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JANUARY 2025 ISSN 1030-2662 01 9 771030 266001 $ 00* NZ $1390 13 INC GST INC GST Compact HiFi Part 2 headphone Amplifier 🔊 🔊 🔊 1W into 16Ω 3.5mm & 6.5mm headphone jack 5MHz 40A Current Probe Class-AB operating mode 9-12V AC plugpack Data Centres, Servers and Cloud Computing Battery-Powered Monarch AA5 Radio Model Train www.jaycar.com.au Contents Vol.38, No.01 January 2025 12 Data Centres & Cloud Computing Whenever you watch a video, send an email, buy a product online etc you are using a data centre. So what makes them so important to how the internet is run, and where does the ‘cloud’ fit into it all? By Dr David Maddison, VK3DSM Computer technology Compact HiFi headphone Amplifier 42 Precision Electronics, Part 3 This series covers the basics of precision electronics design. Building on from last month, we move our shunt to the high side and use an instrumentation amplifier to measure the voltage across it. By Andrew Levido Electronic design 80 TCS230 Colour Sensor This little module senses the colour of any object or light source using an array of 64 photodiodes. It can be controlled via most microcontrollers like an Arduino or Micromite. By Jim Rowe Using electronic modules 84 Extracting Data from Micros Older microcontrollers that store code internally are a problem when repairing old equipment. We describe a method we developed to extract ROM (read-only memory) data from an older microcontroller. By Dr Hugo Holden Data preservation 27 Digital Capacitance Meter This Meter measures capacitors from 10pF to 10,000μF and it does it using only through-hole components. It’s portable too, fitting into a UB1 case and powered via a single 9V battery. By Stephen Denholm Test equipment project 33 Compact HiFi Headphone Amp Our new Headphone Amplifier is easy to build, fairly priced and delivers up to 0.9W into 8Ω, 1W into 16Ω and 140mW into 600Ω. In this final part, we go over the PCB design, construction, testing and then start using it. Part 2 by Nicholas Vinen Audio project 60 40A Current Probe Build your own current probe for a fraction of the cost of a commercial unit. Our 5MHz Current Probe is bi-directional, has an output scaling of 0.1V/A, a maximum current of 40A (35A continuous) and ~30 hours of battery life. By Andrew Levido Test equipment project 68 Battery-Powered Model Train By modifying your model train layouts to be battery-powered, you eliminate the need to keep the tracks clean. This is because it doesn’t need to draw power from the tracks that might be dirty or oxidised. By Les Kerr Model railway project Part 2: Page 33 Precision Electronics Part 3 – Page 42 Page 60 5MHz 40A Current probe 2 Editorial Viewpoint 4 Mailbag 39 Circuit Notebook 48 Jaycar Mini Projects 59 Subscriptions 67 Silicon Chip Kits 77 Online Shop 90 Serviceman’s Log 96 Vintage Radio 101 Ask Silicon Chip 103 Market Centre 104 Advertising Index 1. Current indicator for USB power banks 1. LED voltmeter & ammeter 2. LED handbag light 1. Gesture-controlled USB lamp 2. BIG LED clock Monarch “All-American Five” radio by Ian Batty 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: 9 Kendall Street, Granville NSW 2142 2 Silicon Chip Editorial Viewpoint As expected, the 3G shutdown was messy Australia is one of the first countries to shut down both 2G and 3G mobile services – others have not done it (yet) for good reasons. So many devices rely on the existence of either 2G or 3G. Most countries that have shut down 3G have at least kept 2G (GSM) as a backup. The few exceptions are Singapore (since the end of 2021) and most carriers in the USA no longer have 2G or 3G networks. Virtually all other countries retain one or the other for now. Some have discussed shutting them down in future but most don’t have a specific date yet. One exception is Japan, where major carriers plan to end 3G service by March 2026. Another reason to keep one or the other is that 4G coverage is generally not as good in remote areas, so it’s good to have 2G/3G to fall back on if you’re in an area with poor 4G coverage – something that would seem to be smart in a large country like Australia, with people living in remote areas. However, I guess our politicians are smarter than their peers overseas and can get away with doing this without consequences. (Yes, I’m being sarcastic.) That is bad enough, but it gets worse. 4G is not really a proper standard and many phones implement it (VoLTE) differently. That means that some 4G/5G phones fall back on 3G to make emergency calls. Without a 3G network, they are therefore unable to call 000. It gets worse again. The government’s “solution” to this is to force all the wireless carriers to block all phones from their network if they can’t be 100% sure they are able to call 000 without the 3G network. This has resulted in many phones being blocked that do support 4G and can call 000 simply because it’s so difficult to create a comprehensive list of all supported devices. Some of these blocked devices are relatively new 5G smartphones! The word ‘schemozzle’ is the most appropriate way to describe this situation. Over half a million active devices have been blocked from our networks due to this debacle – most of which are now basically e-waste. Possibly in excess of a million devices are affected, and that’s ignoring those that have already been replaced due to the then-impending 3G shutdown. Discarding millions of otherwise functional devices can’t be good for the environment (something our government pretends to care about). It’s also a big waste of money. Some of these devices cost upwards of a thousand dollars and were perfectly functional before they were made redundant. Some, like the 3G devices integrated into some vehicles, have no obvious upgrade path. A cynical person would say that the telcos must have lobbied for this situation because it now means that they essentially have a monopoly on selling mobile devices in Australia. After all, not only can they block ‘grey market’ phones, they are legally required to do so. Remember when the NBN came along and they got rid of regular telephone lines, forcing many people to switch to 3G for services like back-to-base house alarms, asset tracking and so on? That was before the advent of 4G, so people who were forced to replace those devices about 10 years ago are being (or have been) forced to replace them yet again. Who knows how long those replacement devices will last? Will the 4G network be switched off in the near future, forcing us to replace them all again? I wouldn’t rule it out. You can read more about this debacle at siliconchip.au/link/ac2r by Nicholas Vinen Cover background image: https://unsplash.com/photos/purple-and-blue-light-digital-wallpaper-8bghKxNU1j0 Australia's electronics magazine siliconchip.com.au siliconchip.com.au Australia's electronics magazine January 2025  3 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”. Silicon Chip magazine collection for sale Issues from 1987 to 2023 in excellent condition, the best offer will be accepted. Contact Geoff on 0406 071 544. Geoffrey Orr, Ryde, NSW. 50 years of Stereo FM radio in Australia 2024 is the 50th anniversary of Australian stereo FM broadcasting. A half-century ago, on the 15th of December 1974, the first FM station of this new radio service, 2MBS-FM, commenced broadcasting from makeshift studios in Alexander Street, Crows Nest, NSW. It has remained continually on air since then. It seems fitting that Silicon Chip should mention this 50th Anniversary of FM and perhaps provide its readers with some background information. The history of how Australia eventually managed to get FM broadcasting after most of the world had had the service for decades is long, colourful and troubled. It is really quite an interesting story, and a rather long one at that—but unfortunately much of that history remains scant and poorly documented in official archives. Those records provide precious little information about the key personalities involved. Much of the history covers how the government badly bungled spectrum management by putting TV channels 3, 4 and 5 in the international 88-108MHz FM band. I have first-hand knowledge of how we set the stage for the government to unwind this almighty stuff-up. Our private discussions with Senator (Diamond Jim) McClelland got the ball rolling for the then (Whitlam) Government to establish the FM Royal Commission (the McLean Commission). The commencement of 2MBS-FM represents a major milestone in Australian broadcasting history. It was the first station to broadcast continuously in stereo FM, but it also had to develop all of its original stereo encoding and transmitting equipment (including a 10kW transmitter) from scratch due to its impoverished beginnings, and it did so with volunteer labour. Those volunteers gave up many hundreds of hours of their spare time. It was a major electronics project undertaken by electronics and classical music enthusiasts, many of whom had little or no experience of electronics. 2MBS-FM was also the first new station in Sydney in about 40 years, and that hiatus itself is quite a story. In the days before the internet we, a bunch of amateurs (some hams, others in electronics and allied fields), had to start from scratch with only the specifications of the FM pilot-tone system to hand and develop a 20/25W stereo FM exciter from the ground up. It had to pass the high standards of the Australian Broadcasting Control Board (ABCB) with its rigorous testing procedures. It turned out that our efforts were quite spectacularly successful. It must be remembered that in those days, we had very little information about the technical intricacies of stereo FM. In effect, we were in a clean-room scenario Left-to-right: Sir Francis McLean (deceased), FM Royal Commissioner, ex BBC head of engineering; Grahame Wilson (yours truly) and Max Benyon. Photo taken whilst Sir Francis was inspecting the construction of our high-power bespoke, home-brew transmitter. 