9: this close-up of Fig.8 shows the most densely populated section, so that you can more clearly see the values of the resistors and capacitors there. At this stage, you will have installed all the parts in the low-voltage domain except for the AC-to-DC switch-mode power supply module, MOD2. We can now test this circuitry. Connect a bench supply to the +12V and GND pins of CON17. Make sure the polarity is correct and don’t accidentally connect it to the +3.3V pin! DuPont jumper leads are a good way to make this connection. Set the supply to deliver 12V DC, with the current limit set at around 200mA. When you switch it on, the power supply should not go into current limiting. If is does, there is a short circuit or incorrectly placed component somewhere, so switch off and check the components on the board carefully, including their solder joints. Initial testing The fully assembled PCB; it just needs the fuse cover added, to be mounted in the case and the wiring connected. 80 Silicon Chip Australia's electronics magazine If the current draw is OK, check for 3.3V at the bottom pin of CON17 relative to GND. It should be in the range of 3.1-3.5V. If that is OK, and your microcontroller is not already pre-programmed, now is the time to connect an ST-Link programmer to CON16 and flash the code using the STM32Cube software (a free download). If yours is pre-­programmed, you can skip this step. With the micro programmed, to check for the correct operation of the control circuit, first ensure all the DIP switches are in the off positions and both trimpots are wound all the way anti-clockwise, then apply power. All three LEDs should flash briefly twice, then after about three seconds, the yellow LED (LED2) should come on. If you short the E-Stop & Run pairs of terminals with two wire links and advance the speed trimpot (VR1), the yellow LED should extinguish and the green LED (LED3) should flash while the speed ramps up to the setpoint, at which time LED3 will light steadily. If you turn the speed pot back to zero, the controller should ramp down with the green LED flashing until the yellow LED lights again. Increasing the ramp time using trimpot VR2 should prolong the ramp time. If you close the At-Speed DIP switch and repeat the above process, you should hear relay RLY2 close whenever the green LED stops flashing and lights steadily, then open when it begins to flash again. Don’t forget that siliconchip.com.au The finished VSD, all wired up, including the external control wiring (upper right). you need to cycle the power to read the new DIP switch configuration. You can try opening the Run switch or the E-Stop circuits while the speed controller is running (green LED on or flashing). If Run is opened, the green LED should flash while the speed ramps down to zero, then the yellow LED should light. If the E-Stop switch is opened, the yellow LED should come on immediately. Now you can check pool pump mode. Bridge the E-Stop and Run terminals again, set the speed and ramp potentiometers to about halfway and close the pool pump mode DIP switch (“POOL MD”). On reapplying power, the controller should start and ramp to full speed with the green LED flashing slowly. After about 30 seconds, the speed should ramp down (green LED flashing fast) to the preset speed (green LED on steadily). Trying again with the Pool-Time DIP switch (“POOL TIM”) also closed should extend the pool-pump period to about five minutes. You can check three-phase mode by closing that DIP switch. It should work as described earlier (ignoring the siliconchip.com.au pool pump mode part). If you now short the Reverse terminals while it is running, the speed should ramp down (green LED flashing fast) then stop for two seconds (yellow LED on) and ramp up again to the preset speed. Finally, you can check fault operation by momentarily shorting out the thermistor terminals. The red and yellow LEDs should latch on. Opening and reclosing the E-Stop circuit should reset the fault. If you hit a snag at any point, stop, check the board carefully and fix the problem. Each step above tests a different part of the circuit, so consult the relevant part of the circuit diagram for components to check. Fix any problems and verify it has the correct operation before moving on. If you have an oscilloscope, you can take a look at the PWM motor drive signals on pins 2 to 7 of IC3. They can be a bit difficult to trigger on since the pulse widths are continuously varying, so consider using one-shot mode to capture a snapshot if your ‘scope supports it. There will only be signals on four of these pins if single-phase mode is selected. Australia's electronics magazine The switching frequency should be 15.625kHz (a period of 64µs) and the amplitude about 3.3V. Power electronics Start the assembly of the power components by preparing the heatsink. This is a 100mm length of 40 × 40mm heatsink ‘tunnel’ extrusion. Mine came cut to length from AliExpress. A total of 11 holes need to be drilled and tapped in accordance with the drilling diagram (Fig.10). There is a different arrangement of holes on each face, so take care to get them all in the right orientation with respect to each other. I recommend clearly labelling each face according to the diagram and marking the fan end. Mark the hole positions, but before drilling anything, offer it up to the board to check the marks line up with the IGBTs, Mosfet and diode bridge. Don’t forget to run the tap through the four extruded corner ‘holes’ on each end to make the mounting of the fan and finger guard easier. Use some wet & dry abrasive paper on a flat surface to ensure that the drilled faces are flat and free of burrs so that the power devices make good thermal contact. December 2024  81 Secure the fan to the appropriate end of the heatsink with four M3 × 25mm screws, making sure the arrow denoting the direction of airflow is pointing towards the heatsink. Orientate the fan so that the lead emerges at the corner shown in the photos. Now attach the finger guard together with its filter to the other end of the heatsink, using four M3 × 10mm machine screws. Mount the heatsink assembly to the PCB with two M3 × 10mm screws with spring washers under the heads. The rectifier bridge (BR1) and the discharge Mosfet (Q7) can be mounted next, with a smear of thermal compound between the devices and heatsink to ensure good thermal contact. Use M3 × 10mm screws with spring washers under the heads. Don’t solder the devices to the PCB just yet. Next, mount the six IGBTs (Q1Q6) after carefully bending their centre pins to fit the footprint. Again, use thermal compound, M3 × 10mm screws and spring washers. Tighten all the devices down, making sure they don’t twist too much, then solder and trim all the leads (of Q1-Q7 and BR1). Give all the screws a final tighten – you can’t get to some of them once the DC bus capacitors are installed. Affix the thermistor to the top of the heatsink, again using thermal compound, an M3 × 10mm screw and spring washer. Orient the thermistor lead along the heatsink towards the fan as shown. Trim and strip the thermistor and fan leads, then solder them to the PCB pads provided. The thermistor is not polarised, but the fan is, so make sure the red lead goes to the pad marked by the plus sign. Now you can install all the remaining components. I suggest starting with the shortest and finishing with the five large electrolytic capacitors. Pay attention to the orientation of the filter capacitors – their positive leads must all go towards the top of the board! Be careful also with the AC-DC power modules; they look similar but have different secondary voltages. The 15V one is MOD1 and the 12V one is MOD2. You have finished the PCB assembly at this point, but it’s a good idea to take a bit of time to check your work thoroughly before moving on. Enclosure preparation The enclosure needs to have a square opening cut into the side to accommodate the heatsink exhaust, plus a series of ventilation holes in the top and opposite side and holes for the cable glands in the bottom end. The locations and dimensions of these are given in Fig.11. Making the square opening can be a challenge. It helps to screw the lid firmly onto to the case for this operation, as the opening overlaps both the base and the lid. I applied masking tape in the area of the cutout and marked its edges onto that. I created the opening by chain-drilling a series of holes near, but just inside the marked line and then filing carefully up to it. Fig.10: the heatsink requires a total of 11 M3-tapped holes. They are positioned differently on each face, so be careful to get them all correct with respect to each other. All dimensions are in millimetres, and the diagram is shown at actual size. 82 Silicon Chip Australia's electronics magazine siliconchip.com.au Next, drill the 14 ventilation holes according to the diagram. I used masking tape as before to mark the centres, then drilled pilot holes with a 3mm drill bit, followed by a 10mm bit. You can then drill holes in the bottom end of the enclosure for the cable glands. Two of the glands are required: one for the mains input and one for the motor output cable, but the third one, for control cables, is optional. If you are using the VSD in standalone mode (see the applications section below), this hole may be unnecessary. The hole size should match the glands that you use. Make sure you get the correct sized glands for your cables – they will only provide good strain relief if they are matched to the cable diameter. The enclosure comes with a length of O-ring material which you should push into the slot in the lid, avoiding the area of the fan guard cutout. As a side note, you can get a set of mounting feet for the enclosure that allows it to be mounted on a panel or wall. If you are using those, now is a good time to screw them onto the bottom of the enclosure. Final assembly and wiring You can now mount the PCB assembly into the case with four self-­tapping screws and wire it up to suit your application. For most single-phase applications, an input cable with a three-pin mains plug and an output cable with a matching mains socket should work. An easy way to create these cables is to sacrifice a low-cost extension cord by cutting it in half. Please use something that meets the Australian standards, bought from a reputable supplier and not some random internet find. Feed the cut end of each cable through the appropriate gland, tighten, and then crimp female 6.3mm spade connectors to the conductors. Either use insulated spade connectors for the Active and Neutral (brown and light blue) wires, or add some insulating heatshrink tubing in the appropriate colours over the exposed metal after crimping. We need a direct 10A wire connection between the incoming and outgoing Earth wires to ensure the device can handle a high fault current if something goes wrong with the motor. Therefore, cut a 15cm-long piece of 10A green/yellow striped wire (which siliconchip.com.au Fig.11: the case needs a square opening for the heatsink exhaust, plus a total of 14 10mm ventilation holes as shown. The size of holes for the cable glands depends on the exact glands you are using. can be stripped from 10A mains flex or a spare 10A mains cord) and crimp piggyback spade lugs onto both ends. Plug the incoming/outgoing Earth wire spades onto the tabs on the piggyback connectors and then shrink some 10mm green/yellow striped heatshrink tubing over the piggybacked connectors. They will be close to the Active and motor output spades. While those are also insulated, it doesn’t hurt to have extra insulation! Australia's electronics magazine Plug the piggyback spade lugs onto both Earth connectors on the PCB, then connect up the Active (brown), Neutral (blue) and motor output wires. Double-check the wires are in the right places. The wire with the mains plug on the end (incoming power) must go to the A, EARTH and N spades near the fuse clips, while the one with the socket on the end goes to the EARTH, U and V motor connectors near IC2. Now is also a good time to December 2024  83 The fan and thermistor wires should be cable tied together preventing a loose wire from one of these straying onto any of the U, V or W terminals. We recommend that for safety, you strip back some of the insulation in the middle of the Earth wire (without cutting the conductors) and crimp the copper to an eyelet lug that’s attached to the heatsink via an extra tapped hole (the position isn’t critical) so the heatsink can’t become live if the PCB Earth tracks fuse. Make sure you don’t leave off the 10A Earth wire between the two Earth terminals as it’s vital for fault protection. Also fit an insulating cover over the fuse as seen here for safety. 84 Silicon Chip Australia's electronics magazine insert the 10A slow-blow fuse into the F1 clips and slip the insulating cover over the top. If you are driving a three-phase motor, or building the VSD into another piece of equipment, you may need custom wiring. In any case, it is absolutely mandatory to wire in the mains Earth and to connect the motor Earth to the motor chassis with a proper wire between the two (not relying on the PCB to conduct Earth current!). The PCB Earth connections are for two purposes only: to Earth the heatsink for safety, and as part of the mains EMI filters that each have two Y2 capacitors between the phases and Earth. As mentioned in the adjacent caption, we recommend attaching the Earth wire directly to the heatsink as well. Control wiring This speed controller has been designed to be as flexible as possible. In the standalone configuration, no external controls are required. The E-Stop and Run terminals should be bridged by short lengths of hookup wire, and the internal speed pot selected on S1. In this case, as soon as power is applied, the motor will start and ramp up to the preset speed. The speed and ramp rate are set via the onboard trimpots, VR1 & VR2. When power is removed, the motor will coast to a stop just as it would if switched off when directly connected to the mains. This arrangement could be used to run a single-phase motor at a lower speed than usual, or to run a threephase motor at a fixed speed from a single-phase supply. It could also be used as a ‘soft starter’, to provide a gentle start for sensitive loads or to limit the initial starting current surge. Most pool pump applications will also use this configuration. At the other end of the spectrum, it is possible to use this controller as part of a more complex control system, such as for a machine tool. In such applications, the VSD would normally be mounted in an electrical cabinet, with external controls (run, emergency stop, speed control etc) located on a panel close to the operator. If the machine tool is numerically controlled, these control signals may come from a CNC controller or PLC. siliconchip.com.au You can see from our photos that we built a small ‘remote control’ box to test out the external control functions. It’s little more than three switches and a pot mounted to a Jiffy box and wired to a 9-core alarm cable, run through cable glands into the VSD case, where they connect to the EXT SPEED, ESTOP, RUN and REV terminals of CON8-CON10. We won’t go into details here, as we expect anyone who can build this VSD will be able to figure out the wiring from the PCB labelling. The cable gland outside nuts that are tightened to secure the mains input and output wires should be permanently fixed using super glue on the threads to prevent the glands from being undone from outside the box and the mains wires becoming loose. Using the VSD Using the VSD is straightforward. If the unit trips out when starting, you can extend the ramp rate and/or switch the BOOST DIP switch on. We tested it on a domestic pool pump and found that, with the correct settings, it had no trouble starting the pump under load. If you have one, you can use a current clamp meter around one of the motor power wires to monitor the motor current during startup. The VSD should be able to deliver its full rated current (9A in single-phase mode and 5.5A in three-phase mode) continuously and up to 18A/11A for a few cycles. You will need a clamp meter with a peak hold setting to measure this. If you are wiring the VSD directly to the motor, you will need to work out how to connect it. Single-phase PSC motors have notoriously confusing terminal housings with no discernible standard arrangement. There is usually a diagram inside of the terminal housing lid to help; otherwise, see if you can locate a wiring diagram for your motor online. Don’t forget to connect the Earth wire solidly to the stud provided in the terminal box. The only way to change the direction of rotation of PSC motors is to reverse the sense of the start winding with respect to the run winding. Many motors