4 Silicon Chip Australia's electronics magazine The West Street, North Sydney Post Office tower originally constructed for the experimental monophonic FM service in the late 1940s. In the early days, it became the site for our own FM antenna. It has since been demolished. siliconchip.com.au FREE Download Now! Mac, Windows and Linux Edit and color correct using 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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NO SUBSCRIPTIONS • NO ADS • NO USER TRACKING • NO AI TRAINING and had to develop most of our circuitry from scratch. This was a blessing in disguise, as our first stereo FM exciter turned out to have quite remarkable performance; it was much better than most commercial designs on the market. Back then, it’s likely we had the best performing FM exciter in the world. That first stereo FM exciter still exists! To my knowledge, I am one of only two surviving FM campaigners who put the technical case for FM to the government; the other is Max Benyon. He was a very important player in the FM story. What happened behind the scene that clinched the deal, so to speak, has never been documented publicly. We ultimately achieved political success and solved some quite formidable technical problems to get on air. Grahame Wilson, Glebe, NSW. Modifying single-phase motors to work with a VSD In your “Single-Phase Induction Motors” panel on page 31 of the November 2024 issue, you state that motors with centrifugal start switches are not suitable for use with the Mk2 Variable Speed Drive design. I submitted a Circuit Notebook entry that was published in the February 2015 issue (siliconchip.au/Article/8303), which demonstrated that it is possible to use these motors on the Mk1 VSD. That is done by feeding the start winding from the third phase via the centrifugal switch. This phaseshifted drive provided excellent initial starting torque. Using this technique, I was able to run a pool pump at about 40% of its full speed power without significant impact on filtering efficiency. I think the pump was probably overpowered for the required duty, and the automatic pool cleaner ran better at the reduced power. I did not have to run my pump any longer each day – so I saved about 60% of the running costs. The pump also ran much, much cooler. My experience was that the pump would start reliably and ramp up and down very smoothly through the (large hysteresis) switching point under load, providing the full range of available ramp-rates. As a note of caution, I initially ran the pump at a low speed with a slow ramp-rate on the bench to ensure the pump ran in the correct direction. An abrupt start in the wrong direction could unscrew the impellor, with nasty consequences! Another note: fans and centrifugal water pumps require very little torque to run at low speeds, so with low ramprate speed increases, the inertia torque can also be easily limited. I had no problems at all running my pump below the centrifugal switch-off speed for extended periods, as the supplied voltage is reduced at these low speeds, so the current is quite modest (little heating of the windings). However, I think more caution would be needed with high starting torque devices, like compressors, conveyor belts etc, to avoid extended low-speed, high-torque operation. I have been running my Mk1 VSD-controlled pool pump twice a day for about nine years, only stopped when we decommissioned the pool recently for other reasons. The pump still runs. Now I have a question. I have a 3.3kW three-phase (440V) motor that I would like to control at reduced voltage. It lacks the ability to change from star to delta configuration. Would it likely run at about 1.5kW with the reduced 230V supply? Ian Thompson, Duncraig, WA. 6 Silicon Chip Andrew Levido comments: thanks for the email. You are obviously correct that it is possible to run a capacitor-start single-phase motor from the VSDs I have described in Silicon Chip. You can do it using the third phase as you describe, or by ramping the speed up above the switch opening RPM, then keeping it above the switch closing RPM thereafter. You can also replace the centrifugal switch with a timer that switches the start winding and capacitor out of circuit once the rotor is started. However, none of these methods is suitable for someone with limited experience and understanding. There are so many variations of motors and loads out there that we decided it was best to just say it should not be done. We figured that any sufficiently advanced user like you will know how to do it safely and accept the risks – some of which you point out. As for running a 440V 3.3kW motor on 230V, I honestly have no idea what shaft power you could expect. I’m sure the motor would run, but I couldn’t vouch for the current it would draw or the torque it would produce. The load’s torque-speed characteristic will also impact the operating point. Mains wiring colours worldwide It has been a long time since power cables that plug into Australian power points used red for Active, black for Neutral and green for Earth. The problem with this combination is that 8% of Australian males and 0.5% females are colour blind, virtually all having red/green blindness. So the likelihood of them wiring the Active to a metal case is high. This was the case before Residual Current Device breakers (RCDs) were common. They will switch off the supply when someone touches such a case (although that doesn’t guarantee no harm can occur!). The latest standard used by Australia is IEC60227 – 2024 from the International Electrotechnical Commission, part of the United Nation Standards. To overcome the problem of colour blindness, Active was changed to brown because it appears darker than the green used for Earth and the light blue used for Neutral. Virtually all colour-blind people can detect its hue (brown) compared to other hues. To make it unmistakable, the Earth is green with a yellow stripe. The yellow stripe is easily seen in low light. The Earth is most important because it must be correctly wired to provide a safety shield. In the Americas, green/yellow or green only are acceptable and Neutral is white. They usually have a two-phase 240V supply where 120V can be tapped between either the black and white or red and white wires. For higher power devices, 240V is available between the red and black wires. Our 400V, 50Hz three-phase supply is used in high-power and commercial/industrial applications. 60Hz supply countries also have 480V three-phase but it is only available to commercial and industrial customers. Alan Hughes, Hamersley, WA. Old electrical wiring standards I was recently reminded about the old electrical wiring standard here in Australia. Somewhere around 50 years ago when I lived in NSW, my neighbour’s son came over and asked me if I could help his father with an electrical problem. The light switch near their front door had broken, Australia's electronics magazine siliconchip.com.au “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! Schadenfreude over H. G. Palmer 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. Regarding the letter in the October 2024 issue from Ian Robertson about Stromberg-Carlson and H. G. Palmer’s part in their demise, I was pleased to hear that karma finally got Palmer and that he spent some time as a guest of H. M. My wife was arrested for non-payment of a non-existent debt to H. G. Palmer back in the late 1960s. She was taken to the police station and fingerprinted. Despite her protests that she had never bought anything from the company, she had to appear in court. They finally realised that she could not have been the person they wanted as she was far too young, and she was released. Of course, there was no apology, and the police record remains. She should have sued him, but she was so relieved to have it finished, she just let it go. She will be very pleased to hear that he got his comeuppance! David Coggins, Beachmere, Qld. Beware! The Loop has many twists and turns, facts and figures that inspires your imagination. Unexpected failure in ignition system 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. 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 and they had tried to replace it, but they ran into all sorts of problems. The light switch wiring consisted of three twin red & black cables. They had connected all the red wires together and all the black wires together; who knows how they had connected the switch. Of course, as soon as they switched the power back on, it blew the fuse. (There were no circuit breakers or ‘safety switches’ back in those days). They didn’t know what to do, so they asked for my help. The old standard for light wiring was twin red & black cable with no Earth wire. Power cables had a bare Earth wire. For light switches, black was used as the switch wire – very dangerous! I arrived on the scene and the three cables had been disconnected and were easily accessible. I asked for a light globe and said to switch the power back on. I connected the globe across all three cables and determined which was the live cable. Then I connected the other two cables in turn and asked them if the next light in the circuit was working. This identified the wiring to the other lights, meaning that the other cable was the switch wire going to the light. I got them to switch the power off again and I connected the new switch. With the power back on, everything worked as expected. Although this old wiring standard left a lot to be desired, at least it was nowhere near as dangerous as the cotton-­ covered rubber-insulated wiring of earlier eras. Even power cables back then used rubber insulation; I recently found such a cable when I was sorting out items that had been in storage for decades. After saving the plug and socket, I discarded the cable without a second thought. No way I would even think of using such a dangerous cable. Bruce Pierson, Dundathu, Qld. Silicon Chip E-ISBN 9780645945669 Around eight years ago, you provided me with a lot of assistance in resolving a few teething issues in configuring my Programmable Ignition module (March-May 2007; siliconchip.au/Series/56) for use with a Piranha optical trigger in my Rover P5 V8 coupe. David Parker, with whom I correspond on occasion and who also uses your programmable module on his MGB car, provided me with your email address as I wanted to offer a bit of feedback. Australia's electronics magazine siliconchip.com.au I have been running the module in my dual-fuel (LPG & petrol) Rover P5 Coupe reliably now for eight years. I’ve also written quite a few ignition maps for both fuel options. These changes have slowly improved the performance with each fuel type as I learn how the engine responds to the various changes made. Recently, on a three-day Car Club run returning home, the engine started to act erratically. To cut a long story short, it was misfiring badly. The electronic tacho’s needle would swing violently from whatever RPM the engine was initially running at and back or close to zero RPM. I managed to nurse the car to within 12-15km of home before the engine couldn’t sustain running any longer. An RACV flatbed truck ferried the Rover home from that point. The next day, I removed the module to investigate. I emailed David, who suggested a few possible reasons the module could have failed. David felt that it was probably due to a failed capacitor. I thought the microprocessor chip may have been corrupted in some way. As I had most of the parts in a spare kit, I could swap the chip for a fresh one and also look for evidence of a failed capacitor. David outlined what I should look for in a failed capacitor. In the end, I changed two of the larger capacitors and the chip with no effect. Then I looked at the crystal. It sits there on its own on the circuit board (a 20MHz oscillator). Perhaps the erratic behaviour of the electronic tacho was telling me something. As I had a spare crystal, I thought I’d change it. Bingo; that was the component that had failed. It was quite a surprise to David when I told him that the crystal was the component that failed in the module! He said that they are generally very reliable and not prone to failing. The next day I ordered a few spares as they are only 67¢ each. What are your views regarding the crystal failing? I’d be interested in any feedback. Vincent Stok, East Bentleigh, Vic. Comment: Crystals are generally very reliable. The crystal may have had an internal connection break or become high-impedance due to the constant vibration from the engine and bumps on the road. Then again, even though crystals are mostly reliable, that doesn’t mean they never fail. You might have just been unlucky. You may wish to mount the Programmable Ignition system on rubber mounts to reduce the amount of vibration reaching it. That should increase its overall reliability. AC Bench Supply desired I was recently working on repairing some 1.8m-tall Christmas drummer men. They contained LED lights and some of them also had motors to drive mechanical arms to hit the drum. The power supplies had been lost, and the connectors could not be found anywhere. Some of the internals were extremely difficult to access, and when we did, it looked like most of the internal circuitry had been burnt out. There were no circuit diagrams, so we had to reverse-engineer what everything was doing and what voltages it needed to run at. From this, we determined that the original power supplies were AC plugpacks, which are now no longer as common as they used to be (and much more expensive than the ubiquitous DC switch mode plugpacks now available). siliconchip.com.au Australia's electronics magazine January 2025  9 We had bench DC power supplies with voltage and current limiting; these are commonly available, and Silicon Chip has published such designs. However, we had no similar device for a variable AC power supply. Searching on the internet, I could not find anything that would do that job. This would be very useful for all sorts of devices with small AC motors. My request is for a project design for a bench AC variable power supply with current limiting, to fulfil a similar role to the commonly available DC bench power supplies. It could use a PIC-based waveform generator, perhaps just with sine, square and triangular wave forms at a small range of frequencies; perhaps 30Hz, 50Hz, 100Hz and 1kHz (50Hz being the most significant). I seem to remember that you already have something that might meet this requirement. That would then feed into a modified audio amplifier (perhaps Class-D), perhaps with a 50W to 100W output (whatever meets the price/performance curve). Again, you have already published designs for this, although the low frequency response may need to be tweaked. It would need appropriate variable output voltage and variable current limiting and metering. Or, is there a much simpler solution that I have not thought of? Andrew Hannam, Cornubia, Qld. Comment: all you really need to do is plug an AC plugpack into a variac with a multimeter to measure the current. AC plugpacks are still easily obtainable. For example, Altronics has 10 different models ranging from 9V 1.33A (M9233) to 24V 3A (M6014). Jaycar also have a few, like MP3026 (12V 1A), MP3032 (24V 1A) and MP3045 (24V 6.25A). We have previously published a design that can synthesise an AC waveform, although for a different purpose: the May 2016 Precision 230V/115V 50/60Hz Turntable Driver (siliconchip.au/Article/9930). It could be used as the basis of an AC ‘bench supply’, but it is a lot more complex than the variac approach. Audio amplifiers are not really suitable for driving general loads. For a start, they are usually designed to handle a narrow range of load impedances and won’t like driving highly capacitive or inductive loads. More on the nuclear power debate I notice that in his October 2024 letter in Mailbag, Phil Denniss is less optimistic than Kelvin Jones (July 2024) that nuclear power might be good for Australia. I tried to become better informed on the subject, but it wasn’t easy. Nuclear power is controversial partly because it isn’t one singular technology; it is a whole zoo of technologies, illustrating the many ways to ‘skin a cat’. A group that calls itself the World Nuclear Association has tried to be helpful by defining generations of technologies, a bit like 2G/3G/4G/5G phones. Their help leads to names for the technologies like generation I, II, III, IV etc, but even this naming provides a minefield of opportunities for pedants to quibble. Almost all the currently working power stations are from generation II, but most new power stations will probably be from generation III. That doesn’t help much. Even this subset describes many technologies, and the differences between them are huge. Amusingly, or maybe tormentingly, the editorial in the 10 Silicon Chip Australia's electronics magazine siliconchip.com.au October issue is very prescient. I waded through tables and tables of beeping TLAs (three-letter acronyms). Some were reasonably obvious: a BWR is a boiling water reactor, while a PWR is a pressurised water reactor. It was a bit alarming to discover that the “S” in some acronyms means sodium, and the “L” means lead, given that they are abbreviated Na and Pb in the periodic table, respectively. Oh, goodie! Several tonnes of red-hot liquid metals sploshing about. Weirdly, some protagonists hope that these will make the reactors safer. Part of the reason that some antagonists warn that construction might take decades is that it will take ages to do the paperwork to select a suitable contractor. The selection of the technology risks being more like a guess than a sound technical decision. I enjoy the irony that the big problems will be administrative, not technical. There is a lot of enthusiasm for small modular reactors (SMRs). Some of the definitions of these terms remind me of the average-sized boy who wanted to be the biggest midget in the world. The list of types that someone would like to sell us is another thesaurus of TLAs, except that some of them need four and five letters. About two SMRs have been built, but many are only in various design stages, including ‘cancelled’. Keith Anderson, Kingston, Tas. Comment: as you rightly say, a lot of the difficulties with generating electricity using nuclear fission are administrative (or political) rather than technical. One nuclear engineer had a good point about liquid-metal cooled reactor designs. While they have certain technical advantages, he asks, how do you inspect their internals, especially during operation? At least water is mostly seethrough! More on the pitfalls of high-impedance meters I want to comment on the entry in the November 2024 Serviceman’s Log column entitled “Impediment to Learning”. The letter mentions the downsides of using high-­ impedance meters in the electrical industry. As an electrical apprentice in the late 1970s, I too used an Avo meter with relatively low impedance for all testing in and around a hydroelectric power plant. In later years, as a tradesman training apprentices, I found that the advent of high-impedance digital meters caused all sorts of inductive ‘ghost’ voltages to appear on what should have been isolated circuits; a constant source of confusion for some. The solution I used was a twin extension terminal set that plugged directly into the terminals of the high impedance meter, bridged by two parallel 1MW resistors to reduce the test impedance to around 500kW. The meter leads then piggybacked into the extension terminals. This eliminated the unwanted induced voltages so that the isolated circuits could be confidently tested and worked upon, and was a great learning aid for those in training. I still carry the so-called “induction pack” around with the meter today, in case it is ever needed. Terry Ives, Penguin, Tas. Comment: the resistors would need to be insulated (and for a CATIII rating, insulated to 1000V) Also they should SC be rated for the applied test voltage. 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 January 2025  11 Last month we covered the invisible backbone of the internet, undersea cables. This month we write about another important piece of mostly invisible internet infrastructure: data centres. Like cables, they are rarely seen and little is known of them by the general public. By Dr David Maddison VK3DSM Data Centres, Servers & Cloud Computing E very time you use a search engine, watch an online video, use an email service, use social media, read or write blogs, buy products online, use an AI system, or even read Silicon Chip or most other magazines or newspapers online, you are almost certainly using a data centre. Data centres contain large numbers of computer servers where information is received, stored, managed, processed and disseminated. A server is a computer on which software runs remotely, to ‘serve’ other computers called ‘clients’ over a network. Such software applications include web servers, email servers, databases, custom servers and more. Small companies might start with their own central computer system with an in-house server to store and process their data. As they grow, it might become more economical to move these services to offsite data centres, especially for companies with multiple locations. Companies can: ● pay a data centre to host their own hardware ● rent hardware from a third party but manage the software themselves 12 Silicon Chip ● have their off-site hardware and software managed entirely by a thirdparty or multiple parties More and more these days, individuals also pay companies to manage offsite services and data for them, often referring to those services as being in ‘the cloud’. For example, you might be a Google customer and use Google Docs, Gmail, Google Drive etc; or an Apple customer using iCloud, Apple Mail etc; or a Microsoft customer using OneDrive, Office 365 etc. Those services may use local apps (or run in a web browser) but most of the ‘heavy lifting’ is done in servers located in data centres. In most cases, those servers are distributed around the world, so there will always be a local server for fast access (and also so that the entire service doesn’t go down due to one network outage). In some cases (or in certain areas), it is also necessary to store data locally to comply with local laws. Cloud services providers can be huge; they might contain tens of thousands of servers, or even millions, as they service numerous companies Australia's electronics magazine (and individuals) from all over the world. The origins of data centres Early computers were room-sized, used large amounts of power and needed a specialised environment with air conditioning, raised floors for cables, provision of a large power system and a building capable of taking the weight of the computer. Such computers were known as “mainframes” (see Fig.1). They were typically accessed via a ‘dumb terminal’, as shown in Fig.2. That was the case from the late 1940s through to the 1970s. Only large businesses, government organisations and scientific establishments could afford such computers. Due to the cost, computing was often done through ‘time-sharing’ arrangements, where many users accessed a portion of the power of one large computer through a terminal at their desk or some common location. In the 1970s, the microcomputer was invented, and it was popularised in the 1980s. Software could then be run by individuals from their personal siliconchip.com.au Fig.1: the NASA Mission Control computer room in 1962 which used two IBM 7094-11 computers. Source: https://archive.org/details/S66-15331 computer (PC), which is also where data was stored. Software was developed that did not need specialised training to use (it was more ‘userfriendly’). Unfortunately, having a computer on every desk led to other problems, such as organisations losing control of their IT resources. This created an incentive to again centralise computing resources. Some larger government and corporate entities still maintained special rooms with traditional mainframe computers where critical data was stored, even with the rollout and acceptance of microcomputers. Still, by and large, desktop PCs were widely used throughout the 1980s and 1990s until the internet started to expand rapidly. The expansion of the internet and the resulting vast requirement for data storage and e-commerce created a need for centralised computing and data storage. This coincided with the so-called ‘dot.com bubble’, from about 1995 to 2000, with large investments in IT-related companies. Central data storage was expensive, and eCommerce companies needed a fast internet connection, which at the time was costly. There