have an arrangement of relocatable bridges to allow this to be done without rewiring the whole motor. The terminal arrangement for three-phase motors is usually a little simpler. The VSD can only supply a siliconchip.com.au L1 L1 L2 L2 L3 L3 'STAR' CONNECTION 'DELTA' CONNECTION Fig.12: the windings of small 3-phase motors are normally connected in star configuration for use with the 400V RMS 3-phase mains supply. In this case, each winding is driven with the phase-to-neutral voltage of 230V. By changing how the windings are connected (which can usually be done by moving some jumpers), the motor can be changed to delta configuration, with just one winding between each phase. DUTY CYCLE 1 It can then be driven from a 230V RMS DUTY CYCLE 2 3-phase supply such as the output of this motor controller. PWM 1 Fig.13: this diagram illustrates the difference between traditional edgealigned PWM and centre-aligned PWM (also known as dualramp PWM). With centre-aligned PWM, the leading edge of each pulse moves as the duty cycle changes. This is an advantage because if all outputs switch high at the same time, as with edge-aligned PWM, the total current pulse is larger and so more EMI is generated. PWM 2 EDGE-ALIGNED PWM DUTY CYCLE 1 DUTY CYCLE 2 PWM 1 PWM 2 phase-to-phase voltage of 230V RMS, so it is suitable for motors with 230V or 240V windings (most small induction motors). The rating plate will normally quote the voltage rating as 230V/400V, 240V/415V or something similar. There are usually six terminals for the three windings, with bridges to connect the windings in star (Y) configuration for the higher voltage or delta (Δ) configuration for the lower (see Fig.12). For 230/240V operation, use the delta (Δ) option. Again, the inside of the terminal box lid should have a diagram to help. You can connect the VSD’s U, V & W outputs in any order, although this will Australia's electronics magazine CENTRE-ALIGNED PWM affect the direction of rotation. If the direction is not what you want, swap any two of the leads or use the Reverse control input, which does the same thing electronically. Again, connecting the Earth is mandatory for safety. A word of warning: induction motors often have a shaft-mounted fan that blows cooling air across the fins cast into the housing. This fan will be much less effective at low shaft speeds, so be careful if you intend to run a motor in this way for long periods of time or in very hot environments. If this is a concern for you, consider using an external cooling fan with a separate power source. SC December 2024  85 SILICON CHIP .com.au/shop ONLINESHOP HOW TO ORDER INTERNET (24/7) PAYPAL (24/7) eMAIL (24/7) MAIL (24/7) PHONE – (9-5:00 AET, Mon-Fri) siliconchip.com.au/Shop silicon<at>siliconchip.com.au silicon<at>siliconchip.com.au PO Box 194, MATRAVILLE, NSW 2036 (02) 9939 3295, +612 for international You can also pay by cheque/money order (Orders by mail only) or bank transfer. Make cheques payable to Silicon Chip. 12/24 YES! You can also order or renew your Silicon Chip subscription via any of these methods as well! The best benefit, apart from the magazine? Subscribers get a 10% discount on all orders for parts. PRE-PROGRAMMED MICROS For a complete list, go to siliconchip.com.au/Shop/9 $10 MICROS $15 MICROS 24LC32A-I/SN ATmega328P ATmega328P-AUR ATtiny45-20PU ATtiny85V-10PU PIC10LF322-I/OT PIC12F1572-I/SN PIC12F617-I/P PIC12F617-I/SN PIC12F675-I/P PIC16F1455-I/P Digital FX Unit (Apr21) ATSAML10E16A-AUT High-Current Battery Balancer (Mar21) 110dB RF Attenuator (Jul22), Basic RF Signal Generator (Jun23) PIC16F18877-I/P USB Cable Tester (Nov21) RGB Stackable LED Christmas Star (Nov20) PIC16F18877-I/PT Dual-Channel Breadboard PSU Display Adaptor (Dec22) Wideband Fuel Mixture Display (WFMD; Apr23) 2m VHF CW/FM Test Generator (Oct23) PIC16F88-I/P Battery Charge Controller (Jun22), Railway Semaphore (Apr22) Shirt Pocket Audio Oscillator (Sep20) PIC24FJ256GA702-I/SS Ohmmeter (Aug22), Advanced SMD Test Tweezers (Feb23) Range Extender IR-to-UHF (Jan22) ESR Test Tweezers (Jun24) LED Christmas Ornaments (Nov20; versions), Nano TV Pong (Aug21) PIC32MM0256GPM028-I/SS Super Digital Sound Effects (Aug18) Active Mains Soft Starter (Feb23), Model Railway Uncoupler (Jul23) PIC32MX170F256D-501P/T 44-pin Micromite Mk2 (Aug14), 4DoF Simulation Seat (Sep19) Model Railway Carriage Lights (Nov21) PIC32MX170F256B-50I/SP Micromite LCD BackPack V1-V3 (Feb16 / May17 / Aug19) Train Chuff Sound Generator (Oct22) Advanced GPS Computer (Jun21), Touchscreen Digital Preamp (Sep21) Auto Train Controller (Oct22), GPS Disciplined Oscillator (May23) PIC32MX170F256B-I/SO Battery Multi Logger (Feb21), Battery Manager BackPack (Aug21) Railway Points Controller Transmitter / Receiver (2 versions; Feb24) PIC32MX270F256B-50I/SP ASCII Video Terminal (Jul14), USB M&K Adaptor (Feb19) PIC16F1455-I/SL Battery Multi Logger (Feb21), USB-C Serial Adaptor (Jun24) $20 MICROS PIC16LF1455-I/P New GPS-Synchronised Analog Clock (Sep22) ATmega32U4 Wii Nunchuk RGB Light Driver (Mar24) PIC16F1459-I/P K-Type Thermostat (Nov23), Secure Remote Switch (RX, Dec23) ATmega644PA-AU AM-FM DDS Signal Generator (May22) Mains Power-Up Sequencer (Feb24 | repurposed firmware Jul24) 8-Channel Learning IR Remote (Oct24) $25 MICROS PIC16F1459-I/SO Multimeter Calibrator (Jul22), Buck/Boost Charger Adaptor (Oct22) PIC32MX170F256B-50I/SO + PIC16F1455-I/SL Micromite Explore-40 (SC5157, Oct24) PIC16F15214-I/SN Digital Volume Control Pot (SMD; Mar23), Silicon Chirp Cricket (Apr23) PIC32MX470F512H-I/PT Stereo Echo/Reverb (Feb 14), Digital Effects Unit (Oct14) PIC16F15214-I/P Digital Volume Control Pot (TH; Mar23), Filament Dryer (Oct24) PIC32MX470F512H-120/PT Micromite Explore 64 (Aug 16), Micromite Plus (Nov16) PIC16F15224-I/SL Multi-Channel Volume Control (OLED Module; Dec23) PIC32MX470F512L-120/PT Micromite Explore 100 (Sep16) PIC16F18146-I/SO Volume Control (Control Module, Dec23), Coin Cell Emulator (Dec23) $30 MICROS Compact OLED Clock & Timer (Sep24), Flexidice (Nov24) PIC16LF15323-I/SL Remote Mains Switch (TX, Jul22), Secure Remote Switch (TX, Dec23) PIC32MX695F512H-80I/PT Touchscreen Audio Recorder (Jun14) PIC32MZ2048EFH064-I/PT DSP Crossover/Equaliser (May19), Low-Distortion DDS (Feb20) STM32G030K6T6 Variable Speed Drive Mk2 (Nov24) DIY Reflow Oven Controller (Apr20), Dual Hybrid Supply (Feb22) W27C020 Noughts & Crosses Computer (Jan23) KITS, SPECIALISED COMPONENTS ETC COMPACT HIFI HEADPHONE AMP (SC6885) (DEC 24) CAPACITOR DISCHARGER KIT (SC7404) (DEC 24) PICO COMPUTER (DEC 24) Complete Kit: includes everything except the power supply (see p47, Dec24) Includes the PCB and all components that mount on it, the mounting hardware (without heatsink) and banana sockets (see p36, Dec24) $70.00 $30.00 For full functionality both the Pico Computer Board and Digital Video Terminal kits are required, see page 71 in the December 2024 issue for more details. - Pico Computer Board kit (SC7374) $40.00 - Pico Digital Video Terminal kit (SC6917) $65.00 Separate/Optional Components: - PWM Audio Module kit (SC7376) $10.00 - ESP-PSRAM64H 64Mb SPI PSRAM chip (SC7377) $5.00 - DS3231 real-time clock SOIC-16 IC (SC5103) $7.50 - DS3231MZ real-time clock SOIC-8 IC (SC5779) $10.00 FLEXIDICE COMPLETE KIT (SC7361) Includes all required parts except the coin cell (see p71, Nov24) (NOV 24) MICROMITE EXPLORE-40 KIT (SC6991) (OCT 24) DUAL-RAIL LOAD PROTECTOR (SC7366) (OCT 24) PicoMSA PARTS (SC7323) (SEP 24) COMPACT OLED CLOCK & TIMER KIT (SC6979) (SEP 24) DISCRETE IDEAL BRIDGE RECTIFIER (SEP 24) Includes all required parts (see p83, Oct24) Hard-to-get parts: includes the PCB and all semiconductors except the optional/variable diodes (see p73, Oct24) Hard-to-get parts: includes the PCB, Raspberry Pi Pico (unprogrammed), plus all semiconductors, capacitors and resistors (see p63, Sep24) Includes everything except the case & Li-ion cell (see p34, Sep24) $30.00 $35.00 siliconchip.com.au/Shop/ DUAL MINI LED DICE (AUG 24) AUTOMATIC LQ METER KIT (SC6939) (JUL 24) ESR TEST TWEEZERS COMPLETE KIT (SC6952) (JUN 24) DC SUPPLY PROTECTOR (JUN 24) Complete kit: choice of white or black PCB solder mask (see page 50, August 2024) - Through-hole LEDs kit (SC6849) $17.50 - SMD LEDs kit (SC6961) $17.50 Includes everything except the case & debugging interface (see p33, July24) - Rotary encoder with integral pushbutton (available separately, SC5601) Includes all parts and OLED, except the coin cell and optional header - 0.96in white OLED with SSD1306 controller (also sold separately, SC6936) All kits come with the PCB and all onboard components (see page 81, June24) - Adjustable SMD kit (SC6948) - Adjustable TH kit (SC6949) - Fixed TH kit – ZD3 & R1-R7 vary so are not included (SC6950) USB-C SERIAL ADAPTOR COMPLETE KIT (SC6652) (JUN 24) WIFI DDS FUNCTION GENERATOR (MAY 24) Includes the PCB, programmed micro and all other required parts $35.00 Short-form kit: includes everything except the case, USB cable, power supply, labels and optional stand. The included Pico W is not programmed (SC6942) - Optional laser-cut acrylic stand pieces (SC6932) - 3.5in LCD touchscreen: also available separately (SC5062) $50.00 Complete kit: Includes the PCB and everything that mounts to it, including the 49.9Ω and 75Ω resistors (see page 38, May24) $45.00 Both kits include the PCB and everything that mounts to it (see page 83, Sep24) - All through-hole (TH) kit (SC6987) $30.00 - SMD kit (SC6988) $27.50 10MHz to 1MHz / 1Hz FREQUENCY DIVIDER (SC6881) (MAY 24) ESP-32CAM BACKPACK KIT (SC6886) (APR 24) PICO GAMER KITS (APR 24) $50.00 $10.00 $17.50 $22.50 $20.00 $20.00 $95.00 $7.50 $35.00 $40.00 Includes everything to build the BackPack, except the ESP32-CAM module - SC6911: everything except the case & battery; RP2040+ is pre-programmed - SC6912: the SC6911 kit, plus the LEDO 6060 resin case - SC6913: the SC6911 kit, plus a dark grey/black resin case - 3.2in LCD touchscreen: also available separately (SC6910) *Prices valid for month of magazine issue only. All prices in Australian dollars and include GST where applicable. # Overseas? Place an order on our website for a quote. $100.00 $3.00 $42.50 $85.00 $125.00 $140.00 $30.00 PRINTED CIRCUIT BOARDS & CASE PIECES PRINTED CIRCUIT BOARD TO SUIT PROJECT HIGH-POWER BUCK-BOOST LED DRIVER ARDUINO PROGRAMMABLE LOAD SPECTRAL SOUND MIDI SYNTHESISER REV. UNIVERSAL BATTERY CHARGE CONTROLLER VGA PICOMITE SECURE REMOTE MAINS SWITCH RECEIVER ↳ TRANSMITTER (1.0MM THICKNESS) MULTIMETER CALIBRATOR 110dB RF ATTENUATOR WIDE-RANGE OHMMETER WiFi PROGRAMMABLE DC LOAD MAIN PCB ↳ DAUGHTER BOARD ↳ CONTROL BOARD MINI LED DRIVER NEW GPS-SYNCHRONISED ANALOG CLOCK BUCK/BOOST CHARGER ADAPTOR AUTO TRAIN CONTROLLER ↳ TRAIN CHUFF SOUND GENERATOR PIC16F18xxx BREAKOUT BOARD (DIP-VERSION) ↳ SOIC-VERSION AVR64DD32 BREAKOUT BOARD LC METER MK3 ↳ ADAPTOR BOARD DC TRANSIENT SUPPLY FILTER TINY LED ICICLE (WHITE) DUAL-CHANNEL BREADBOARD PSU ↳ DISPLAY BOARD DIGITAL BOOST REGULATOR ACTIVE MONITOR SPEAKERS POWER SUPPLY PICO W BACKPACK Q METER MAIN PCB ↳ FRONT PANEL (BLACK) NOUGHTS & CROSSES COMPUTER GAME BOARD ↳ COMPUTE BOARD ACTIVE MAINS SOFT STARTER ADVANCED SMD TEST TWEEZERS SET DIGITAL VOLUME CONTROL POT (SMD VERSION) ↳ THROUGH-HOLE VERSION MODEL RAILWAY TURNTABLE CONTROL PCB ↳ CONTACT PCB (GOLD-PLATED) WIDEBAND FUEL MIXTURE DISPLAY (BLUE) TEST BENCH SWISS ARMY KNIFE (BLUE) SILICON CHIRP CRICKET GPS DISCIPLINED OSCILLATOR SONGBIRD (RED, GREEN, PURPLE or YELLOW) DUAL RF AMPLIFIER (GREEN or BLUE) LOUDSPEAKER TESTING JIG BASIC RF SIGNAL GENERATOR (AD9834) ↳ FRONT PANEL V6295 VIBRATOR REPLACEMENT PCB SET DYNAMIC RFID / NFC TAG (SMALL, PURPLE) ↳ NFC TAG (LARGE, BLACK) RECIPROCAL FREQUENCY COUNTER MAIN PCB ↳ FRONT PANEL (BLACK) PI PICO-BASED THERMAL CAMERA MODEL RAILWAY UNCOUPLER MOSFET VIBRATOR REPLACEMENT ARDUINO ESR METER (STANDALONE VERSION) ↳ COMBINED VERSION WITH LC METER WATERING SYSTEM CONTROLLER CALIBRATED MEASUREMENT MICROPHONE (SMD) ↳ THROUGH-HOLE VERSION SALAD BOWL SPEAKER CROSSOVER PIC PROGRAMMING ADAPTOR REVISED 30V 2A BENCH SUPPLY MAIN PCB ↳ FRONT PANEL CONTROL PCB ↳ VOLTAGE INVERTER / DOUBLER 2M VHF CW/FM TEST GENERATOR TQFP-32 PROGRAMMING ADAPTOR ↳ TQFP-44 ↳ TQFP-48 ↳ TQFP-64 K-TYPE THERMOMETER / THERMOSTAT (SET; RED) DATE JUN22 JUN22 JUN22 JUN22 JUL22 JUL22 JUL22 JUL22 JUL22 AUG22 SEP22 SEP22 SEP22 SEP22 SEP22 OCT22 OCT22 OCT22 OCT22 OCT22 OCT22 NOV22 NOV22 NOV22 NOV22 DEC22 DEC22 DEC22 DEC22 JAN23 JAN23 JAN23 JAN23 JAN23 FEB23 FEB23 MAR23 MAR23 MAR23 MAR23 APR23 APR23 APR23 MAY23 MAY23 MAY23 JUN23 JUN23 JUN23 JUN23 JUL23 JUL23 JUL23 JUL23 JUL23 JUL23 JUL23 AUG23 AUG23 AUG23 AUG23 AUG23 SEP23 SEP23 SEP23 OCT22 SEP23 OCT23 OCT23 OCT23 OCT23 OCT23 NOV23 PCB CODE 16103221 04105221 01106221 04107192 07107221 10109211 10109212 04107221 CSE211003 04109221 04108221 04108222 18104212 16106221 19109221 14108221 09109221 09109222 24110222 24110225 24110223 CSE220503C CSE200603 08108221 16111192 04112221 04112222 24110224 01112221 07101221 CSE220701 CSE220704 08111221 08111222 10110221 SC6658 01101231 01101232 09103231 09103232 05104231 04110221 08101231 04103231 08103231 CSE220602A 04106231 CSE221001 CSE220902B 18105231/2 06101231 06101232 CSE230101C CSE230102 04105231 09105231 18106231 04106181 04106182 15110231 01108231 01108232 01109231 24105231 04105223 04105222 04107222 06107231 24108231 24108232 24108233 24108234 04108231/2 Price $5.00 $5.00 $7.50 $7.50 $5.00 $7.50 $2.50 $5.00 $5.00 $7.50 $7.50 $5.00 $10.00 $2.50 $5.00 $5.00 $2.50 $2.50 $2.50 $2.50 $2.50 $7.50 $2.50 $5.00 $2.50 $5.00 $5.00 $5.00 $10.00 $5.00 $5.00 $5.00 $12.50 $12.50 $10.00 $10.00 $2.50 $5.00 $5.00 $10.00 $10.00 $10.00 $5.00 $5.00 $4.00 $2.50 $12.50 $5.00 $5.00 $5.00 $1.50 $4.00 $5.00 $5.00 $5.00 $2.50 $2.50 $5.00 $7.50 $12.50 $2.50 $2.50 $10.00 $5.00 $10.00 $2.50 $2.50 $5.00 $5.00 $5.00 $5.00 $5.00 $10.00 For a complete list, go to siliconchip.com.au/Shop/8 PRINTED CIRCUIT BOARD TO SUIT PROJECT PICO AUDIO ANALYSER (BLACK) MODEM / ROUTER WATCHDOG (BLUE) DISCRETE MICROAMP LED FLASHER MAGNETIC LEVITATION DEMONSTRATION MULTI-CHANNEL VOLUME CONTROL: VOLUME PCB ↳ CONTROL PCB ↳ OLED PCB SECURE REMOTE SWITCH RECEIVER ↳ TRANSMITTER (MODULE VERSION) ↳ TRANSMITTER (DISCRETE VERSION COIN CELL EMULATOR (BLACK) IDEAL BRIDGE RECTIFIER, 28mm SQUARE SPADE ↳ 21mm SQUARE PIN ↳ 5mm PITCH SIL ↳ MINI SOT-23 ↳ STANDALONE D2PAK SMD ↳ STANDALONE TO-220 (70μm COPPER) RASPBERRY PI CLOCK RADIO MAIN PCB ↳ DISPLAY PCB KEYBOARD ADAPTOR (VGA PICOMITE) ↳ PS2X2PICO VERSION MICROPHONE PREAMPLIFIER ↳ EMBEDDED VERSION RAILWAY POINTS CONTROLLER TRANSMITTER ↳ RECEIVER LASER COMMUNICATOR TRANSMITTER ↳ RECEIVER PICO DIGITAL VIDEO TERMINAL ↳ FRONT PANEL FOR ALTRONICS H0190 (BLACK) ↳ FRONT PANEL FOR ALTRONICS H0191 (BLACK) WII NUNCHUK RGB LIGHT DRIVER (BLACK) ARDUINO FOR ARDUINIANS (PACK OF SIX PCBS) ↳ PROJECT 27 PCB SKILL TESTER 9000 PICO GAMER ESP32-CAM BACKPACK WIFI DDS FUNCTION GENERATOR 10MHz to 1MHz / 1Hz FREQUENCY DIVIDER (BLUE) FAN SPEED CONTROLLER MK2 ESR TEST TWEEZERS (SET OF FOUR, WHITE) DC SUPPLY PROTECTOR (ADJUSTABLE SMD) ↳ ADJUSTABLE THROUGH-HOLE ↳ FIXED THROUGH-HOLE USB-C SERIAL ADAPTOR (BLACK) AUTOMATIC LQ METER MAIN AUTOMATIC LQ METER FRONT PANEL (BLACK) 180-230V DC MOTOR SPEED CONTROLLER STYLOCLONE (CASE VERSION) ↳ STANDALONE VERSION DUAL MINI LED DICE (THROUGH-HOLE LEDs) ↳ SMD LEDs GUITAR PICKGUARD (FENDER JAZZ BASS) ↳ J&D T-STYLE BASS ↳ MUSIC MAN STINGRAY BASS ↳ FENDER TELECASTER COMPACT OLED CLOCK & TIMER USB MIXED-SIGNAL LOGIC ANALYSER (PicoMSA) DISCRETE IDEAL BRIDGE RECTIFIER (TH) ↳ SMD VERSION MICROMITE EXPLORE-40 (BLUE) PICO BACKPACK AUDIO BREAKOUT (with conns.) 