was also the need for backup power for the computers and dedicated staff to maintain the systems. It thus became preferable for organisations to subcontract their data storage and computing requirements to an external organisation, such as a siliconchip.com.au data centre, where economies of scale helped to minimise costs. In a way, the modern data centre represents a return to the earliest days of computing via centralised systems with dedicated staff. Fig.2: a typical way to interact with a computer in the early 1960s was via a printing Teletype, such as this ASR-33 model, introduced in 1963. Source: https://w.wiki/B5fn A data centre is a dedicated facility that houses computers, storage media, networking, power & telecommunications infrastructure, cooling systems, fire suppression systems, security systems, staff facilities and anything else required to run networked computers. the customer has to do is access it. It is a somewhat nebulous concept (like a cloud!). Clouds may be public, such as many of the services operated by Microsoft, Google and Apple, or ‘private’, where only specific customers with contracts can access them. Hybrid clouds contain a mix of public and private data and/or services. Macquarie Data Centres (https:// macquariedatacentres.com) hosts a lot of data and services for Australian companies and the federal government. What is “the cloud”? Service delivery models This expression is often used in reference to computers running in data centres. ‘The cloud’ represents the availability of computing resources to an end user anywhere that an internet connection exists. That generally implies that the resources are located in one or more data centres. While cloud resources could be hosted in one central location, more likely, they will be distributed over a range of locations for redundancy, to reduce bandwidth requirements over long-distance connections and to reduce latency (access time). Most commonly, a ‘cloud’ service is a type of Software as a Service (SaaS), as per the following section on delivery models. That means that both the cloud hardware and software (including the operating system, applications etc) are managed by a third party. All A data centre or cloud can be managed in various ways, as shown in Fig.3. It can either be completely in-house, or with infrastructure as a service (IaaS), platform as a service (PaaS) or software (applications) as a service (SaaS) representing reducing levels of customer management and increasing levels of data centre or cloud provider management. For those who are curious, the Silicon Chip website (and some of our other software) use the IaaS model. We do this to retain maximum control over our systems, without us having to worry about provisioning high-speed internet, backup power, cooling etc. It also saves money because we only need a fraction of the power of a computer, so we can share hardware with others to split the costs. Tenancy refers to the sharing of What is a data centre? Australia's electronics magazine January 2025  13 Fig.3: four different data centre service delivery models (related to the concept of tenancy). Original source: https://w.wiki/B5fq resources. Multi-tenancy is popular on public cloud services, such as Microsoft Azure. In this case, an individual customer’s data remains invisible and inaccessible to others, but they share hardware, networking, other infrastructure, databases and memory. In that case, there are limited possibilities for the customisation of application software. Examples of multi-tenancy software providers include Google Apps, Salesforce, Dropbox, Mailchimp, HubSpot, DocuSign and Zendesk. With single-tenancy, there is no sharing of resources, which means maximum control over the software – see Fig.4. Virtual machines and servers A virtual machine or virtual server is an emulated version of a physical computer running within a physical computer. To put it another way, from the customer’s perspective, they have access to an entire computer, with which they can do whatever they like. But it doesn’t exist as a physical computer; instead, it is software running on a physical computer, alongside many other customers’ virtual machines. Businesses can create their own virtual server, which can run software and operating systems, store data, perform networking functions and do other computing functions as though it was a real physical computer. This virtual server runs under a software layer known as a ‘hypervisor’, which manages the memory, CPU, storage, networking and other physical resources of the physical computer and allocates them to the virtual machines as required. ● lower costs (due to economies of scale) ● lower latency and faster transfer speeds ● hardware maintenance performed by third parties with access to experts and parts ● multi-tenancy allows costs and resources to be shared among a large pool of users ● data centres typically have a lot of redundancy, making them resistant to power outages and natural or human-induced disasters Why use the cloud? These reasons include those for using a data centre, plus: ● device independence; applications can be typically via a web browser, so will work from any operating system, including mobile devices ● software maintenance, including updates, performed by expert third parties ● performance monitoring and security by expert third parties ● scalability and elasticity so resources can be increased as required How many data centres exist? According to Cloudscene (https:// cloudscene.com), there are 308 data centres in Australia (mostly in Sydney, Melbourne and Brisbane) with international connectivity to the rest of the world by numerous subsea cables. There are 81 in New Zealand, mostly in Auckland and Wellington. Worldwide, there are approximately 11,000 data centres, with the United States of America having the most at 5387. Data centre infrastructure Data centres have major network infrastructure to connect the data centre to the outside world with plenty of bandwidth. The internal network is also handy for transferring data between multiple computers operated by the same customer (and sometimes even different customers, eg, web crawlers for search engines). There is also significant storage infrastructure for storing data and software; it may be integrated with the computing nodes, or separate and accessed through internal high-speed networking. Of course, there are plenty of computing resources for data processing with onboard memory, with connections to data and applications storage, plus internet infrastructure. These are supported by cooling systems, power supplies and fire suppression Why use a data centre? We touched on this earlier when we explained why we use IaaS, but there are other reasons, including: 14 Silicon Chip Fig.4: the single tenancy vs multi-tenancy models for data centres. DB is short for database. Original source: https://resources.igloosoftware.com/blog/multitenancy-database Australia's electronics magazine siliconchip.com.au Fig.5: the NVIDIA GH200 Grace Hopper platform, based on the Grace Hopper Superchip. This board is capable of four petaflops (4 × 1015 floating point operations per second) and includes 72 ARM CPUs, 96GB of HBM3 memory for the CPUs plus 576GB for the GPUs. Source: TechSpot – siliconchip.au/link/ ac19 systems. The work of a data centre is done in various forms of processing units: CPUs (central processing units) CPUs are at the heart of traditional computers and generally continue to be, including in data centres. They may be supplemented by GPUs, TPUs and DPUs (each described below) to improve performance or provide new capabilities. An example of a CPU designed for data centres is the fourth-generation AMD EPYC based on the x86 architecture, as used in most PCs and servers (Fig.7). It is designed to be energy efficient, secure and give high performance. Each of these processors may include up to 128 Zen 4 or Zen 4c cores, allowing each server to potentially handle thousands of requests at any time. GPUs (graphics processing units) GPUs are special processors to accelerate the rendering of images, including 3D scenes. They are also capable of image processing. While they were originally designed for graphics applications, they are highly suitable for non-graphics applications such as parallel processing, accelerated computing and neural networks as needed in machine learning and artificial intelligence (AI). As such, they are commonly found in AI systems. The term ‘accelerated computing’ refers to using specialised hardware such as GPUs to more efficiently performing complex computing tasks than traditional CPUs can. An example of a GPU used in accelerated computing and AI data centres is the NVIDIA Grace Hopper Superchip processor, which forms part of the GH200 Grace Hopper platform (Fig.5). It is specifically designed for accelerated computing and generative AI, primarily in data centres. It utilises the latest HBM3e high bandwidth memory technology that provides 10TB/sec of memory bandwidth. TPUs (tensor processing units) TPUs are proprietary ASICs (application specific integrated circuits) by Google, optimised for neural network machine learning and artificial intelligence. Various versions have been produced since 2015. They are designed for high computational throughput at low precision, handling numbers with as few as eight bits. The chips (see Fig.6) are designed Fig.7: a range of AMD fourth-generation EPYC processors designed specifically for data centre applications. Source: www.amd.com/en/products/processors/ server/epyc/4th-generation-9004-and-8004-series.html siliconchip.com.au Australia's electronics magazine Fig.6: Google’s v5p TPU chip. Source: https://thetechportal.com/2024/04/09/ google-ai-chip specifically for Google’s TensorFlow framework for machine learning and artificial intelligence, and are incorporated into ‘packages’, as shown in Fig.8. A notable application was Google’s use of TPUs to find and process all the text in the pictures of Google’s Street View database in under five days. Google has developed what they call the Cloud TPU v5p AI Hypercomputer (Fig.9). DPUs (data processing units) DPUs, also called infrastructure processing units (IPUs) or SmartNICs (NIC stands for network interface controller) are used to optimise data centre workloads and to manage networking, security and storage. They relieve system CPUs of these workloads. An example is the SolidNET DPU, an ARM-based software-defined DPU with a PCIe half-height-half-length (HHHL) format. It is based on an offthe-shelf 16-core NXP LX2161A System on Card (SOC) and uses open standards (see Fig.10). For more information, see siliconchip.au/link/ac0b Power supply A typical data centre power system includes: Fig.8: Google’s TPU v4 board. It has 4 PCIe connectors and 16 OSFP connectors. Source: https://w.wiki/B5fr January 2025  15 ● transformer(s) to reduce the utility voltage, if necessary ● automatic switching gear to switch to backup power sources such as a generator in the event of a utility failure ● a UPS (uninterruptible power supply) supplied by a battery bank to provide backup power in the event of a utility failure, until the generator starts, as well as to condition power and remove voltage spikes in normal operation ● power distribution units (PDU), an electrical board to distribute power from the UPS to equipment locations ● a remote power panel (RPP), an electrical sub-board to distribute power from the PDU to individual rack-mounted power distribution units (rPDU) rPDUs are much like power boards. Individual servers or other equipment are plugged into them. Some of these components may be absent, depending on the size and sophistication of the data centre. All of the above has cables, wiring, circuit breaker boards etc. Some data centres use flywheel energy storage rather than a battery-­ based UPS (see siliconchip.au/link/ ac1b). They can be slightly more costly to install, but they don’t degrade over time as much as batteries do. Power consumption Data centres, especially AI data centres, use an enormous amount of electrical power. That’s both to power the computers themselves, particularly their CPUs, GPUs and TPUs, as well as their cooling systems. So it is important that these be designed to be as efficient as possible to minimise power consumption. Data centres need access to inexpensive, reliable 24/7 power supplies. They consume a significant amount of the world’s electrical power; one estimate is 1%-1.5% (siliconchip.au/ link/ac0i). According to another estimate (siliconchip.au/link/ac0j), AI currently uses 8% of the world’s electrical energy. The IEA predicts that data centres will consume 6% of electrical power in the United States by 2026, and 32% in Ireland by 2026, up from 17% in 2022 (siliconchip.au/link/ac0k). A typical ‘hyperscale’ data centre consumes up to 100MW according to Oper8 Global (the largest is up to 960MW). But that is just internal consumption. Given a power usage effectiveness (PUE) of 1.3, 130MW will need to be provided from the grid. At a time when dispatchable (on demand) power capacity is diminishing in many countries and being replaced with intermittent solar and wind production, plus the energy demand for charging electric vehicles, it is not clear where all this power will come from. The shortage of power has been recognised. According to the CBRE Group (siliconchip.au/link/ac0l): A worldwide shortage of available power is inhibiting growth of the global data center market. Sourcing enough power is a top priority of data center operators across North America, Europe, Latin America and Asia-­ Pacific. Certain secondary markets with robust power supplies stand to attract more data center operators. Data centres are being set up in New Zealand with access to 200MW of relatively inexpensive hydroelectric, gas and geothermal energy, from which 79% of New Zealand’s total production is derived (siliconchip.au/link/ac0m). In the United States, Equinix, a data centre provider, signed a 20-year non-binding agreement with Oklo to purchase up to 500MW of nuclear power (siliconchip.au/link/ac0n). Microsoft is proposing to use nuclear power for its data centres (see siliconchip.au/link/ac0o), as is Google (siliconchip.au/link/ac0p). Amazon purchased a nuclear-powered data centre in Salem Township, Pennsylvania, USA (siliconchip.au/link/ac0q). It consumes an almost unbelievable 960MW of electrical power. According to Funds Europe, the rapid growth of data centres is putting an unsustainable strain on the European electrical grid (siliconchip. au/link/ac0r). They already use 2.7% of their power, expected to increase to 3.2% by 2030. It has been suggested they use small modular reactors (SMR) and micro modular reactors (MMR) to power data centres. There is a growing interest in using nuclear power for AI data centres: siliconchip.au/link/ac0j Cooling One of the most critical aspects of a data centre, apart from the computing resources, is the provision of cooling. This is because the vast majority of the enormous amount of power used by data centres ultimately gets converted into heat. Data centres are cooled by air conditioning the rooms the computers are in, and also possibly some type of liquid cooling of the servers themselves. A data centre can be designed with hot and cold aisles between server racks to help maximise the efficiency of the cooling system. Cold air may Fig.9: inside part of Google’s ‘hypercomputer’ based on v5p TPUs arranged into ‘pods’. Each pod contains 8960 v5p TPUs. Source: Axios – siliconchip.au/ link/ac1a Fig.10: a SolidRun SolidNET Software-Defined DPU (data processing unit). Source: www. storagereview.com/news/ solidrun-solidnet-software-defineddpu-for-the-edge-unveiled 16 Silicon Chip Australia's electronics magazine siliconchip.com.au be delivered from beneath perforated floor tiles and into the server racks before being discharged into the hot aisles (see Fig.12). Alternatively, hot air may be collected at the top of the server racks rather than being blown into an aisle. Some data centres are using emerging technologies such as immersing the computer equipment in a fluid to efficiently remove heat (Fig.13). In two-phase cooling, a volatile cooling liquid boils and condenses on a coil which is connected to a heat exchanger to remover heat, after which it drips down into the coolant pool. We published an article in the November 2018 issue on the DownUnder GeoSolutions supercomputer in Perth that was immersed in an oil bath for cooling (siliconchip.au/ Article/11300). Water usage Some data centres, especially those used for AI, consume water for cooling and hydroelectric generation as well. One would think that cooling a data centre would mostly involve a closed loop system, like a typical car. But apparently that is not always the case, as many data centres use large amounts of water. Nature magazine states: ...in July 2022, the month before OpenAI finished training the model, the cluster used about 6% of the district’s water. As Google and Microsoft prepared their Bard and Bing large language models, both had major spikes in water use — increases of 20% and 34%, respectively, in one year, according to the companies’ environmental reports... demand for water for AI could be half that of the United Kingdom by 2027 – https://doi.org/10.1038/ d41586-024-00478-x Details of Microsoft’s water consumption for AI is at siliconchip.au/ link/ac0u About 2/3 of the water used by Amazon data centres evaporates; the rest is used for irrigation (siliconchip.au/ link/ac0v). That source also states that the amount of water to be consumed by a proposed Google data centre is regarded as a trade secret! Fig.11: part of the elaborate plumbing for the cooling system for the Google data centre in Douglas County, Georgia. Source: www.google.com/about/ datacenters/gallery Fig.12: one possible configuration of a data centre using the concept of hot and cold aisles between rows of servers. Original source: www.techtarget.com/ searchdatacenter/How-to-design-and-build-a-data-center Fig.13: the concept of twophase immersion cooling for server equipment Source: www.gigabyte. com/Solutions/ liquidstack-twophase Fire detection and suppression Due to the very high electrical power density inside a data centre, if a fire breaks out, it could get serious very quickly. Fire detection systems need to give early warning to prevent major siliconchip.com.au Vapor condenses on coil or lid condenser Fluid recirculates passively to bath Vapor rises to top Heat generated on chip and fluid turns into vapor Australia's electronics magazine January 2025  17 Fig.14: a comparison of the VESDA early warning smoke detection to conventional fire detection systems. Source: https://xtralis.com/product_ subcategory/2/VESDA-Aspirating-Smoke-Detection Fig.15: an artist’s impression of the Victaulic Vortex fire suppression system in operation, discharging a water and nitrogen fog. Source: https://youtu.be/ qmhO7E4c0tM Fig.16: the entry lobby of a Google data centre uses a Circlelock door and retinal scan, emphasising the high security requirements of data centres. Source: www. google.com/about/datacenters/gallery 18 Silicon Chip Australia's electronics magazine damage, and fire extinguishing systems need to cause minimal damage to electrical equipment. VESDA (Very Early Smoke Detection Apparatus) is a highly sensitive smoke detector (Fig.14), at least 1000 times more sensitive than a typical smoke alarm. It sucks air through perforated pipes that are routed around a protected area, then analyses the sample for the presence of smoke with sensitive detectors. It is an Australian invention in use in many data centres for the early detection of fires. Victaulic Vortex is a fire suppression system used in many data centres (Fig.15). It is a combined water and nitrogen fire extinguishing system. Tiny droplets of water and nitrogen gas, like a fog, are discharged from nozzles to absorb heat, reduce oxygen and extinguish the fire. It causes minimal or no wetting and therefore no equipment damage, avoiding a costly clean-up. After rectifying the fire damage, the data centre can be quickly returned to operation. Security Physical security, data security, environmental security (avoiding flooding, earthquakes etc) and power supply security are all important considerations for data centres. Human entry usually requires some type of biometric system (like a retinal scan) via a secure doorway – see Fig.16. That shows a Circlelock door, which is described at siliconchip.au/link/ac0c Server racks Server racks are standardised frames (typically made from metal) that hold computer servers, network switches or other equipment. They help to organise wiring, airflow or plumbing for cooling, provide access for service & maintenance, and sometimes physical security – see Fig.17. Server racks are mounted together in single or multiple rows in whatever number is required, as shown in Fig.18. An important feature of server racks is that they allow a very high density, with up to 42 individual systems in one standard rack, or over 100 with a ‘blade’ configuration. A server rack is designed to accommodate equipment that is 19 inches (482.6mm) wide; that standard was established in 1922 by AT&T. The height of equipment is standardised in heights representing multiples of 1.75 siliconchip.com.au Fig.17: this server rack is mostly populated with network switches and patch panels. Source: Fourbs Group – siliconchip.au/link/ac1c Fig.18: a group of server racks in a data centre. Source: https://kpmg. com/jp/en/home/insights/2022/03/ datacenter-business.html Fig.19: removing a 1U rack-mounted server mounted with sliding rails. Source: https://youtu.be/fWaW9lA_ pA0 inches (44.45mm). A single-height unit is designated 1U (see Fig.19), double height 2U etc. Equipment might be mounted on rails so it can easily be slid out for service. Alternatively, and more simply, it may be bolted to the edges of the rack using ‘rack ears’. Almost all aspects of server racks are covered by CEA, DIN, EIA, IEC and other standards. The so-called 19-inch rack is used for many other types of equipment as well. There are some other rack standards. One example is Open Rack, an initiative of the Open Compute Project. This rack was specifically designed for large-scale cloud deployments and has features such as a pair of 48V DC busbars at the rear to power the equipment. It is designed for equipment that is 21-inches (538mm) wide instead of 19in (482.6mm), with a vertical spacing of 1.89in (48mm) instead of 1.75in (44.45mm) to improve cooling. The racks are strong to accommodate the extra weight of equipment, all cables connect at the front rather than the back, and IT equipment is hot pluggable. See Fig.20 for a typical Open Rack configuration. According to Seagate (siliconchip. au/link/ac0d), over 90% of online data stored in data centres is on hard disk, with the remainder on SSDs. Western Digital sells a drive intended for use in data centres, the Ultrastar DC HC680, with a capacity of 28TB. Seagate’s Exos X series of hard drives have capacities up to 32TB. Tape drives are also used in data