8-CHANNEL LEARNING IR REMOTE (BLUE) 3D PRINTER FILAMENT DRYER DUAL-RAIL LOAD PROTECTOR VARIABLE SPEED DRIVE Mk2 (BLACK) FLEXIDICE (RED, PAIR OF PCBs) SURF SOUND SIMULATOR (BLUE) DATE NOV23 NOV23 NOV23 NOV23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 JAN24 JAN24 JAN24 JAN24 FEB24 FEB24 FEB24 FEB24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 APR24 APR24 APR24 MAY24 MAY24 MAY24 JUN24 JUN24 JUN24 JUN24 JUN24 JUL24 JUL24 JUL24 AUG24 AUG24 AUG24 AUG24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 OCT24 OCT24 OCT24 OCT24 OCT24 NOV24 NOV24 NOV24 PCB CODE 04107231 10111231 SC6868 SC6866 01111221 01111222 01111223 10109231 10109232 10109233 18101231 18101241 18101242 18101243 18101244 18101245 18101246 19101241 19101242 07111231 07111232 01110231 01110232 09101241 09101242 16102241 16102242 07112231 07112232 07112233 16103241 SC6903 SC6904 08101241 08104241 07102241 04104241 04112231 10104241 SC6963 08106241 08106242 08106243 24106241 CSE240203A CSE240204A 11104241 23106241 23106242 08103241 08103242 23109241 23109242 23109243 23109244 19101231 04109241 18108241 18108242 07106241 07101222 15108241 28110241 18109241 11111241 08107241/2 01111241 Price $5.00 $2.50 $2.50 $5.00 $5.00 $5.00 $3.00 $5.00 $2.50 $2.50 $5.00 $2.00 $2.00 $2.00 $1.00 $3.00 $5.00 $12.50 $7.50 $2.50 $2.50 $7.50 $7.50 $5.00 $2.50 $5.00 $2.50 $5.00 $2.50 $2.50 $20.00 $20.00 $7.50 $15.00 $10.00 $5.00 $10.00 $2.50 $5.00 $10.00 $2.50 $2.50 $2.50 $2.50 $5.00 $5.00 $15.00 $10.00 $12.50 $2.50 $2.50 $10.00 $10.00 $10.00 $5.00 $5.00 $7.50 $5.00 $2.50 $2.50 $2.50 $7.50 $7.50 $5.00 $15.00 $5.00 $10.00 COMPACT HIFI HEADPHONE AMP (BLUE) CAPACITOR DISCHARGER PICO COMPUTER ↳ FRONT PANEL (BLACK) ↳ PWM AUDIO MODULE DEC24 DEC24 DEC24 DEC24 DEC24 01103241 9047-01 07112234 07112235 07112238 $7.50 $5.00 $5.00 $2.50 $2.50 NEW PCBs We also sell the Silicon Chip PDFs on USB, RTV&H USB, Vintage Radio USB and more at siliconchip.com.au/Shop/3 SERVICEMAN’S LOG All washed up Dave Thompson It’s always difficult when an appliance in the household breaks down because we are obliged to at least take a look, even if we have no idea what we are doing. It’s the Serviceman’s Curse, and I’m guessing we all suffer from it to some extent. So of course, when our washing machine chucked a wobbly, I felt that I had to troubleshoot it, to see if I could do anything before calling in an actual (and likely really expensive) repair person to have a proper look at it. I’m sure you are all aware that I am not an expert on washing machines. I mean, I know basically how they work: water, soap of some type, agitation, rinse, repeat and then they shake themselves all over the floor while they turn into a huge salad spinner. The rest is timers, valves, sensors, solenoids and motors; how many of each usually depends on the complexity of the individual machine. Ye olde clothes washer I’ll always have an image of my mother standing in our laundry in the 1960s, working on what was then a pretty modern machine. I can’t recall the brand exactly, but it was likely a Fisher & Paykel wringer type, popular at the time. It was basically a big round basin with a motor underneath and a spindle in the middle, or, and I’m going out on a limb here, an ‘agitator’. The agitator went backwards and forwards and shredded the clothes and sheets, or whatever was in it. A big lever on the side engaged the thrasher and meant you could load it and get everything ready before kicking it into gear. Mounted on top was a fearsome ‘wringer’: two rollers that were driven by the motor and engaged by that lever on the side, which disengaged the agitator. Those rollers could be tensioned with a large, cast-iron knob at the top. A shaped drain at the bottom of the rollers guided any water wrung from the washing back into the main tub. The whole wringer assembly also had a safety mechanism built into it so that when dumb kids like my brother put his hand in there, it would pop open. That would release the downward pressure on the rollers, allowing him to pull his hand or arm back out. As a system, it was a simple and actually brilliant design. The machine worked well, aside from ruining just about anything washed in it finer than denim. Still, this pressure-­ sensitive wringer safety system used to trip all the time with mum just putting things like sheets or heavier wet fabrics through it. So in the end, she cranked that thing down so tight that it wouldn’t trip at all. The obvious concern is that your arm would come out like a pancake if you were silly enough to get it caught in there. And going by anecdotal evidence at the time, plenty of people did! I was always too afraid to get anywhere near it, but mum was braver and fed the clothes and linens through it, with her fingers ending up dangerously close to the rollers! I suppose she knew what she was doing, having worked the thing every other day for a long time. Of course, things have moved on a lot from those days. Aside from the Hoover-matic style twin-tub horrors of the 1970s, most subsequent machines have been more efficient and more reliable. Indeed, some appear to have lasted forever, if the washers I came across in flats I rented were anything to go by! Not to be sexist, but housewives of the day wouldn’t put up with something that didn’t work properly or would make their lives harder, preferring to utilise tools that made their lot easier. Front vs top loaders We now use a front-loading style of washing machine in our household. These use a lot less water than top loaders and are generally more efficient with power and soap use. Most European countries use front loaders, and as my wife is from Europe, we went with what she knows. Happy wife, happy life! The reason for this in Europe is water usage (households there pay for the water they use, something most cities in New Zealand do not do). Moreover, there’s the simple and practical fact that many people living in apartments have the washing machine in the kitchen, and it sits under a counter or bench like a dishwasher, so top loading is not typically an option there. Our machine is from a well-known Korean manufacturer of home electronics, smartphones and whiteware. It 88 Silicon Chip Australia's electronics magazine siliconchip.com.au is the second model we’ve had from that maker in the past 25 years, so I’d call that reliable (although I’d like to think mum’s first F&P is likely still going somewhere!). The first machine simply wore out, and as is typical, was too expensive to replace all the bits that needed replacing – at least, those that were still available. So, we invested in a flash new model with a few other bells and whistles on it, not that it turns out we use any of them anyway (marketing works!). We basically use the same program for everything we do. I didn’t know we had a pool This has been a very good machine too. Except one morning, I got up to go and hang out the washing we’d done the night before, and there was a swimming pool in the laundry. Great! Just what I needed. I also could not open the door of the washer, an electronically applied safety feature designed to prevent, well, swimming pools on the laundry floor. Usually, the last thing you want with a front loader half full of water is to open the door! This is one of the few advantages of a top loader, in my opinion; you can stop it mid-cycle, open the top and do what you need to do without a disaster in the laundry. My first thought for the water egress is that the big, circular door seal had gone. These seals work like an oil seal in the gearbox of a car (or the diff, you choose). The pressure and weight of any water behind it ensures a decent seal on the surface, in this case the glass front window, which of course is opened and closed all the time to load and unload the washer. That opening and closing can wear it out. Any grit or anything else that gets through the filters can damage the fragile seal as well. Usually, the door will not open until the pump has evacuated all the water, and only then does the door lock deactivate. In the old days this was a simple mechanical lock, but now, of course, is a much more complex electronic type of arrangement utilising sensors and solenoids and likely smoke, mirrors and ball bearings, knowing modern designers. Maintaining that seal on the door glass is likely where millions of research dollars were spent (and probably more than a few buckets and mops!). They do seem to have gotten the hang of it though, as our machines have never leaked from there. siliconchip.com.au But leak this one has. And cleaning up water spills is time-consuming; there’s also the potential damage to all the cabinetry in the laundry to worry about. The Weet-Bix wood this stuff is made from is like a sponge and will soak up any moisture it comes into contact with, despite the rock-hard melamine laminate on the outside. Once the lake was dealt with, I turned my attention to the machine itself. The water didn’t seem to have come from the door – there were no tell-tale watermarks. In fact, the whole cabinet appeared to be stain-free. My amateur washing-­ machine repair brain thought the obvious: a clogged filter? Fortunately, this is something the owner can usually fix without too much difficulty, with panels provided for access to the filter(s). The problem here was that the panel was very tightly fitted, and I had to prudently use a spudger type tool to pop it clear. Once off, I could clearly see the filter assembly and it was dry all around it, so I guessed the water didn’t come from there. I still opened it, took the filter out and cleaned it. We have three cats (I know) and though short-haired, they still drop a large amount of hair. Relentless vacuuming and sweeping typically keeps it at bay, but clothes love gathering hair, and that is what filters are for in washing machines (well, that and the tissues or bits of paper you might have forgotten in pockets). There was surprisingly little detritus in the filter, but I cleaned it out anyway and replaced it. The cover popped back in with a satisfying click, and I could see there was no chance of it falling off even with the most unbalanced of washing loads. So, not the filter then. This was fast becoming something beyond my scope of abilities. Time to bite the projectile and get in someone who actually knows what they are doing. Getting some help The next challenge was finding a repair provider who knows these machines. Several I called told me they weren’t familiar with them, which is madness because the brand is one of the most famous in the world. I guess the manufacturer might have a watertight (har!) service policy with repair agents, a la Apple and their guys, but I got the impression that these people just hadn’t worked on one of these types before; Australia's electronics magazine December 2024  89 Items Covered This Month • Treading (un)familiar water • Faults take many shapes and sounds • A rattling fan bearing • Repairing an automatic HDMI switch for PVRs Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Cartoonist – Louis Decrevel Website: loueee.com not an ideal situation. I finally did get hold of one outfit that was an official repair agent for these machines, and he said he had a good idea as to what might have caused this problem. I wasn’t going to be presumptuous and ask him what that might be – even I respect a serviceman’s right to make a living! The fact is, I was happy to pay for someone who knew what they were doing to come and fix it. It seemed to me the water was coming from inside somewhere, and I wasn’t about to break open the sides and covers just to find out I couldn’t fix it anyway. So, I made a time for this guy – which exposes another annoying aspect of this type of repair. Afternoon or morning? I mean, when I had vans on the road I would nail a time down on the phone long before we got there, and unless something like a natural disaster happened, we’d be there at that time. These people seem to operate in half-days. You’d think that after years of repairing whiteware, they’d have a better idea of time management. This would mean I’d have to sit around waiting for the guy to turn up any time between 1pm and 5:30pm. Being self-employed, taking time off work is not a huge deal, but imagine if I had to come back from my job and take time off just to accommodate a repair guy? That doesn’t seem right to me. These days, they can always text when they are on their way, but it is still disruptive and generally why I dislike relying on third parties. Still, he arrived with his toolkit and started with the usual troubleshooting procedures. He also asked what detergents and additives we used. I showed him the liquid soap and softener we used in the machine. We’ve used them since day one, so I couldn’t really see the relevance as far as the soap goes, but we’ll come to that later. I was more concerned about a sensor that wasn’t detecting water levels or a valve that wasn’t closing properly, something along those lines anyway. Well, this guy seemed to know what he was doing, so I left him to it. I know how annoying a hovering customer can be! He pulled the machine out (easy enough to do on the two wheels in the front) and whipped the case off with practised ease. I was keeping a little look-out, but it all looked alien to me inside it. A simple resolution After having a good look around, he dismantled the soap dispenser and discovered it was completely gooped up. I know this is a technical term, so bear with me. It seems that some detergents and washing powders don’t break down so easily with the water in our pipes and the residue builds up over time, causing problems. The guy showed me the soap tray. It’s just a plastic thing with many holes in it, and usually the soap and water mixture drains through it. This one, however, was coated in a thick slime of residue. The holes were blocked and, of course, nothing could get through them. The water fills up as part of the wash cycle and should drain through this assembly. However, if the holes are blocked, it just fills up and pours over the top, then down through the appliance until it floods the floor. Because this fills up with water several times a cycle, and it has nowhere to go, it just pours over the top of the dispenser and ends up on the floor of the laundry. So that at least explains why we had a lake in the laundry. Then what is the answer to this problem? Different detergents? Removing and flushing out the dispenser regularly? You would think that the manufacturer would be well aware of these problems, but there are no alerts or advisories, no cautionary tales on their social media. I guess it all comes down to the different water in certain countries, whether it is ‘hard’ or ‘soft’ and how it reacts with the various products and soaps that are available. At the end of the day, it was a simple fix: clean out the gunk. However, when I was looking at the machine, I was thinking all manner of potential problems; the Serviceman’s Curse was at play again. Overthinking it is typical for me. The thing is I don’t really know how these things work, and blocked drain holes in the water/soap dispenser is not really a logical thing for me to think of if the machine starts leaking. The repair guy knew (I think) pretty much right away what was likely to be wrong. He basically told us to avoid a certain brand of fabric softener that is known (in this machine in this country at least) to cause problems. If it doesn’t break down completely in water, it can’t be all that good for clothes anyway, which is something else he hinted at. I guess we won’t be using it any longer. A good enough ‘fix’ then, and well worth getting a professional serviceman in – I would have never known of this problem unless he’d told me. Faults have many causes I recently repaired a studio monitor speaker. These are fairly common items for home studios and often have a 6-inch (15cm) or 8-inch (20cm) woofer and a dome tweeter. The more expensive units are bi-amped, meaning they have separate amplifiers for the two drivers. Often, the amplifiers 90 Silicon Chip Australia's electronics magazine siliconchip.com.au are built around power amplifier ICs to simplify construction, as was the case with this unit. The fault was no sound, but the power indicator was lit. Given that it has separate amplifiers for the woofer and tweeter, it seemed unlikely the fault was there. However, they are usually connected to a common mute circuit, so the fault could be there, or in the preamp or power supply. The first thing I noticed on opening it up was glue covering a lot of the circuit board. This is put there by the manufacturer to secure the large capacitors to the board. I checked the +15V and -15V supplies to the preamp and discovered the +15V rail was slightly negative. So there was a problem with the series regulator feeding this rail. A quick check showed the resistor feeding the base of the regulator transistor was open-circuit. Pulling it from the board revealed why. The underside of the resistor was covered in glue, which limited its ability to dissipate any heat. The resistor was a 0.5W type, and a quick calculation showed it was dissipating about 0.3W. I replaced it with a 1W type and stood it up off the board a little to help get some air around it. I also replaced the resistor doing the same function in the -15V regulator, the underside of which was also covered in glue. I removed as much of the glue from the board as I could while doing so. In other units, I have found the same glue, which starts life as a honey colour, has turned dark brown or black with heat and becomes conductive and corrosive. In some cases, the glue gets into the through-hole vias on double-sided boards and corrodes the connection. Such faults are not easy to find. In this case, just two replacement resistors restored normal operation. My next repair relates to the Styloclone project in the August 2024 issue (siliconchip.au/Article/16415). I was aware of the original Stylophone as I owned one; it was a horrible beast. The sound it produced was basically square waves and the noise becomes grating after a short while. I note the Styloclone has a capacitor filter to help with this. The unit pictured here is called “Wasp” and was made in the late 1970s by an English company called Electronic Dream Plant (EDP). I have been servicing music electronics for more than 50 years, and I have never seen one of these before. This unit does not use a stylus; instead, the keys react to touch. I thought it was somewhat of a toy at first, but once I got it going, I found it to be a capable synthesizer with nice tone. The customer helped me find the first fault as he said there was a wire off inside and he was right. The filter section has a rotary switch to select between HPF, LPF or Band Pass Filter and this switch had worked loose, allowing it to rotate and break wires off. Only one wire was broken, which should have been connected to the wiper of the switch. With the wire repaired and all the switches and pots tightened to the board, we now had sound. The controls were all noisy; a small squirt of switch cleaner in each fixed that. All was looking good until I tested all the functions and found the Filter Envelope Generator was not working. To my surprise, an internet search located a service manual. The circuit diagram was hand-drawn, difficult to read and rather complicated. Further on in the manual, parts of the diagram had been redrawn nicely, including the part I was interested in. siliconchip.com.au The studio monitor PCB with glue on the capacitors. An internal shot of the “Wasp”. Australia's electronics magazine December 2024  91 The Envelope Generator uses three ICs to do its job: a 4013 flip-flop, a 4016 quad CMOS switch and an LM3900 Norton (current feedback) op amp. They are not commonly used anymore. I was aware of them but had to look up a data sheet to refresh my memory how they work. A few measurements allowed me to determine the LM3900 was the culprit. I remember having some in stock decades ago but figured they are probably obsolete. A deep dive into several parts boxes uncovered a bag with one left in it. This solved the problem and this instrument was ready to go. The customer also had the matching sequencer for this, called The Spider, which was dead. I found a faulty 5V regulator, but it still would not work properly after replacing it. I had to give up on that, as it appeared the RAM was faulty and was also obsolete. P. M., Christchurch, New Zealand. Fanning out troubles I have had a Vulcan Tangi fan heater since 1974. I bought it when I joined the RAAF as some of the base accommodation at the time was poorly heated. It has served me well over the years but has not seen continuous use. The fan motors in these are small shaded-pole induction types with bronze bearings, which tend to wear, causing the fan to rattle. I purchased a spare motor in the 70s, replacing the original in 1989. That motor developed the same rattle, but by 2020, spares had become impossible to find, so I just put up with the noise until I could find a replacement. As luck would have it, I was perusing some exhaust fans on display at my local Mitre 10, and one looked like it had an identical motor. It was worth a punt for the bargain price of $16. On removing the old motor, it was apparent that this was an exact replacement, except that the drive shaft was 2.5cm longer and the diameter was 2mm larger. Neither was a problem, as the fan connects to the motor by a grommeted hole that could accommodate the diameter. The extra length would be hidden within the tangential fan. This worked well, with the bearing rattle gone, but an occasional different rattle remained, and I could not determine the source. Then, one day recently, there was a bang. The fan stopped immediately and a burning smell emanated until the over-temperature cut-out operated. The selector switch has a couple of RFI suppression capacitors on it, and I suspected one had failed, but that didn’t explain the stopped fan. Dismantling the heater showed that a fan blade had separated and had stopped the fan while the capacitors were intact. I checked the internet for spares but none were available. There was a fan for a Westinghouse oven that looked identical, but the supplier did not return my requests for dimensions, and at $95, it would not be worthwhile. Could it be repaired? The fan was made of aluminium and was attached to the steel drive disc by mechanical straking. The broken blade had separated at the joint. It was a weak point and impossible to weld, so I tried some JB Weld epoxy resin around the blade. To keep the fan in balance, I applied some around the other blades at that end. A coat of sprayed black paint and the fan was more rigid than when new. After reassembling the heater, test runs showed no signs of rattling at all. This heater is nearly 50 years old and undoubtedly has The “Wasp” styloclone made by Electronic Dream Plant (EDP), this time with its case on. 92 Silicon Chip Australia's electronics magazine siliconchip.com.au exceeded the life that the designers at Vulcan intended. It is a testament to the quality of goods we made here in Australia at the time. R. W., Hadspen, Tasmania Automatic HDMI switch repair We have two personal video recorders (PVRs) in our lounge room. Each can record two channels at the same time. The main one is used for most of the recording and playback, while the second one is used for the odd occasion when there might be three programs on at the same time that we want to record. Several years ago, when I set everything up, I bought an automatic HDMI switch on eBay. It automatically switches the video and sound from the active HDMI cable input to the TV. So, when one PVR is on, the signal is routed to the TV automatically. This works well; the only time the switch needs attention is if the second PVR happens to turn on while using the first or vice-versa. In that case, it’s necessary to push the button on the switch to change back to the other PVR. This system worked well for several years until the automatic HDMI switch stopped working. At first, I was not sure why I was not getting a signal to the TV, but when I looked at the HDMI switch, I could see that the LED was not illuminated. I replaced it with a remote-controlled HDMI switch I’d picked up recently, but it was unreliable. So I decided to have a look at the automatic HDMI switch to see what was inside it. It comes apart easily by removing four screws on the back and lifting it apart. Then the circuit board just comes out, as it’s held in by both case halves. I was unsure if the main IC had failed, so I decided to test the two 100µF 10V electrolytic capacitors and sure enough, one had an ESR of 9.2W. I looked in my salvaged capacitors, but I could not find one that was tall and thin; I found a shorter, larger diameter one that I fitted to the board (the one in the lower-right corner of the board). Sure enough, this put the HDMI switch back into working order, so I could ditch the unreliable remote-controlled one. SC B. P., Dundathu, Qld. 500 POWER WATTS AMPLIFIER Produce big, clear sound with low noise and distortion with our massive 500W Amplifier. It's robust, includes load line protection and if you use two of them together, you can deliver 1000W into a single 8Ω loudspeaker! PARTS FOR BUILDING: 500W Amplifier PCB Set of hard-to-get parts SC6367 SC6019 $25 + postage $180 + postage SC6019 is a set of the critical parts needed to build one 500W Amplifier module (PCB sold separately; SC6367); see the parts list on the website for what’s included. Most other parts can be purchased from Jaycar or Altronics. Read the articles in the April – May 2022 issues of Silicon Chip: siliconchip.com.au/Series/380 siliconchip.com.au Australia's electronics magazine December 2024  93 Dallas Arbiter “Fuzz Face” Vintage Pedal T he Fuzz Face was first released by small British manufacturer Arbiter in 1966, using a very similar circuit to the ‘tone bender’ products on the market at the time. Manufacturers such as Vox and Sola Sound were already offering near-identical products, as can be seen by comparing Figs.1-3. The Fuzz Face therefore owes its popularity not to a novel design but rather to its uptake by prominent musicians of the era. Pete Townshend (The Who), Paul McCartney (The Beatles), and Hendrix are all known users. If you aren’t aware, distortion effects are widely used by guitar players (including bass guitar) to alter and enrich their sounds. They may be looking to create a unique sound for themselves, create different sounds from the same guitar in different sections of a piece, or just ‘beef up’ their sound with some extra harmonics. Soon after the release of the Fuzz Face, Arbiter was purchased by Dallas, who continued production as the “Dallas Arbiter Fuzz Face”. In 1993, American conglomerate Dunlop took over manufacturing, offering versions with either silicon or germanium transistors. The input stage All images have been reproduced with permission from Pre Rocked Pedals (www.prerockedpedals.com). The Fuzz Face has used many different transistors over the years. This example employs NKT275s, but it was not uncommon for early models to use AC128s or SFT363Es, all PNP germanium types. These substitutions were likely made for part availability reasons. In the era, it was more common for germanium transistors to be offered in PNP, in contrast to modern silicon transistors, which are more typically NPN. Both types can be made with both semiconductors, but NPN transistors require higher crystal purity and can be trickier to dope correctly, so in those early days, manufacturers preferred to stick with the easier-to-make PNP types. This circuit therefore has a positive ground, with a negative Vcc from the 9V battery power source. The guitar connects via a ¼-inch (6.35mm) input TRS (tip, ring, sleeve) jack. The signal is AC-coupled by the 2.2μF electrolytic capacitor before being applied to the base of PNP transistor Q1, which operates as a common-­ emitter voltage amplifier Australia's electronics magazine siliconchip.com.au Popularised by Jimi Hendrix, the Fuzz Face (from 1966) is considered by many the gold standard for foot pedal distortion effects. While it is a simple circuit, it is unusual by modern standards. The topology offers an insight into the compromises circuit designers had to make when working with early semiconductors. Vintage Electronics by Brandon Speedie 94 Silicon Chip with a 33kW collector load and no degeneration resistor. Many readers will note this is a poor choice for an input stage; the common-­ emitter configuration has a low input impedance, which will strongly load the relatively high output impedance of guitar pickups. Oddly, the Fuzz Face has gained a reputation for only sounding good when plugged directly into a guitar, not after other signal processing that might present a lower output impedance. This goes against conventional wisdom but is likely due to the interaction between the Fuzz Face and the guitar pickup resistance/reactance. As is typical in audio electronics, the ear is the litmus test. The decision to omit an emitter degeneration resistor is another ‘poor’ choice by modern standards. Adding one would raise the low input impedance mentioned previously but, more importantly, stabilise the stage against gain variations due to manufacturing differences and temperature changes, among other things. So why would the designer opt for such a crude topology? To understand this choice, we need to be aware of the limitations of early transistors. Germanium is directly under silicon in group IV of the periodic table and therefore shares many of the same properties (eg, both are semiconductors). However, as it is a larger atom, its outer shell is further from the nucleus and therefore not as tightly bonded. Thus, its electrons tend to break free more easily, increasing conductivity. Therefore, Germanium devices have lower forward voltages but are more ‘leaky’ than their modern silicon counterparts, meaning they are more prone to conduction without any base drive. This leakiness was exacerbated by manufacturing tolerances, which were not as tight as we might expect with a modern semiconductor fabricator. Lower purity of the feed stock and imperfections introduced in the manufacturing line contribute to additional charge carriers in the germanium, also increasing conductivity. These impurities serve to lower the effective gain of an amplifier built around a germanium transistor. The circuit designer is therefore compensating in this case, trying to maximise the available gain by omitting siliconchip.com.au Fig.1: the Fuzz Face circuit is deceptively simple, using just two PNP transistors (the types varied over the years of production) and a handful of passives to create an effective and popular adjustable distortion pedal. The distortion was created by a high gain combined with asymmetric limiting and clipping. Power is switched on when an input plug is inserted. Fig.2: the Vox Tone Bender circuit configuration is almost identical to the Fuzz Face, although many of the component values are different, as are the transistor types. Pressing S1 feeds the input signal straight to the output. Fig.3: the SolaSound Tone Bender again uses a virtually identical circuit to the Fuzz Face but with OC75 PNP germanium transistors this time. Some later pedals used NPN silicon transistors in a similar circuit, but they are not considered to sound as good. Australia's electronics magazine December 2024  95 the emitter degeneration resistor. This compromise makes the Fuzz Face sensitive to temperature changes and transistor hfe variations, which can differ significantly between devices. The AC128 data sheet lists an acceptable gain range of 55 to 175 for a newly manufactured device, an enormous variation of more than three to one. For this reason, Jimi Hendrix was known to purchase 10 Fuzz Faces at once and play each to determine the best one or two from the batch. He was experimentally determining which products had good transistors, with adequate gain and reasonable matching between the pairs. Their sound will also fluctuate due to ambient temperature changes – one of the many difficulties sound engineers and musicians faced back then. The output stage 96 Silicon Chip The transistors, capacitors and resistors were mounted on a small phenolic PCB. Much of the assembly work would have been in wiring up the stomp switch, sockets and potentiometers. The output signal of the first stage feeds directly into the base of Q2, another common-emitter amplifier, except this time, a 1kW potentiometer acts as the degeneration resistor. The wiper of this pot connects to ground via a 20μF bypass capacitor, which provides a low-impedance path for AC signals. This potentiometer is therefore the ‘fuzz’ control. With the control set at minimum, AC signals must pass through the degeneration resistor, providing a lower gain and less distortion. With the pot at the maximum setting, AC signals are fully bypassed, so gain and distortion are maximised. Negative feedback is applied to the input stage via the 100kW resistor, which also biases the DC operating point. Australia's electronics magazine siliconchip.com.au The reader may note that this voltage will be quite low and won’t bias Q1 fully into conduction. That is by design. Negative-going peaks from the guitar will be cut off earlier than their positive-going counterparts, providing an asymmetry to the distortion. This gives a more progressive effect, a characteristic musicians enjoy. The series combination of the 470W and 8.2kW resistors forms the collector load. Two resistors are used here to divide the output signal to obtain a more appropriate signal level. The output signal is AC-coupled through the 10nF capacitor and applied to a 500kW log taper volume potentiometer, another voltage divider to provide final control of the output signal level. Considered in its entirety, with the fuzz control at maximum, the circuit offers the highest gain configuration possible from a two-transistor solution, aside from the modest effect of the 100kW feedback resistor. Why early germanium transistors were mostly PNP types In the early days of semiconductor electronics, germanium was the material of choice for manufacturing transistors, predating silicon. PNP types were far more common among these early transistors than NPN types due to germanium’s inherent material properties and the era’s technological limitations. As a semiconductor, germanium has a higher hole mobility than electron mobility. That makes it easier to manufacture PNP transistors, where the current is primarily carried by holes moving from the p-type (positive) areas to the n-type (negative) area. In contrast, NPN transistors rely on electron mobility, which is less efficient in germanium. The doping process involves adding impurities to a semiconductor material to change its electrical properties, with the type of impurity