centres for archiving data and backups. They have great durability and longevity, and can provide an ‘air gap’ (no physical connection to the rest of the system) to protect stored data against hacking attempts and ransomware. They are also low in cost for their high capacity. Enterprise and Datacenter Standard Form Factor (EDSFF) is a specification designed to address the limitations of the 2.5-inch and M.2 sizes for solid-­ state drives. EDSFF drives provide better signal integrity, can draw more power and have higher maximum read/write speeds. Data storage While there is a general move to solid-­state drives (SSDs) for data storage, hard disk drives (HDDs) retain some advantages over SSDs such as lower price, especially for higher capacities; they last longer, with little degradation with constant read/write cycles; and data recovery is easier for certain failure modes. siliconchip.com.au Standards for data centres Various international standards exist for the design of data centres and their security and operational efficiency. Examples include: ● ISO/IEC 22237-series ● ANSI/TIA-942 ● ANSI/BICSI 002-2024 ● Telcordia GR-3160 Data centre ratings Data centres can be rated according to the TIA-942 standard: Rated-1: Basic Site Infrastructure The data centre has single-capacity components, a non-redundant distribution path for all equipment and limited protection against physical events. Rated-2: Redundant Component Site Infrastructure The data centre has redundant capacity components, but a non-­ redundant distribution path that serves the computer equipment. Fig.20: a typical configuration for an Open Compute Project V2 rack. Original source: Mission Critical Magazine – siliconchip.au/link/ac1e Australia's electronics magazine Rated-3: Concurrently Maintainable Site Infrastructure The data centre has redundant capacity components and redundant January 2025  19 distribution paths that serve the computer equipment, allowing for concurrent maintainability of any piece of equipment. It also has improved physical security. Fig.21: the Google Cloud TPU v5e AI infrastructure in a data centre. Source: https://cloud.google.com/blog/products/compute/announcing-cloud-tpu-v5eand-a3-gpus-in-ga Rated-4: Fault Tolerant Site Infrastructure The data centre has redundant capacity components, active redundant distribution paths to serve the equipment and protection against single failure scenarios. It also includes the highest level of security. A ‘hyperscale’ data centre is one designed to accommodate extreme workloads. Amazon, Facebook, Google, IBM and Microsoft are examples of companies that use them. Artificial intelligence (AI) Fig.22: the Microsoft Azure infrastructure that runs ChatGPT. Source: https:// news.microsoft.com/source/features/ai/how-microsofts-bet-on-azure-unlockedan-ai-revolution Fig.23: inside a small section of the Google data centre in Douglas County, Georgia, USA. Source: www.google.com/about/datacenters/gallery 20 Silicon Chip Australia's electronics magazine Some data centres are specialised for AI workloads. AI data centres are much like regular data centres in that they require large computing resources and specialised buildings. However, the resource requirements for AI are substantially more than a conventional data centre. According to Australia’s Macquarie Data Centres, conventional data centres require around 12kW per rack, but an AI data centre might require 60kW per rack. Oper8 Global (siliconchip. au/link/ac0w) states that an ‘extreme density’ rack can have a power consumption of up to 150kW! An AI data centre requires far more computing resources. Instead of mainly using CPUs, it will also contain a significant number of GPUs and TPUs. Deep learning & machine learning AI data centres can use either machine learning or deep learning. Machine learning uses algorithms to interpret and learn from data, while deep learning uses similar algorithms but structures them into layers, within an artificial neural network simulating how a brain learns. A neural network is hardware and/ or software with architecture inspired by that of the human (or other) brains. It is used for deep learning, a form of artificial intelligence. Large versions of these are run in data centres. Machine learning does not necessarily use neural networks (but it can). Machine learning is best for structured tasks with small datasets, with thousands of data points, but may siliconchip.com.au Fig.24: Google Cloud (Cloud CDN) locations (dots) and their interconnecting subsea cables. Source: https://cloud.google.com/ about/locations#network require human intervention if a learned prediction is incorrect. Deep learning is best for making sense of unstructured data with large datasets and millions of data points. Deep learning can determine for itself whether a prediction is wrong or not. Machine learning is relatively quick to train but less powerful; deep learning can take weeks or months to train, like a person. CPUs have advantages for implementing recurrent neural networks (RNNs). Typical applications for RNNs are for translating language, speech recognition, natural language processing and image captioning. GPUs have advantages for some fully connected neural networks. They are probably the most common type of processor used for neural networks, hence the huge stock value of companies that make GPUs like NVIDIA, which at the time of writing is one of the most valuable publicly listed companies in the world at US$2.6 trillion. Fully connected neural networks are suitable for deep learning and have applications in speech recognition, image recognition, visual art characterisation, generating art, natural language processing, drug discovery and toxicology, marketing, medical image analysis, image restoration, materials science, robot training, solving complex mathematical equations and weather prediction, among others. TPUs have advantages for convolutional neural networks (CNNs). Applications for CNNs include pattern recognition, image recognition and object detection. Fig.21 shows part of the Google Cloud TPU data centre artificial siliconchip.com.au intelligence infrastructure. Also see the video titled “Inside a Google Cloud TPU Data Center” at https://youtu.be/ FsxthdQ_sL4 Rack (mentioned previously), energy-­ efficient power supplies and network switches based on SONiC (Software for Open Networking in the Cloud). ChatGPT This popular AI ‘chatbot’, developed by OpenAI, is hosted on a Microsoft Azure cloud computing data centre infrastructure (see Fig.22). It runs on tens of thousands of NVIDIA’s H100 Tensor Core GPUs with NVIDIA Quantum-2 InfiniBand networking. Underwater data centres Google data centres Google is among the largest owners of data centres, storing vast amounts of the world’s data. Fig.23 shows the inside of a part of a Google data centre, while Fig.24 shows the location of Google Cloud data centres and their interconnection via undersea cables. The locations of data centres for delivering media such as videos (such as for YouTube) can be seen at siliconchip. au/link/ac1d Open Compute Project (OCP) The OCP (www.opencompute.org) was founded in 2011 with the objective of sharing designs for data centre products and practices. Companies involved include Alibaba Group, Arm, Cisco, Dell, Fidelity, Goldman Sachs, Google, Hewlett Packard Enterprise, IBM, Intel, Lenovo, Meta, Microsoft, Nokia, NVIDIA, Rackspace, Seagate Technology and Wiwynn. Their projects include server designs, an accelerator module for increasing the speed of neural networks in AI applications, data storage modules (Open Vault), Open Australia's electronics magazine Because of the significant cooling requirements of data centres and the need for physical security, experiments have been made in placing data centres underwater. They would be constructed within a pressure-resistant waterproof container, with only electrical and data cables coming to the surface. They would not have any staff. With no people, there is no need for a breathable atmosphere, so it can be pure nitrogen to reduce corrosion of connectors and other parts. There is also no possibility of accidental damage such as people dislodging wires etc. Also, there would be no dust to clog cooling fans or get into connectors. The underwater environment has a stable temperature, resulting in fewer failures than when the temperature can vary a lot. It is much easier and more efficient to exchange heat with a fluid such as water than with air, reducing the overall power consumption. An underwater environment also provides protection from some forms of nuclear radiation, which can cause errors in ICs, as water is a good absorber of certain types of radiation. Water can also absorb electromagnetic pulses (EMP) from nuclear explosions. The fact that the electronics are also effectively housed in a Faraday cage will also help with disaster resistance. January 2025  21 Fig.25: cleaning Microsoft’s underwater data centre after being on the seabed for two years, off the Orkney Islands in Scotland. Source: https://news. microsoft.com/source/features/ sustainability/project-natickunderwater-datacenter Fig.26: an IBM modular data centre built into a standard 40ft (12.2m) long shipping container. Source: https://w.wiki/B5ft Physical security is improved as being underwater, even if a diver could get to it, there would be no practical way to get inside without flooding the whole container. An underwater data centre can also contribute to reduced latency (response time) because half the world’s population lives within 200km of the sea, so they can be optimally placed near population centres and possibly undersea cables. Underwater data centre projects include: ● Microsoft Project Natick (https:// natick.research.microsoft.com), an experiment first deployed in 2015 with a data centre built within a pressure vessel 12.2m long, 3.18m in diameter, and is about the same size as a standard 40ft (12.2m) shipping container – see Fig.25. Its power consumption was 240kW. It had 12 racks containing 864 standard Microsoft data centre servers with FPGA acceleration and 27.6 petabytes of storage. The atmosphere was 100% nitrogen at one bar. Its planned operational period without maintenance was five years. ● Subsea Cloud (www.subseacloud. com) is proposing to put data centres 3km below sea level for physical security. ● Chinese company Highlander plans to build a commercial undersea data centre at the Hainan Island free trade zone, with a facility for 100 airtight pressure vessels on the seabed. Modular data centres A modular data centre is designed to be portable and is built into a structure like a shipping container – see Fig.26. They might be used to supplement the capacity of an existing data centre, for disaster recovery, humanitarian purposes or for any other reasons where a data centre has to be moved into a place it is needed. Fig.27: looking like somewhere where Superman might live, this 65-storey data centre is proposed to be built in Iceland. Source: www.yankodesign. com/2016/04/01/the-internetsfortress-of-solitude Iceland data centre A 65-storey data centre has been proposed to be built in the Arctic (see Fig.27). It was designed by Valeria Mercuri and Marco Merletti. If built in Iceland, it could take advantage of inexpensive geothermal energy and be close to international cable networks. The low temperatures would minimise cooling costs, and the vertical design would minimise land usage. SC 22 Silicon Chip Australia's electronics magazine siliconchip.com.au January altronics.com.au Workbench DEALS Our yearly workbench is now on! Only until January 31st. Great for cleaning the car too! T 1345 299 Vehicle Jump Starter & Power Bank $ Don’t get stuck with a dud battery! Suits 12V battery vehicles. 