determining whether the semiconductor becomes n-type or p-type. The dopants used to create the p-type material in germanium transistors were more readily available and easier to work with than those needed for n-type material. Elements such as indium and gallium, used for p-type doping, could be more easily incorporated into germanium during manufacturing. This was partly because the processes developed early on were optimised for the materials and dopants that were most accessible and well-understood at the time. Thus, in the early days, when manufacturing processes were less refined, PNP transistors offered better performance and were easier to produce with the available technology. Germanium transistors are more temperature-sensitive than their silicon counterparts, influencing operational stability. By virtue of their construction and the nature of germanium, PNP transistors had better temperature stability than NPN types in the early transistor designs. That made PNP germanium transistors better for applications where thermal stability mattered. Silicon, with its superior thermal stability, higher electron mobility, better resistance to environmental degradation and much greater abundance, became the preferred material for transistors. This shift was facilitated by improvements in manufacturing technologies that allowed for the efficient production of high-performance NPN transistors in silicon. Silicon transistor variants More recently, Dunlop offered the Fuzz Face with silicon transistors such as the BC108 or BC109. These are NPN devices, so the battery is swapped to a more conventional negative ground arrangement. While these more modern transistors have much more stable gain, their differing characteristics from germanium (mainly the higher Vbe of 0.7V compared to 0.3V for germanium) make for a fundamentally altered tone. These variants are known for their harsher and less progressive distortion and are not held in very high regard. A modern silicon version can be purchased for $200, much cheaper than the $10,000 (yes, ten thousand) early germanium versions fetch. Of course, many readers of this magazine will be more than capable of building one version for a fraction of that. If you can find the right vintage germanium transistors, you could easily make one with the ‘vintage sound’ for a small fraction of what a genuine SC early pedal costs! siliconchip.com.au ◀ The Fuzz Face case has an attractive shape that betrays its origins in the mid-1960s. Modern pedals generally come in ‘wedge’ shaped cases; this disc shape appears to be quite ergonomic but would probably be more expensive to manufacture. Australia's electronics magazine December 2024  97 Vintage Electronics by Don Peterson MicroBee 256TC Restoration This article documents my restoration of a nearly 40-year-old computer, a MicroBee 256TC. It was the last of the original MicroBee computers and incorporated several updates since the original kit version from 1982, including a faster processor, more RAM and a colour video display. I ran into significant challenges restoring it but I overcame most of them! T he MicroBee 256TC computer was released by Applied Technology (NSW) in 1987. It incorporated a Z80 8-bit processor running at 3.375MHz, 256KiB of RAM and onboard colour graphics. Like many computers of the era, it was built into a plastic case with the keyboard (see the lead photo). I still remember the excitement of assembling my original MicroBee kit back in 1982. I used it extensively over the next few years for both learning and fun. I still have that machine in working condition, so when I saw a rare 256TC on eBay, it appealed to me as an example of the other end of the MicroBee era. The unit was advertised as not working, apparently due to a power supply fault, but it looked in good condition otherwise. Importantly for this model, Photo 1: the 12V to 5V DC switching power supply. Both the tantalum capacitor (at lower left) and four-way connector were damaged. 98 Silicon Chip Australia's electronics magazine the RTC (real-time clock) battery was reported to be in good condition with no leakage. I didn’t think a power supply problem would present too much of a challenge to repair, so I bought it. The new machine arrived unscathed a few days later. The case was in good condition – slightly yellowed, but no more than you would expect for a computer of this vintage and with virtually no surface marks or damage. The keyboard appeared to be brand new. The case was held together with a variety of mismatched screws. Removing the lid revealed a pair of Chinon F-354L 3.5-inch floppy drives. One drive was floating freely, but the other was screwed to the aluminium mounting bracket as intended. Both drives looked fine and undamaged. Also attached to the drive mounting bracket was the standard 12V DC to 5V switching power supply, which looked to be in reasonable condition except that there was half a charred tantalum capacitor where a whole one siliconchip.com.au should have been (Photo 1). The fourway mainboard connector socket also appeared to have badly overheated at some point and was missing most of one contact internally. The keyboard wasn’t plugged in, so I lifted it away to reveal the rest of the mainboard. The board was a bit dusty, but all the bits appeared to be in the right places. The two EPROMs even boasted stylish custom duct-tape window covers (Photo 2). A close look at the battery area showed clear evidence of past leakage and widespread corrosion as a result (Photo 3). The battery leakage and associated fumes had a few different effects, the worst of which was the corrosion of PCB tracks and pads. The solder mask seems to have done a good job of generally protecting the tracks, but corrosion had definitely set in where the mask was missing around component pads or vias. Some tracks close to the battery area had also turned black and were siliconchip.com.au difficult to see, so the mask obviously hadn’t protected everything. Any exposed metal around the battery had also corroded, including component leads, IC sockets and the keyboard connector contacts. The white component silkscreen had detached from the PCB, and some of it seemed to have floated away, ending up in odd random places around the board. The identification labels on the top of many ICs around the battery had faded, some so badly as to be unreadable. The machine had some good points (case, keyboard, floppy drives), and all the major parts seemed to be there, but the mainboard and component damage did not look encouraging. Could it be repaired? Photo 2 (above): the MicroBee 256TC mainboard before any work had been done on it. Photo 3 (left): a close-up of the battery section of the above PCB. There was evidence of battery leakage and corrosion of the PCB tracks and pads. Australia's electronics magazine December 2024  99 Cleaning up the mess A few days later, I decided I might as well remove the battery and clean up the immediate area to see how bad the damage was. I also did some research online about how best to deal with the leaked NiCad electrolyte. Unfortunately, 50% of the hits said that the battery residue is alkaline and to neutralise it with a weak acid solution (eg, vinegar), while the other 50% said it’s acidic and to use a weak alkali instead (eg, bicarb). It can’t be both! In the end, I reasoned that since the NiCad electrolyte is an alkali (potassium hydroxide), the best course was to use a vinegar solution initially and then just run lots of plain water over the whole area to remove anything that was left, including the vinegar. To do that properly, I’d need to remove the components; otherwise, there would be no proper way to clean the PCB underneath. I removed all the socketed ICs to get them out of the way and checked them for damage at the same time. Most were located far enough away from the battery to be unscathed, including both PAL (programmable array logic) chips, which would have been tricky to replace. The keyboard microcontroller (M3870) and the Screen and Attribute RAMs (TMM2015BP) are located close to the battery and did show some surface corrosion on the adjacent pins. Still, their sockets had taken most of the damage, and the ICs themselves looked like they would probably be reusable. Sadly, the Colour RAM and RTC (146818) were not socketed, and since they were also closest to the battery, they were both write-offs. It seemed odd to me at the time that these two ICs would not be socketed, particularly when the other adjacent Screen and Attribute RAMs were, but it wasn’t until much later that I found the reason for that when it came back to bite me. Next, I removed the battery and some other nearby components I didn’t want to get wet. The board looked even worse with the battery gone. I decided on a 1:2 ratio of vinegar:water and used cotton buds dipped in that mix to liberally wipe over the whole area several times, removing as much of the visible battery residue as possible. I wore latex gloves for this part, as it was pretty messy, and 100 Silicon Chip I wanted to avoid contact with anything nasty. When the buds finally started coming away relatively clean, I ran lots of cold water over the affected area, using a small brush to get into all the nooks and crannies. Much of the silkscreen in the affected area wasn’t bonded to the PCB anymore and was simply washed away. This turned out to be a useful way of gauging how far across the board the damage had spread – whenever I reached an area of the board where the silkscreen wasn’t detached, I figured I was at the damage boundary and could stop. I then sat the board in the sun for a couple of hours to dry out. It was looking a bit better now, and I could see the extent of the track damage. The worst area was within a 50mm radius from the battery. Numerous tracks there had gone black, presumably indicating corrosion underneath the solder mask. Surprisingly, many of those tracks still measured OK, but who knows how long that would last. Other tracks measured as open circuits, and a bit of digging showed that most of those had corroded through at a component pad or via, ie, where the solder mask wasn’t present. There was also widespread corrosion of component leads in the same area, and at least one of the keyboard connectors was a lovely shade of green internally, an obvious write-off. When I eventually downed tools for the day, I was seriously thinking that this machine was beyond repair. I suspect I was visited by the Obsession Fairy again because by the next morning my mind was made up that this was now my very own MicroBee 256TC and I would fix it, no matter what! I set about removing all the damaged components, working outwards from the worst affected area. I’m lucky to have a good desoldering station, which usually makes this sort of job reasonably straightforward. However, I was finding that while most joints desoldered OK, there were always a few on each IC that would not desolder no matter how many times I tried. The problem seemed to be that I couldn’t get enough heat into the joint to fully melt the solder. This is a fourlayer PCB, and I suspect that the inner layers consist of large areas of copper for power distribution and/or shielding that sink much of the heat if you try to desolder a pin tied to either VCC or GND. I decided that I would have to cut all the pins from each IC, then heat each pin individually with the iron and pull it out using pliers rather than trying to desolder all the pins and extract each IC intact. This method worked better, but perseverance and even heating the pin alternately from each side was sometimes needed to extract the more difficult ones. Photo 4: after removing the ICs, I cleaned up the PCB using rags and methylated spirits. The job wasn’t finished but it was a good start. Australia's electronics magazine siliconchip.com.au Once each IC was removed, I could do a better job of cleaning the board underneath using a rag and metho (see Photo 4). I also decided to replace all of the original IC sockets, no matter where they were located on the board, as well as many of the connectors and all the unsealed variable resistors and capacitors, which may have had internal corrosion that I couldn’t see from the outside. The final task was to clear each hole of solder, ready for the installation of replacement components. The method I used for this was to stand the board vertically and apply heat to each hole from both sides simultaneously – the soldering iron on one side and the desoldering iron on the other. This conquered the heatsinking problem nicely, and after a couple of seconds, a quick flick of the desoldering pump trigger would generally be enough to clear even the most problematic hole. Photo 5 shows the final result, ready at last for the rebuild phase. Fixing the power supply The power supply board is a simple switch-mode design that takes an unregulated 12V input and delivers a regulated 5V output. A single four-way connector is used for both the input and output. Oddly, this connector is not polarised, and it is not obvious which way around it should go! Photo 6: I replaced the four-way connector on the power supply (shown in Photo 1) with a safer, polarised connector. It definitely needs to be connected the right way around, and I suspect that the blown capacitor and melted connector in this machine are the result of someone getting that wrong in the past. The PCB had a grey compressed fibre sheet glued to the solder side to insulate it from the mounting bracket. The glue was stuck quite well, but only in a couple of places, so it could be torn away without doing too much damage. The glue could also be lifted from the PCB with some persistence, and I cleaned up what was left with metho, leaving an undamaged PCB surface. When I eventually reattached the power supply to its bracket after restoration, I glued the fibre sheet to the bracket rather than the PCB, making any future work easier. The blown capacitor is a 10μF tantalum across the 12V input. There was no damage to the PCB from the failure, and the capacitor was easily replaced. I also replaced all the electrolytic capacitors at the same time. Axial electrolytics have become less common, so I used radial leaded units as replacements instead. I fitted the large 2200μF capacitor horizontally to keep it within the original profile and secured it to the board with a dollop of hot-melt glue. The large inductor (L1) is supposed to be glued to the PCB, but the original glue had failed, so I added a bit more hot-melt glue to re-secure it. The final restoration step for the power supply was replacing the melted mainboard connector and its wiring. I used the Jaycar HM3434, a beefy four-way connector with the same pin pitch as the original, rated at 7A. Importantly, this new connector is polarised (see Photo 6). I connected the board to a current-­ limited benchtop power supply and Photo 5: the board after cleaning off the corrosion, removing all components to be replaced and clearing solder out of the through-holes. Putting it back together was the next step. December 2024  101 Photo 7: a closeup showing the worst section of the board to repair. Many of the tracks were damaged and required replacement point-to-point wiring to fix. slowly increased the voltage to 12V. The unloaded output measured just over 5V, and there was no sign of smoke. I applied a decent load to the output and rechecked the voltage, confirming it was still measuring close to 5V. Next, I decided to have a go at powering up the mainboard for the first time, using my newly repaired supply board. Obviously it wouldn’t work with half of the components missing, but the undamaged section included a couple of clock circuits that looked like they should still operate. There was a small problem, though. The pins on my spiffy new power connector were slightly too large to fit into the PCB holes. This connector has pins with a square section; the answer was to use a small file to remove the Photo 8: the underside of the MicroBee 256TC mainboard. Multiple thin replacement wires were required due to damaged tracks on the top side. The photo insert below shows the ECO modification made later, but provides an example of how the replacement wiring is attached. 102 Silicon Chip corners from the PCB end of each pin, turning the square section into an octagon. The connector then slotted nicely into the PCB. I attached the supply board and, using my benchtop supply as the power source again, applied power to the Bee through the 5-pin DIN connector. I initially set the current limit low to ensure things wouldn’t get out of control and then gradually increased the voltage and current until a steady 5V was measured at VCC on the mainboard. The benchtop supply was delivering 200-300mA, which seemed reasonable considering how unpopulated the mainboard was. I used a DSO (digital storage oscilloscope) to check if the mainboard clocks were working and was very pleased to find a steady 13.5MHz for the system clock and 4MHz for the floppy controller clock. Mainboard repair The 256TC Technical Reference Manual has a component overlay diagram and a complete parts list. Where possible, I checked that the parts on the board were correct. There were a few variations – for example, some 74ALS157s on the board were supposed to be 74HC157s, according to the manual. Wherever there was a difference, I went with the installed part in preference to what the manual said. Most of the 256TC components are still readily available from mainstream electronics suppliers, but some were a bit more tricky to find. Ǻ Screen/Colour/Attribute SRAM: the original ICs were 2KiB TMM2015BP-10 types in 24-pin SDIP (skinny) packages, but the board will also accept 8KiB 28-pin SDIP ICs. Neither are available from mainstream suppliers, but the 2KiB units at least can still be found on eBay. I decided to use 28-pin IC sockets for future flexibility but ordered 2KiB HT6116 ICs from eBay, equivalent to the TMM2015BP. Ǻ RTC: the original IC is a type 146818 in 24-pin DIP, but a more modern Dallas DS12C887 would probably also work. I had already ordered a 146818 from eBay, so I decided to stick with that, but I picked up one of the Dallas modules to try later. Ǻ Keyboard connectors: there are two FFC (flat flexible cable) connectors on the mainboard, one 8-way and one 15-way. They seem largely obsolete in the 2.54mm pitch that the 256TC requires. While 8-way units are still available from mainstream suppliers, I couldn’t find the 15-way unit anywhere in reasonable quantities. 16-way devices are available, so I went with that, and it worked out well. The board can easily accommodate the slightly longer connector, and the metalwork for the 16th pin is easily removed by gripping the solder tail with pliers and pushing it back up into the connector housing, leaving an empty hole that’s easily visible after the connector has been installed. It’s then just a matter of ensuring that the keyboard cable is aligned to the correct end of the 16-way connector when inserted – ie, the end without the empty hole. The parts I used are Mouser Cat 571-5-520315-8 (8-way) and 571-6-520315-6 (16-way). I ordered most of the remaining parts from Mouser, but a handful of the more common parts I wanted quickly or had neglected to include in the order came from my local Jaycar store. Once the parts arrived, I started by replacing all the IC sockets I’d removed from the undamaged sections of the board. I added a new one for the optional sound IC (SN76489AN) that I plan to try one day, and a couple of 28-pin SDIP sockets for a future 16KiB to 32KiB PCG RAM expansion. PCG is the Programmable Character Generator that is used for displaying graphics and customised characters. I then moved on to the damaged area. The 256TC board has provision for many more bypass capacitors than were actually fitted out of the factory, and since I’d already cleared the solder from all the holes in the damaged area, I thought it wouldn’t hurt to just fit all of them as I went along. I used 100nF multi-layer ceramic capacitors for those but stuck with 10nF for the original factory-fitted bypass capacitors. I decided to use IC sockets for everything that needed replacing in the damaged section. I started work at the edges of the damaged area, gradually moving towards the centre, where I knew it would get harder. The edges were quite straightforward as most of the tracks were still in good condition; it was mainly just a matter of fitting the new components and moving on. As I approached the middle, I came across tracks that looked dodgy or tested as being open-circuit. For each of these, I ran a parallel replacement wire on the solder side and tested the connection. Photo 7 is a close-up of the worst section to repair. You can see the damaged tracks, meaning lots of replacement wires were needed on the other side, as shown in Photo 8. Pretty much every track in that area needed replacing and testing. It’s easy to make a mistake when running replacement wires like this, as you’re working with a mirror image of the component pads on the reverse side. The key I found is just to be methodical, work on only one component at a time, and double-check everything as you go. Probably only about ¼ of the black wires that you can see in Photo 8 replace tracks that actually tested as open circuit – the others correspond to tracks that tested OK but looked dodgy enough that I paralleled them anyway. The blue wires are re-implemented factory mods detailed in section 5.20 of the 256TC Technical Manual. Photo 9 is a top view of the completed board with all replacement components fitted. Troubleshooting At this point, I was busting to power up the machine again to see what happened. I didn’t expect it to work yet because there were just too many opportunities for missed track damage or wiring mistakes. Still, there was only one way to tell! I connected the screen, speaker, and power supply and switched it on. The result was a short beep and the display shown in Photo 10. This was obviously not right, but the display was pretty stable and much closer to working than I had expected. There was even a partly legible clock in the right place for a 256TC kernel boot screen. I was starting to think that this machine was going to live again. The next thing I did was break out the DSO and have a poke around. I Photo 9: the top side of the completed mainboard with all the replacement components fitted. December 2024  103 was hunting for any missing or odd-­ looking signals that might indicate open-­circuit tracks, missing replacement wires or misrouted wiring that might be joining pins and signals that weren’t meant to be joined. I had seen some bridged signals in other boards in the past, so I knew they would likely show up as distorted or superimposed waveforms that would hopefully stand out as being wrong. Unfortunately, after a couple of hours, I had largely drawn a blank. I hadn’t found any missing signals anywhere; while I did see a few odd-­ looking ones, I couldn’t trace any of them to a specific fault or wiring mistake. The main problem was that I didn’t have a working machine for comparison, so what looked odd to me might have been perfectly normal for a 256TC or vice versa. I needed a more structured approach than randomly poking a probe around the circuit and hoping that something would jump out at me. I noticed that there was a brief period during the power-on process Photos 10 & 11: the top screenshot shows the display when first powered on, while the lower image shows the screen after it was fixed. as output if I could get a program to run. A good way to load a program would be to burn a spare EPROM and install it in place of the standard kernel ROM. To paraphrase Red Dwarf, this was an excellent plan with only two minor flaws: I don’t have an EPROM programmer that can burn the type 27128 EPROMs the 256TC uses, and I didn’t have any 27128 EPROMs. Could I load the program from disk instead? The 256TC will automatically boot from a floppy disk during power-up if it finds one. So, if I wrote a small test program and put it on the first sector of a disk in place of the normal CP/M bootloader, my program ought to get loaded and run automatically at power-on, without needing any keyboard input. It was worth a try! First, though, I would need to calibrate the 2793 floppy controller. The 256TC presents the floppy alignment signals at a convenient six-way header (X8) next to the adjustment controls RV1, RV2 and CV1. The 2793 test jumper is also presented at X8, so I connected it to GND, got the DSO ready and switched it on, ready to start the adjustments. I could set RPW and WPW with no problems, but the 250KHz DIRC signal I needed to adjust with CV1 was missing entirely. The disk controller and associated support components are located in a largely undamaged part of the board, and the controller seemed to be getting all the right inputs, but the DIRC test signal simply refused to appear despite numerous resets and power cycles. Perhaps I had a broken 2793? I tried installing a known good controller to test this but got the same result. I came across the answer at www.pdp-11.nl/ homebrew/floppy/diskstartpage.html It seems that you must set the test jumper after applying power and after the 2793 has completed its internal initialisation. All I had to do was power off, disconnect the test jumper, power on, then reconnect the jumper, and the DIRG signal appeared as expected. The CV1 frequency adjustment was then straightforward; phew! I reinstalled the original disk controller chip and repeated the process without problems. I finally had a fully-­ calibrated and hopefully functional floppy controller. Next, I needed to write a bootloader program that I could use for the test. Australia's electronics magazine siliconchip.com.au 104 Silicon Chip when some of the graphics that form part of the normal 256TC kernel boot screen would display correctly, only to quickly disappear shortly afterwards and be replaced by the corrupted display. I had a couple of theories as to what might be causing that. It could be that the boot program was crashing, and the CPU was writing rubbish all over the place. Or perhaps the CPU was doing the right thing, but something was going wrong with the Screen, Attribute or PCG RAM. I figured that the CPU would be a good place to start, so I needed to load and execute a program I controlled to see if it ran correctly. If it didn’t, then that would mean I should concentrate my efforts on the processing parts of the circuit. The problem was that I didn’t have a working screen to write any output to, nor did I have a working keyboard yet, because the new FFC connectors hadn’t arrived. I had a working speaker; I could hear it beep quietly during poweron, so I could presumably use that What I came up with is shown in Listing 1: # Listing 1 – assembly # language test program ORG 00080h ROMDisplay: EQU 0E00Ch Start: LD SP,080h Sound: LD C,007h CALL ROMDisplay CALL BeepDelay CALL BeepDelay JR Sound BeepDelay: LD BC,0FFFFh BeepDelayLoop: DEC B JR NZ,BeepDelayLoop DEC C JR NZ,BeepDelayLoop RET All this program does is call the kernel ROM to produce a beep sound, wait a second or so, and repeat indefinitely. I used a HEX editor to paste the code into the first sector of a DS80 disk image and used it to boot a MicroBee emulator (ubee512). After a bit of debugging, I eventually had a working disk image. I then produced an HFE file from the same image. I loaded that into my GoTEK floppy emulator to simulate a real floppy drive with my custom bootloader disk mounted, ready for some physical machine testing. I tested this setup on a known-good machine first, then moved the GoTEK over to the 256TC, switched it on, and was rewarded with a nice steady heartbeat sound. Yay! The beeps were stable for as long as I could stand to leave it running, so things were looking good. This simple test actually proved quite a bit of functionality. The CPU, RAM (at least the part that contains my program), kernel ROM, PIO and disk controller were all OK. I still had a broken display and didn’t know whether the keyboard worked, but a large part of the machine was working fine. Screen resolution By now, I was quietly confident that the cause of the screen corruption was in the display handling part of the circuit. The video circuitry is concentrated in the area of the PCB that had taken the most battery damage. What I needed now was a way of siliconchip.com.au running some controlled tests of the various video functions so I could narrow the problem down. Ubsermon/ ubsertool is a toolset I’d used previously for automating software testing on real hardware; its core functionality is a MicroBee resident monitor program that is controlled and operated via a serial connection to a remote PC. The PC then acts as a serial terminal, providing keyboard input and screen output for the monitor part running on the machine under test. Ubsermon would allow me to run all sorts of tests easily; all I needed was a serial cable and a way of loading ubsermon into the 256TC. For that part, I needed to write a new bootloader, shown in Listing 2: # Listing 2 – # ubsermon bootloader ORG 00080h Start: LD SP,00080h LD DE,00001h LD HL,08000h-00080h LD BC,01400h CALL 0E039h JP 08000h This is a modified and cut-down version of the standard bootloader. It sets some parameters, then calls a kernel ROM routine (at 0xE039) that does all the hard work of actually reading the disk. Typically, the bootloader loads CP/M from the disk into RAM and then jumps to it, but I modified it to load and run ubsermon instead. The version of ubsermon I used runs from RAM location 0x8000 (0x means hexadecimal, so that’s 32768 in decimal). The bootloader simply reads the first 0x1400 bytes from disk and writes them to RAM starting at address 0x7F80 since the first 0x80 bytes are the bootloader itself. Once that’s done, we jump to 0x8000, the entry point for ubsermon. Next, I needed to create a disk image containing both the bootloader and ubsermon. For this, I started with a blank RAW DS80 disk image and used a hex editor to paste in the bootloader program and ubsermon. The RAW image was then easily converted into an HFE file for use with my GoTEK drive. I soon had my PC communicating with ubsermon on the 256TC and was ready to run some tests. I started with some simple read and write tests to the PCG RAM and found no problems – whatever I wrote could Australia's electronics magazine be read back unchanged. I also had the 256TC display output visible while doing this, and what was shown on the screen coincided with the data I was writing to the PCG. That was a big tick for PCG functionality. The Screen RAM test was a different story. I could read and write individual bytes OK, but sometimes, writing a single byte would cause two bytes to be modified. The target byte consistently wrote OK, but more often than not, another byte at a seemingly random place within the screen RAM map would also get updated. Reads didn’t seem to be a problem. As long as I didn’t actually write anything, the contents of the screen RAM remained stable. I then tried the same tests with Colour and Attribute RAM and found that Attribute RAM had precisely the same problems, but Colour RAM appeared to be working fine. Hmmm. I spent some time with the DSO examining signals associated with the Screen and Attribute RAM, looking for crossed address lines or other weirdness, but I didn’t find anything particularly wrong. I did see an odd-looking WE (write enable) signal on these chips – more on that later. While I was doing this, I was starting to recall something about a screen corruption problem being experienced with the MicroBee Premium Plus kit (PP+) that I had built a few years prior, and an ECO (engineering change order) being released at the time to deal with it. I had automatically applied that ECO when I built the kit, so I never saw what the problem looked like, but now I was wondering if it might be relevant to what was going on here. I dug out the ECO document to have a read. Apparently, there was a “timing problem in the combinatorial logic” associated with the faster RAM the PP+ uses; the penny was now starting to drop. I had fitted three new 2KiB SRAMs as part of the board repair, and these were 70ns parts (HT611670), compared with the original 100ns parts (TMM2015BP-10). Could that be the problem? Two of the original RAM chips were still in reasonable condition as they had been socketed, so I removed my new 6116s from the Screen and Attribute positions, fitted the old original chips and switched on. Bingo! A perfectly normal kernel boot screen appeared, as shown in Photo 12. December 2024  105 That left me with two questions. Firstly, what to do about the Colour RAM, which was apparently working OK with a 70ns part. Why should Colour be magically OK when the other two clearly were not? Just because I couldn’t trigger the Colour RAM to misbehave didn’t mean that there wasn’t some condition that would, and I didn’t have a 3rd serviceable 100ns RAM chip. Secondly, what was the underlying problem? I decided to try to work it out and hopefully get to the point where the machine would function with the faster SRAMs in all three positions. Scope 1 shows the odd-looking WE signal I mentioned earlier. The yellow trace is a WE signal for one of the video SRAMs during a single-byte write operation. All three SRAMs (Screen, Attribute and Colour) have a similar signal during writes. The blue trace is one of the SRAM address lines. Note that the first 100ns negative pulse is followed by a much shorter pulse. My theory is that this extra pulse is why faster SRAMs have a problem with writes – they are fast enough to react to that presumably unintentional pulse, while the slower RAMs are not. That could explain why an extra seemingly random byte gets updated. The WE signal for each of the video RAMs is generated by the Gold PAL (U52), and one of its inputs is the CO1 clock. PP+ ECO 20120714-1 (Rev 2) involves inserting a 1.5kW resistor into the CO1 clock line, which, in combination with the input capacitance of U52, causes the clock signal to the PAL to be delayed slightly. The 256TC is technically very similar to the Premium, except for the different keyboard, so I decided to try applying this ECO and see what happened. Scope 2 shows the same two signals after the ECO was applied. Note that the second WE pulse in the yellow trace has vanished. I removed the old 100ns parts, refitted the new 70ns SRAMs and switched it on. Success! There was no longer any display corruption. Applying ECO 20120714-1 (Rev 2) to the 256TC is relatively straightforward and just involves cutting a single track that runs to U52 pin 13 and inserting a 1.5kW resistor across the cut. I put the resistor inside heatshrink tubing to prevent accidental shorts against adjacent pads. A fly in the ointment Real life got in the way at this point, so it was several more months before I could return to finish this project. However, when I fired up the 256TC again, all was definitely not well. Instead of the colourful kernel boot screen that I had seen months earlier, now I just had a monochrome display filled with a mix of ASCII characters 0x00 and 0x02. The display was flickering rapidly and seemed to be cyclically redrawing itself several times a second; the machine wouldn’t do anything else. It wouldn’t boot from a floppy, either at power-up or following a manual reset. I had a quick look at all my track repair wiring on the back of the board to see if anything had come adrift, but it seemed OK. I also had a poke around with the DSO, but nothing stood out. Whatever was going on, there seemed Scope 1: the yellow trace shows the WE signal for one of the video SRAMs during a 1B write operation, while the blue trace is one of the SRAM address lines. 