24000mAh rated battery provides up to 2400A peak output when cranking. A 90W USB PD output is provdided for your laptop (use it like a giant battery bank!). It also has a 600 lumen LED torch in built. 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B 0001 Project by Stephen Denholm This straightforward piece of test equipment measures capacitor values over a wide range, from about 10pF to 10,000μF (10mF). It’s easy to assemble with all through-hole parts, fits into a UB1 Jiffy box, and won’t break the bank either. I Digital Capacitance Meter occasionally need to measure values of large electrolytic capacitors (up to at least 6800µF) but have been restrained by the limited capacitance measurement ranges of the DMMs I have. To overcome this, I’ve resorted to setting up a test circuit using a digital oscilloscope to measure a capacitor’s value by measuring their charge time. This worked well but was time consuming. I explored Silicon Chip magazines looking for a relatively simple capacitance meter project that I could expand my development skills on and build. I found quite a few articles on the subject, ranging from very simple to quite advanced designs. Of particular interest was the Circuit Notebook item “PIC capacitance meter measures charging time” by William Andrew (July 2008; siliconchip. au/Article/1874). It was a little too siliconchip.com.au basic for my requirements, but I liked the relatively simple design concept, which appeared to work. I therefore decided to develop a similar design that was also PICbased, would use the charging time measurement concept, was relatively simple to build and compact, covered a range from about 10pF up to about 10,000µF, and was powered by a standard 9V battery. I was also inspired by Jim Rowe’s article on low cost 1.3-inch OLED displays in October 2023 (siliconchip. au/Article/15980). I thought I would have a go at also incorporating one of those low-power display modules into my design. Circuit details As shown in Fig.1, my circuit uses an 8-bit enhanced mid-range Australia's electronics magazine PIC16F1847 microcontroller unit (MCU). It has three capacitance ranges selected by switch S1 and shows the measured value of the capacitor under test (Cx) on the 1.3-inch OLED display (MOD1). The OLED is also used to display any over/under range or battery voltage warning messages that are necessary. The measurement operating sequence is commenced by pressing pushbutton switch S3. The MCU will then first ensure that Cx is fully discharged by switching on Q4 for a short period, then off, discharging it via the 33W resistor. It then starts charging capacitor Cx via one of Mosfets Q1, Q2 or Q3 and the associated series resistance. At the same time, it starts the MCU’s 16-bit Timer1, which operates with a counting interval of 1µs. The charging January 2025  27 voltage developed across Cx is then measured by the MCU’s Comparator1 positive input (C1IN+, pin 2) and compared to the voltage applied on its negative input C12IN0− (pin 17). As soon as the charging voltage exceeds the voltage at C12IN0−, the comparator stops Timer1, initiates a program interrupt and passes control back to the main program, where the Timer1 count register values are used to calculate the capacitance. As the source voltage for charging Cx is the 5V Vdd supply, the comparator C12IN0− input is set to 63.2% of Vdd, nominally 3.16V. This ensures that the comparator operation and hence measurement time will always be equivalent to one RC time constant of the capacitor under test. That simplifies the calculation to Cx = Timer1 count (µs) ÷ selected range series resistance, scaled accordingly. For the Lo, Mid and Hi capacitance ranges, the MCU calculations use series resistance values of 2MW, 25kW or 500W, respectively. It also means that, even if the output of the 5V regulator drifts with temperature or time, the measurements should remain accurate. The actual values used in the circuit are provided by the fixed/variable resistance combinations VR1 + 1MW, VR2 + 12kW and VR3 + 500W, which are switched into or out of circuit by the MCU via Mosfets Q1, Q2 and Q3. I used P-channel SMD devices as, particularly for the Hi range, they need 28 Silicon Chip low on-resistances to slightly improve the measurement accuracy. Suitable PNP transistors such as BC858s with base resistors of say 1kW to 3.9kW may work reasonably well, but with a small reduction in measurement performance. However, I have not tried that arrangement. re-compiling the code and uploading it to the MCU if necessary. I did briefly think about adding an auto-­ zeroing function to the meter design but decided it wasn’t worth the extra effort for my particular requirements, especially if I always stick to using the same meter leads. Performance Construction Performance-wise, my meter has been providing quite accurate and repeatable results across all three ranges. I have confirmed this occasionally by checking the meter’s range extremities against the calibration capacitors that I now keep for such a purpose. On the Lo range, it is necessary to keep the meter leads short to minimise any stray capacitance. In the MCU program code, I have allowed compensation for zero-offset in the Lo range calculations, which significantly improves the capacitance measurements for values below 1nF and surprisingly allows the meter to achieve quite accurate and consistent results down to about 10pF. This zero-offset value compensates for some inherent MCU program instruction cycle time, which starts to dominate the measurements for very short capacitance charging durations. It also compensates for the stray capacitance inherent in the physical construction of the meter and the short leads I use. The zero-offset value is hard-coded, but it is not too difficult to change by The board, coded 04111241 and measuring 80 × 100mm, is a double-­ sided design, but there are only a few top-layer tracks that can easily be replaced by wire links, as you will see in the photo of my prototype. So if you are etching the board yourself, start by fitting the four wire links you can see in that photo; they are also visible as top-layer tracks in the overlay diagram, Fig.2. Also note that there are four SMD components that mount on the underside: Mosfets Q1-Q3 and regulator REG1. They are shown in ‘X-ray’ fashion in Fig.2. Start by soldering them in place while the board will still fit flat on your bench. Q1-Q3 are all the same types and REG1 is in a different package, so it should be obvious which goes where and in what orientation. Do make sure that the leads are sitting flat on the board before soldering and not sticking up in the air, which would indicate that the part is upside-down. Tack each part by one pin and check that all the leads are over the matching PCB pads. If not, remelt that Australia's electronics magazine siliconchip.com.au Fig.1: the circuit diagram for the Capacitance Meter. S1 is used to switch the capacitance range. joint and gently nudge it into place. Once it’s properly aligned, solder the remainder leads and then refresh the first joint. Next, flip the board over and solder all the resistors in place. They are mounted with the leads bent quite close to the bodies. Follow the overlay diagram to see which values go where. There is just one diode, so fit that now, making sure its cathode stripe goes towards the top edge of the board as shown in Fig.2. You don’t have to use a socket for IC1, but it makes it easier to swap that chip if that ever becomes necessary. Solder either the socket or IC1 directly to the board, but in either case, make sure it is orientated with its notched (pin 1) end towards the top of the PCB. Solder terminal block CON1 in place now. We recommend that its wire entry holes are kept towards the left-hand side, although you can insert the wires from either end. Next, fit the headers (CON2-CON6), 100nF capacitor (which is not polarised) and transistor Q4 (orientated as shown). Note that CON4 is only required if you plan on (re)programming IC1 in-circuit. You could leave the other headers off and solder wires directly to the board, but we suggest using headers to make assembly (and if required later, disassembly) much easier. Mount the four trimpots next, making sure the adjustment screws all go towards the bottom of the board as per Fig.2: the overlay/wiring diagram for the Digital Capacitance Meter. Check your OLED pinout before wiring it up; the 5V pin is at the top of CON5. siliconchip.com.au Australia's electronics magazine January 2025  29 Fig.2. They are all different values, so don’t get them mixed up. Now solder the two electrolytic capacitors in place, ensuring that the longer (positive) lead goes into the bottom hole in each case. The negative striped ends of the cans should be near the top edge of the PCB. PCB pins for test points TP1 and TP2 are not strictly required if you have a double-sided board, as you can simply insert DMM probes into the plated through-holes. If you have a single-­ sided board, you will need to solder PCB pins into the two test point holes. Rotary switch The last part to mount directly to the PCB is the rotary switch. It is a twopole type. As supplied, it will probably have six positions, but we only need three. To change that, undo the nut and remove the washer from the shaft. Prise up the stop washer and rotate the switch fully anti-clockwise, then re-insert the stop washer with its pin going into the second hole between the moulded “3” and “4”. Check that it now only switches through three possible positions. If not, change the position of the stop washer and try again. Once it’s correct, put the lock washer back over the shaft and tighten the nut on top. In my build, the switch shaft length as supplied was just long enough to Figs.3 & 4: the cutting diagrams for the base and lid of the Jiffy box. You have some flexibility with the locations cutouts on the lid, as they’re mounted off the board. All diagrams are shown at actual size, and all dimensions are in millimetres. 30 Silicon Chip Australia's electronics magazine siliconchip.com.au pass through the front panel with enough poking through to attach the knob. The exact length required depends on the height of the spacers used to mount the PCB in the box and the knob you’re using. Ideally, you should temporarily mount the PCB in the box so you can check how much to cut off (if any). To do that, you will first need to drill PCB mounting holes in the base of the box and at least one hole in the lid (for the rotary switch shaft). The PCB mounting hole positions are shown in Fig.3 and the lid holes in Fig.4. With the shaft cut to length, remove the PCB from the box and solder the switch to it. There are two possible orientations, so match the switch to the photos and overlay. The next job is to mount the remaining parts on the front panel/lid and solder wires with female DuPont headers ready to plug into the headers on the PCB. If you haven’t already, finish making the holes in the lid as per Fig.4, after reading the next two paragraphs. Regarding the OLED screen, you can see from the photos that I used countersunk head screws, Nylon washers and nuts to mount it to a clear acrylic sub-panel, then glued that panel to the inside of the lid using epoxy. I did it this way as the acrylic panel provides some protection for the OLED screen; the screw heads are hidden under the front panel label. You could use the same approach, or mount the OLED directly to the lid using the holes shown in Fig.4. However, if you do that, note that even if you countersink the holes on the outside, the screws will probably still project above the surface of the lid due to its thinness. You may be able to cover them with a label but it’s better to use my approach, if possible, if you want a flat panel label. If you use my approach, use washers to space the OLED screen from the acrylic panel so the screen isn’t crushed when you tighen the screws. Strip off pairs of DuPont jumper wires from the ribbon for the 9V battery snap and switches S2 & S3. Strip off a set of four for the OLED. Cut them so that you have bare wires on one end, then solder them to the panel-­ mounting parts (check the OLED pinout with reference to Fig.2). For the two banana sockets, use medium-duty hookup wire (or similar) in two different colours instead. 