106 Silicon Chip to be no attempt to access the disk controller chip, so there was no chance of using ubsermon again to help with the debugging. I thought about it over the next couple of days and decided that, since it was displaying reasonably ordered screen content, it was probably starting to execute the kernel ROM OK. Somewhere in the ROM code, the CRT controller gets programmed, and that’s when the random screen RAM data at power-up would be replaced by the more ordered data I was seeing. My plan was to disassemble the ROM and start tracing through the code. I would compare what it said should be happening with what I was seeing on the screen or as signals in the circuit using the DSO. When I reached a part of the code that I couldn’t see working, that might give me a clue what the problem was and where to look. The code starts by setting what looks like a flag in high Screen RAM to 0xFF. It then performs some basic setup steps before starting to program the CRTC (cathode ray tube controller). I could see that this code was working, both by what I could see on the screen and by checking for a CRTC chip selection signal with the DSO. Next, it initialises the contents of the Colour RAM. This part looked to be working, too, because the content on the screen was all one colour, and I could see an active WE signal on the Colour RAM chip. It then moves code from ROM to RAM and calls another ROM routine to clear the screen. It looked like the screen might have been briefly cleared Scope 2: the same two signals shown in Scope 1, but after the ECO was applied (by cutting a single track and soldering a 1.5kW resistor along that cut). Australia's electronics magazine siliconchip.com.au as part of the cyclic flickering I could see. I could also see an active WE signal on the Screen RAM chip, so it seemed to be getting that far, at least. Next, it calls another ROM routine to initialise the Attribute RAM, and this is where it starts to get interesting. This routine fills the Attribute RAM with zeros, then overwrites a block of that with 0x02. This block is intended to point to a “256TC” PCG graphic that’s displayed towards the top right corner of the kernel boot screen. Two things were interesting about this part of the code. Firstly, the DSO was showing zero activity on the Attribute WE signal, so the RAM wasn’t getting written to, despite what the code said should happen. Secondly, the data this routine was trying to write was the text I could see on the screen, so it seemed it was actually writing this data to Screen RAM instead of Attribute RAM. Screen RAM and Attribute RAM occupy the same address space and are swapped by writing to the Video Memory Latch port (0x1C). So, it seemed something was going wrong with the Video Memory Latch; it wasn’t switching between Screen and Attribute RAM as it should. Returning from the Attribute RAM initialisation routine, the next significant action is to program the PIO. The DSO showed that the PIO was being selected, so I could only assume that part was working. Lastly, it checks the Screen RAM flag that was set at the start, and as long as it’s not zero, it attempts to boot from the floppy. Unfortunately, by this point, the flag has been set to zero by the malfunctioning Attribute RAM routine, so it skips over the floppy boot function. That explains why there was no attempt to access the disk controller chip. I stopped looking through the ROM code because I now had a good lead to follow; all the evidence pointed to a Video Memory Latch problem. Looking at the circuit diagram, CPU access to Attribute RAM data is via a bus transceiver (U84), enabled at pin 19. The DSO showed no activity on that pin, and tracing back through some logic gates showed that the LV4 signal was permanently low. LV4 is derived from pin 6 of U64, the Video Memory Latch, and is supposed to go high when bit 4 is set during a write to port 0x1C. So why wasn’t this happening? siliconchip.com.au Photo 12: the fully working computer showing off its colour display capabilities. The time and date need updating though! A rising signal on pin 11 of U64 triggers the latching of data, but looking at that pin with the DSO showed it to be permanently high, so no latching could occur. Tracing this signal back through an OR gate showed that pin 7 of U88 was permanently high. U88 is a 74HC138 used as a port decoder, and pin 7 is supposed to go low whenever port 0x1C is accessed. I could see other outputs from this chip going low in response to activity on other ports, but there was definitely nothing happening on pin 7. All the inputs to U88 seemed OK, so could U88 itself be the problem? I had not replaced U88 during the rebuild a few months ago, but it is located right on the border of the section of chips that did get replaced. Being an original meant that it was securely soldered in place (ie, no socket), so it was not easy to swap it. I did have a spare 74HC138, so I Australia's electronics magazine decided to set it up on a breadboard first and feed it with all the same input signals as the original to see what happened with pin 7. It took some fiddling to get all the signals hooked up, but eventually, I did. Pin 7 on the original was still stuck high, but pin 7 on the spare was behaving quite differently and regularly pulsing low in response to activity on port 0x1C. I think I must be an expert in removing chips from a 256TC PCB now, so it didn’t take long to remove the old chip, fit a shiny new 16-pin IC socket and insert my spare 74HC138. Success! The normal colourful 256TC kernel boot screen was back, and there were no problems booting from a floppy. I wonder if this machine is deliberately setting out to give me new challenges! Postscript I’m a fan of IC sockets and am still December 2024  107 glad I decided to use them as part of this repair, but there are a couple of consequences to that decision that I didn’t realise at the start. The first problem is that the 256TC power supply mounts underneath the floppy drive mounting bracket and sits very close to the main PCB when the machine is assembled. I’m sure the lack of clearance in this area is why the RTC and Colour RAM ICs didn’t have sockets fitted originally, whereas the Screen and Attribute RAMs, located right next door, did. The main problem is the power supply inductor, L1, which fouls against the side of a socketed U84 on the mainboard. I solved this by detaching L1 again and refitting it slightly further to the rear and as close as possible to the PCB. The second problem is that the drive mounting bracket ends up resting on top of the row of socketed ICs immediately to the rear of the keyboard connectors (U95, U82, U11 etc). This is less of a concern because it contacts the insulated top surface of the ICs rather than any pins, but I wasn’t happy to leave it that way because I thought it might eventually cause problems with those ICs or their sockets. My solution was to modify the drive bracket slightly. It is installed on top of a couple of plastic case posts at either end of the bracket. Putting a kink in the bracket at those two points causes it to be lifted a few millimetres higher and gives good clearance from all the underlying ICs. I suppose this solution is a bit agricultural, but it does the job and doesn’t cause any problems with the floppy drives or their presentation through the case openings. Floppy drives The twin floppy drives that came with the machine both looked to be in good condition on the surface, but unfortunately, I was not able to get either of them to work reliably. Drive #1 would read OK during testing using the top head but refused to read anything via the lower head. Looking closely at the problematic head showed what looked like a single fine hair on the surface, but no amount of cleaning would shift it. Thinking it might be a scratch instead, I gently ran my fingernail along the surface to see if I could feel anything, and it quickly became evident what the real problem was when part of the head came away, as shown in Photo 13. I have no idea what could have happened to cause this damage, but I was clearly wasting my time on this drive; nothing short of a head replacement was going to get it working again. Drive #2 also had a problem with read reliability. The bottom heads seemed to work OK under testing, but the top head would misread random sectors. This problem improved somewhat with cleaning, but not enough to be reliable. The problem may be related to the media I’m using (HD as opposed to DD), but the same media works OK in other machines. A future job might be to make one good drive from the pair, but for now, I’ve installed drive #2 as the B drive just to fill the hole in the case. On the other side, I have installed a new Photo 13 (above): the damaged floppy disk drive head. Photo 14 (right): a GoTEK floppy drive emulator was installed as drive A. 108 Silicon Chip Australia's electronics magazine GoTEK drive emulator as drive A, which works very nicely (see Photo 14). Assembly Assembling the 256TC is a bit of a jigsaw puzzle and can be a struggle if you don’t do everything in the correct order. I’ve found this technique works well: 1. Attach the rear panel to the mainboard using the D socket posts. Install the board/panel combination into the case base and insert all screws. Three screws attach the rear bracket to the case, and there are another two at the front of the mainboard. You need to support the thin edge of the case at the rear with one hand as those three screws go in. Tighten all screws. 2. Attach the power supply and floppy drives to the mounting bracket and plug in all drive cables. 3. Facing the front of the machine, hover the bracket roughly where it should go and plug the main power supply cable and both floppy drive power cables into the mainboard underneath. The 34-way floppy drive cable is best left unplugged for now. Lower the bracket into its usual resting place. 4. Lay the keyboard upside-down on top of the floppy drives with the cables facing forward. Run the keyboard cables down through the open slot between the mounting bracket and the power supply. 5. Lift the front edge of the mounting bracket/power supply and reach underneath to plug in the keyboard cables. Lower the mounting bracket again, then roll the keyboard forward so it is the right way up and in its proper place. 6. Plug in the 34-way floppy cable and put the case top in place. The bracket or keyboard location might need shifting slightly to get the top to fit correctly. Squeeze the whole package together at the sides and hold it together tightly while turning it upside down so that the screws can be installed. 7. Install all screws loosely, starting with the centre screws on each side that run through the drive mounting bracket. Check that everything stays aligned as each screw goes in, then tighten them all, working outwards from the two centre screws. 8. Turn the machine over and you’ve finished. SC siliconchip.com.au ASK SILICON CHIP Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line and we’ll answer your question. Send your email to silicon<at>siliconchip.com.au Long SSID support for WiFi Time Source Thank you for the prompt delivery of the Compact OLED Clock/Timer kit (September 2024 issue; siliconchip.au/ Article/16570). Construction is going well. The problem I have is the setting up of the Raspberry Pi Pico W. The programming went smoothly, but when I was entering my WiFi passphrase, I could only enter 48 characters. My passphrase is 63 characters long. I had a look at the source code, and found that only the first 33 characters are saved. So the changes I have made are as follows. In util.h, I changed #define SCAN_LEN (50) to #define SCAN_LEN (65) and added #define PASSLEN (70) after #define SSIDLEN (40). I also changed char savePASS[SAVE_ COUNT][SSIDLEN] to char savePASS[SAVE_COUNT][PASSLEN] and in util.c, on line 299, changed if(strlen(s)<33) to if(strlen(s)<64). I trust that these changes do not affect other parts of the code. Should that do the trick? Thank you for a wonderful magazine, keep up the good work. (A. L., Watsonia, Vic) ● Our understanding of the C SDK is that it (or its underlying libraries) only supports SSIDs and passphrases up to 32 characters long, so we are not sure if longer passphrases would work, even with modified code. We suggest simply changing SSID­ LEN and SCAN_LEN to 70 (and also modify the length check in util.c), rather than introducing a new #define. The SSIDLEN define is also used in the struct that stores the SSID/passphrase in flash memory, so we suspect the changes you are suggesting would end up corrupting or wrongly reading the flash memory. A.L. has reported back that our suggested changes have worked and he can now enter his 63-character passphrase. Question about using Ideal Bridge Rectifier Would any of the Ideal Rectifiers published recently be suitable for the old-fashioned choke input filter with capacitor for 25V at 20A in bridge configuration? (B. P., Toowoomba, Qld) ● Phil Prosser responds: I do not have a lot of experience with choke input power supplies, but can offer the following. I am assuming this configuration has a transformer with a single secondary winding and not a centre-tapped secondary. If my assumption is erroneous, then the recommendation changes. The IC version (December 2023; siliconchip.au/Article/16043) would be the appropriate design of the two, as it is intended for a full bridge rectifier on a single secondary winding. The discrete/dual rail design (September 2024; siliconchip.au/Article/ 16580) is designed for use with a centre-­tapped transformer and would not be the best choice in a single secondary rectifier application. The 20A current is substantial. If that is a continuous rating then serious consideration needs to be given to heatsinking and I would be looking to the TO220-device based designs. On the other hand, if that is a peak value, the surface-mount options might work. Still, you would want to be certain you have a solid understanding of the average current that would be drawn. I must admit to interest in what this power supply is doing! A sketch would be helpful; feel free to send something to us for further consideration. How does this LED flasher circuit work? Can you help me to understand the operation of this circuit? It often appears in various forms on the internet as a simple LED flashing circuit. It produces short flashes from the LED at around 2Hz. The pulse length is about 1.8ms and the LED current is limited by the 10W resistor. There is no resistor between the collector of NPN transistor T1 and the base of PNP transistor T2. WARNING! Silicon Chip magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of Silicon Chip magazine. Devices or circuits described in Silicon Chip may be covered by patents. Silicon Chip disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. Silicon Chip also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable. siliconchip.com.au Australia's electronics magazine December 2024  109 Breadboarding the circuit and placing a resistor between the collector and base with values as high as 1kW, the circuit continues to work. My first question is: why isn’t a resistor needed between the collector of T1 and the base of T2? Surely, when T1 is switched on, a short-circuit exists through the base emitter junction of T2 and collector to emitter of T1? When breadboarding this circuit, I noticed a variation on the internet with the PNP transistor’s emitter and collector reversed, ie, the PNP’s collector went to the +4.5V rail, meaning it was used backwards. To my surprise, the circuit worked as well as it worked before the reversal of the collector and emitter, except the pulse width of the flash was reduced to 250μs. I did not expect it to work in that configuration. Experimenting further by reversing the NPN’s emitter and collector, the circuit also works, but with a much dimmer LED flash. Can you or your readers explain what’s going on? (B. M., Minto, NSW) ● There’s no need for a resistor between T1’s collector and T2’s base because T1 has a finite current gain and its base current is limited by the 1MW resistor. So its collector will never be able to sink more than about 4.5V ÷ 1MW × hfe = 1mA or so (assuming an hfe of 200, which is about average). transistors designed to be used in both orientations that have reasonably high gain even if you swap the collector and emitter. Of course, this doesn’t work for very high voltages because the B-E junction reverse breakdown voltage is usually only about 7V. Test Tweezers cell accidentally reversed To put it another way, T1’s current sinking ability is limited when it has a relatively low base drive current, so there will be no short-circuit as such. If you look at circuits using compound transistors (eg, Sziklai pairs), they are often connected like this with no base resistor. The overall current gain is the product of the current gain of the two transistors, so provided the base current of the first transistor is suitably limited, the total collector current will remain modest. Bipolar transistors will usually work if the collector and emitter are reversed. After all, they are both P/N junctions of the same polarity. The resulting current gain is typically low (due to the difference in doping of the two junctions), but they will function. There are some special bipolar Extending Filament Dryer runtime I really enjoyed the first part of your article on the 3D Printer Filament Heater (October & November 2024; siliconchip.au/Series/428). However, I have problems with the filament being hygroscopic, and I need a heater which can remain on a medium temperature all the time as I would rather not have to wait eight hours or so before each print. Would it be possible to modify your design to do this? (R. T., Hove, UK) ● Phil Prosser responds: of course, it is possible to make the Filament Dryer run continuously, but the timer is an integral safety control for this system. A slight modification to the microcontroller code can make it run for 24 or 48 hours, which is still reasonable and does require you to check in on the dryer every day or two. This simply requires the period of the timer to be extended. A further tweak would be to allow yourself to press the start button at any time and ‘recharge’ the 24/48-hour period. Simply popping the following into the “DRYER_ STATE_COUNTING:” state will achieve that: if ( PortA_Read & Time_24h ) { Dryer_Data.Timer_Runtime = Time_24hr_Runtime; } else if ( PortA_Read & Time_48h ) { Dryer_Data.Timer_Runtime = Time_48hr_Runtime; } I recently built the Advanced Test Tweezers (February 2023; siliconchip. au/Series/396) and found soldering the SMD components quite challenging. However, I managed to complete the task and visually inspected it for bridges etc. Having established that all was well, I inserted the cell (unfortunately the wrong way round) and nothing seemed to happen. After putting the cell back with the correct polarity, it was still lifeless. I checked the supply voltage on the IC pins, and it was okay. Do you think I should replace the IC, or will I have to get a whole new kit? (G. F., Salamander Bay, NSW) ● We’ve had one or two other readers who have done the same thing, inserting the cell in reverse; in at least one of those cases, it didn’t seem to cause any damage, so we would be hopeful. The coin cell can only provide a modest current, and the two schottky diodes in the circuit would hopefully shunt most of it in this case. We suggest you try a fresh cell since the one that was reversed would have suffered an effective short circuit. We wonder if a soldering problem you missed is causing it to be lifeless. If you can email some clear photos of your construction, we can take a good look and make some suggestions. The OLED is about the only other component that could be easily damaged, but it is only powered when IC1 is active, so we doubt it would be damaged. PICs are pretty tough and have been known to survive worse abuse than this, although if any component was damaged, it would be IC1. Does the type of ferrite bead matter? If you choose to dispense with the timer safety control, there is a two-pin jumper header on the PCB labelled “No Micro”. If you put a jumper on this, the system will operate without the microcontroller. As a result, it will not ventilate, but the heater will run full-time. The safety controls for over-temperature and fan failure will still operate. We do not recommend that you use the system in this way, but it would have the effect you are asking for. I am building the Dual Hybrid Power Supply (February & March 2022 issues; siliconchip.au/Series/377) and have a question about the ferrite bead (FB12). There doesn’t seem to be any instruction on the type or how to install it. Australia's electronics magazine siliconchip.com.au 110 Silicon Chip MARKET CENTRE Advertise your product or services here in Silicon Chip KIT ASSEMBLY & REPAIR FOR SALE DAVE THOMPSON (the Serviceman from Silicon Chip) is available to help you with kit assembly, project troubleshooting, general electronics and custom design work. No job too small. Based in Christchurch, New Zealand, but service available Australia/NZ wide. Email dave<at>davethompson.co.nz LEDsales KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith: 0409 662 794 keith.rippon<at>gmail.com LEDS, BRAND NAME AND GENERIC LEDs, filament LEDs, LED drivers, heatsinks, power supplies, kits and modules, components, breadboards, hardware, magnets. Please visit www.ledsales.com.au PMD WAY offers (almost) everything for the electronics enthusiast – with full warranty, technical support and free delivery worldwide. Visit pmdway.com to get started. Do I just run a wire through the bead on the PCB? (B. L., Melbourne, Vic) ● Yes, you just run a piece of wire through the middle; see Fig.13 in part two of the article. It can be a component lead off-cut, a length of tinned copper wire, a piece of solid-core insulated wire – anything that fits should work. The type of ferrite bead is not particularly critical. There’s space to use a fairly long one (up to maybe 12mm). Altronics L4710A should work; L4810A might also fit. Jaycar LF1250 should also work, but in that case, we’d be tempted to thread the wire through two beads since they are fairly short. Still, one would likely be sufficient in this application. ● We mentioned using JLCPCB for 3D printing in the Pico Gamer project article (April 2024; siliconchip.au/ Article/16207). Some public libraries also offer 3D printing services. 3D printing service recommendations Adjusting 2.5GHz Freq. Counter to 10MHz I was impressed with the excellent finish you achieved on the case for the Shirt Pocket Oscillator kit I purchased recently (September 2020; siliconchip. au/Article/14563). Can I obtain the necessary printed items for the ‘plastic tub’ version of the 3D Printer Filament Dryer project from you? If not, can you recommend a 3D printing service? (J. A., Townsville, Qld) I have a question about the trimming of VC1 on the 2.5GHz Frequency Counter (December 2012 & January 2013; siliconchip.au/Series/21). I am feeding a 1PPS signal in on channel A, set in period mode for one second and have a reading of 1000003μs, which is close but not on the money. When I try to trim to the correct reading of 1000000μs, I can only increase siliconchip.com.au 1kHz sinewave generator circuit I need a 1kHz sinewave generator. Have you published one? (R. M., Melville, WA) ● The Roadies’ Test Oscillator project (June 2020 issue; siliconchip. au/Article/14466) can be set up as a 1kHz oscillator. Just replace the 6.8kW resistors with 15kW values for a 1kHz output. Australia's electronics magazine PCB PRODUCTION PCB MANUFACTURE: single to multilayer. Bare board tested. One-offs to any quantity. 48 hour service. Artwork design. Excellent prices. Check out our specials: www.ldelectronics.com.au Advertising in Market Centre Classified Ad Rates: $32.00 for up to 20 words (punctuation not charged) plus $1.20 for each additional word. Display ads in Market Centre start at $82.50 per column centimetre per insertion. All prices include GST. Closing date: 5 weeks prior to month of sale. To book, email the text to silicon<at> siliconchip.com.au and include your name, address & credit card details, or phone (02) 9939 3295. it to 1000010, so 1000003 is the lowest it goes to. Is this normal? I have read that replacing the 39pF ceramic capacitor with a 47pF type will help in trimming, but I think it’s odd that I only have 7μs adjustment in the first place. Do you have any ideas what’s going on here? (E. B., Meadow Springs, WA) ● A trimmer capacitor in a crystal oscillator circuit will always have a fairly limited adjustment range. We would try changing the value of the fixed 18pF capacitor in parallel with the trimcap; it likely needs to be a lower value, like 12pF, to achieve calibration. Different crystals will have a different adjustment range. It’s possibly yours is particularly narrow, in which case swapping it for a different type with more ‘pull’ might help. Unfortunately, there is often no pull figure given in the crystal data sheet. Replacement relay for Soft Starter In the Soft Starter for Power Tools (July 2012 issue; siliconchip.au/Article/ 601), you used a relay from element14 that is no longer available. It was rated at 16A with a 24V coil. Can you December 2024  111 recommend a replacement? (F. C., Maroubra, NSW) ● That is a common style of relay made by many manufacturers. We believe that Altronics S4199, which is currently available, is virtually identical. Jaycar’s SY4051 is similar but rated at 10A. That should be sufficient, given that it isn’t switching the full load current (the parallel thermistor carries some). There’s also element14 4228168, which looks to be compatible, although it is 5mm taller and doesn’t have the NC pin (which is not used in that project). We think it will still fit in the box despite the extra height. Advertising Index Altronics.................................29-32 Beware! The Loop......................... 8 Blackmagic Design....................... 7 Dave Thompson........................ 111 DigiKey Electronics....................... 3 Emona & RIGOL Contest.............. 9 Emona Instruments.................. IBC Jaycar............................. IFC, 55-58 Keith Rippon Kit Assembly....... 111 LD Electronics........................... 111 LEDsales................................... 111 Microchip Technology.............OBC Mouser Electronics....................... 4 OurPCB Australia.......................... 5 PCBWay....................................... 11 PMD Way................................... 111 SC Advanced Test Tweezers...... 53 Silicon Chip 500W Amp............ 93 Silicon Chip PDFs on USB......... 37 Silicon Chip Shop.................86-87 Silicon Chip Songbird................ 26 Silicon Chip Subscriptions........ 13 The Loudspeaker Kit.com.......... 10 Wagner Electronics..................... 12 Next Issue: the January 2025 issue is due on sale in newsagents by Monday, December 30th. Expect postal delivery of subscription copies in Australia between December 30th and January 13th. 112 Silicon Chip Improvements to Relay Selector circuit I’m an avid reader of Silicon Chip! Recently, I came across the Pushbutton Relay Selector circuit in the Circuit Notebook section of the January 2006 issue (siliconchip.au/Article/2537). Looking at the circuit and reading the text, the basic principle is the 4017 counter/decoder counts up and sequentially brings O1 through O9 high. It does that until it connects with the switch being pressed, which then stops the clock from being fed to CP0. I think there is a problem since each of these are sequentially cycled through. For example, pressing S5, before Q4 is fed a +5V signal, Q1 through Q3 will have been fed that voltage as well in sequence. It just will not have been routed to IC1c to stop the clock pulses. This means if any except the first button connected to O1 is pressed, all before it in sequence will be pulsed before settling on the pressed channel. If these are big relays, that will make quite a bit of chatter, but more significantly it will turn on potentially unintended channels, as would be the case in my application where line-level sources are to be selected. I have a fix for it in my application, which could be adapted to the author’s application as well. I am using latching relays and the driver IC has an ENABLE pin. This can be connected through a 4069 inverter to pin 10 of IC1c to disable the relays until the clock stops, at which point only one will be activated. A similar action can be achieved in the author’s example by connecting the positive end of all the relays to the collector of a power Darlington PNP like a TIP107. Its emitter would be tied to +12V, then its fed base via a 10kW resistor by pin 10 of IC1c. When that pin goes low after a selection is made and the clock stops, all relays are enabled. The approach used in this circuit is elegant and achieves many of the unique attributes of an interlocked mechanical push-button array at a much lower cost. Is there a reason this was not considered with the original circuit? (H. H., Chapel Hill, North Carolina, USA.) ● Your suggestions are certainly interesting variations on that circuit. We think the reason that they were not incorporated in the original circuit is Australia's electronics magazine that it probably cycles too fast for the relays to actuate. It looks like the cycle frequency is around 20kHz. That means each transistor will be on for around 50μs. A relay normally needs several milliseconds to actuate. Consider that the circuit uses small Mosfets to switch the relay coils with 100W series gate resistors. The resistors and gate capacitances (around 60pF each) will form a low-pass filter with a time constant of 6ns. By increasing the resistance and/ or adding capacitors from each gate to ground, you can increase the time constant enough that the Mosfets can’t switch on while the 4017 is cycling. It would have to stop to provide a long enough pulse to switch the Mosfet on. Increasing the resistors to 10kW and adding 100nF capacitors from the gates to ground will give a time constant of 1ms, which is far longer than the 50μs on-time during cycling, but short enough not to notice when you are purposefully activating a relay. Sourcing or substituting OPA2134PA op amps I’m trying to find a replacement for the OPA2134PA op amp that was used in the Studio Series Stereo Preamplifier design (October 2005; siliconchip. au/Article/3203). I have built the preamp but am unable to source the op amps. I am going to use it as a replacement for the Series 5000 preamp (still working) when it finally dies. Any suggestions would be appreciated. (V. P., McLoughlins Beach, Vic) ● OPA2134PA ICs are still available from multiple online retailers, and they appear to still be current devices. For example: • RS 285-8069 • DigiKey OPA2134PA-ND • Mouser 595-OPA2134PA There is another variant available, the OPA2134PAG4, which is essentially identical. We can’t think of any reason you couldn’t use NE5532s or LM833s instead, as we did in later designs with similar circuits. They have similar if not superior performance (eg, slightly lower noise). The main difference is that the OPA2134 is a FET-input op amp, which is important in some applications, but it won’t make much difference in the Studio Series Preamp. 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