1 single- or double-sided PCB coded 04111241, 80 × 100mm 1 UB1 Jiffy box 1 panel label, 100 × 160mm 1 1.3-inch (33mm) 128×64 pixel I2C OLED display module (MOD1) [Silicon Chip SC5026 or SC6511] 1 3mm clear acrylic sheet of ~43 x 41mm (for mounting the OLED module) 1 2-pole sealed rotary switch (S1) [Altronics S3022, Jaycar SR1212] 1 miniature panel-mount SPST toggle switch (S2) 1 panel-mount momentary NO pushbutton switch (S3) [Altronics S0960, Jaycar SP0700] 1 small-to-medium knob to suit S1 1 2-way 5.08mm pitch terminal block (CON1) 3 2-pin headers, 2.54mm pitch (CON2, CON3, CON6) 1 5-pin header, 2.54mm pitch (CON4; optional, for ICSP) 1 4-pin header, 2.54mm pitch (CON5) 1 red panel-mount binding banana socket 1 black panel-mount binding banana socket 1 pair of banana plug to crocodile clip test leads 1 2MW top-adjust multi-turn trimpot (VR1) 1 20kW top-adjust multi-turn trimpot (VR2) 1 500W top-adjust multi-turn trimpot (VR3) 1 50kW top-adjust multi-turn trimpot (VR4) 1 18-pin DIL IC socket (optional) 1 9V battery snap 1 9V battery retaining clip 1 9V battery 5 M3 × 6mm panhead machine screw 8 M3 × 6mm countersunk machine screw 4 M3 × 10mm tapped spacers 4 Nylon M3 washers 5 M3 hex nuts 10 short (~100mm) female/female DuPont jumper leads, joined in a ribbon 2 100mm lengths of medium-duty hookup wire (red & black) 1 100mm length of 1.5mm diameter black/clear/white heatshrink tubing 2 PCB stakes/pins (optional) Semiconductors 1 PIC16F1847-I/P 8-bit microcontroller programmed with 0411124A.HEX, DIP-18 (IC1) 1 AMS1117-5.0 or similar 5V 1A LDO linear regulator, SOT-223 (REG1) 3 AO3401(A) or SQ2351ES P-channel logic-level Mosfets, SOT-23 (Q1-Q3) 1 BC337 45V 800mA NPN transistor, TO-92 (Q4) 1 1N5819 40V 1A schottky diode (D1) Capacitors 1 470μF 10V radial electrolytic 1 100μF 10V ±5% tantalum [Vishay Sprague 293D107X5010D2TE3] 1 10μF 50V radial electrolytic 1 2.2μF 50V ±5% MKT [TDK B32529D0225J000] 1 100nF 50V ceramic or multi-layer ceramic 1 100nF 63/100V ±5% MKT [Altronics R3025B, Vishay BFC237012104] Resistors (all ¼W 1% axial) 1 1MW 1 27kW 1 22kW 1 15kW 1 12kW 10 10kW 1 4.7kW 1 1kW 1 270W 1 33W siliconchip.com.au Australia's electronics magazine Parts List – Digital Capacitance Meter January 2025  31 You can then plug everything into the headers on the PCB, using Fig.2 as a reference, and screw the two banana socket wires into the terminals of CON1. Ensure the wire routing is correct for the 9V battery, OLED screen and wires to CON1. With IC1 out of its socket, switch on power and check the voltage between pins 5 and 14 of that socket. You should get a reading between 4.5V and 5.5V. If not, switch off and check for faults. Assuming it’s close to 5V, switch off and insert IC1 in its socket, ensuring it has the correct orientation and that none of the leads fold up under the body when you do so. If IC1 has not been programmed, you can now power the device back on and connect an in-circuit programmer to CON4, with its pin 1 marking to the left as shown. Use software like Microchip’s free MPLAB IPE to load the HEX file, which you can download from siliconchip.au/Shop/6/532 You can then switch it back on and check that the screen display comes up normally. If so, you can proceed with calibration. Otherwise, power it off and check your soldering and parts placement. Calibration To initially calibrate the meter, set the voltage at test point TP1 (IC1’s negative comparator input voltage) to 3.16V by adjusting trimpot VR4. There is no ground test point; you could use negative (bottom) terminal of CON1. Next, for each range in turn, make repeated capacitance measurements of a calibration capacitor of known value while adjusting the selected range trimpot (VR1-VR3) to progressively obtain a calibrated value very close to the known capacitances. The parts list includes suggestions of three low-cost 5% tolerance capacitors that could be used, although sourcing the larger values may not be easy (DigiKey and Mouser have suitable parts). Cycle through the ranges and adjust each to get the correct measurement until you are only making minimal adjustments. In operation, once the measurement and calculation of the capacitance is completed, the MCU displays the value on the OLED in units of either pF, nF or µF depending on the range selected and size of the capacitor under test. If the measured value is out of range, a warning is shown to select a higher or lower range if possible. Also, before any measurement of Cx commences, the MCU checks the battery voltage and a warning message appears if it is low. If the voltage is too low (less than about 7V), a message to replace the battery is displayed and measurement stops. Conclusion Having built, tested and calibrated my meter, I decided to check my stock of electrolytic capacitors. 32 Silicon Chip The finished Digital Capacitance Meter with crocodile clips attached (shown below). Our version of the front panel label (shown here at 50% actual size) will be available to download from our website at siliconchip.com.au/Shop/11/585 I found some relatively new, unused electrolytic capacitors with values nowhere near their labelled value and not within the specified tolerance. In fact, I would say these capacitors had been incorrectly labelled or manufactured, as they were that far out! This was rather concerning as these components had been sourced from reputable suppliers. Buyer beware, as they say! I built the Touchscreen Wide-Range RCL Box (June 2020; siliconchip.au/ Series/345) a few years ago now. I’ve found it to be a very handy device. When I first built it, I thoroughly checked all the resistance values and found these to be well within the ±1% tolerance, which was great. However, I did not check the C and L values. So, out of interest, I decided to do a quick check on the capacitance values with my new meter. Surprisingly, I found two capacitors well outside (>30%) the ±10% tolerance I was expecting, even though I’m sure I had purchased SMD capacitors with specified tolerances of ±10% or better. I also performed a check with a DMM on capacitance range and got very similar results. I’m now waiting on a rainy day to do some further diagnostics on the RCL box. SC siliconchip.com.au Part 2: by Nicholas Vinen Complete Kit (SC6885; $70): includes the case but not a power supply Compact HiFi headphone Amplifier Introduced last month, our new Stereo Headphone Amplifier fits in a neat package and has two sets of inputs with individual volume controls. Having described its performance and how it works, we’ll go over some notes on the PCB design before getting into construction and testing. T he Headphone Amplifier circuit is fairly basic and uses all low-cost and common parts, but it delivers great performance in a small package. It’s suitable for relative beginners, with nothing being terribly tricky during the assembly process. Despite that, it still gives a very professional result. PCBs for hifi circuits are always a bit challenging to design due to the tiny levels of distortion and interference that are required to achieve good performance. So let’s take a brief look at what was involved in designing this one. PCB design It was a little tricky to fit everything into a relatively small (148 × 80mm) PCB using through-hole components, but we managed that, and the result is shown in Fig.8 and the photos. The power supply section has been kept on the left side, with the input section in the middle and the amplifier section on the right. The incoming signals arrive at the RCA connectors at the top of the board, flow down through the filtering and coupling components to the buffer op amps at lower middle, then to the volume control pots. They go to the mixer op amp to the right, and up to the transistor buffer section above, then right to the output filter and down to the output sockets. This arrangement keeps all the signal tracks relatively short, to minimise siliconchip.com.au the chance of picking up EMI or magnetic/electric fields from other parts of the PCB. It also keeps the component arrangement neat. As power needs to flow from the supply on the left side to the transistors at upper right, the positive and negative supply tracks are kept fairly wide and close together so that the magnetic loop is small. That reduces the amount of supply-ripple-induced distortion entering the sensitive signal tracks in the middle of the PCB. The output transistors have local 100μF bypass capacitors (shared between the channels) to help reduce the effect of the resistance and inductance of those supply tracks. All major ground returns are kept separate back to the power supply common point (similar to star Earthing) so that half-wave rectified currents don’t get into the signal grounds and increase distortion. If you’re wondering why only the NPN output transistors have small heatsinks attached, it definitely isn’t because we didn’t check whether there would be enough room for all four heatsinks to fit side-by-side on the PCB! Actually, during testing we found that even with reasonably high quiescent currents, the output transistors didn’t get terribly warm. Four resistors were added between VR1 & VR2 in the final version. Australia's electronics magazine January 2025  33 Still, as there was room to fit small heatsinks to the NPN output transistors (Q3 & Q5), we did so. That’s because these transistors have the Vbe multipliers (Q7 & Q8) mounted on top, so they won’t be able to dissipate heat as effectively as the PNP output transistors (Q4 & Q6) will. Also, the PCB is designed to draw heat away from all the transistors that are mounted on it (including those in the power supply). However, as the PCB’s ability to absorb, distribute and radiate heat is limited, we figured that by keeping Q3 & Q5 cooler with small heatsinks, that will reduce the total heat load on the board and thus effectively improve cooling for Q4 & Q6 as well. The heatsinks are actually sandwiched between each NPN output transistor and its associated Vbe multiplier transistor, with thermal paste in between. As the thermal resistance of the heatsink is low, that shouldn’t have any significant impact on thermal tracking for the Vbe multipliers. While we’re on the topic of output transistor ratings, we also need to keep in mind their continuous current limits of 1.5A each, especially during plugging and unplugging headphones. The output transistors have an hFE (current gain) of around 50 times at their limit of 1.5A, regardless of the junction temperature. That means, to exceed their 1.5A current limit would require a base drive of over 30mA (1.5A ÷ 50). While the NE5532 data sheet says it can typically source or sink 38mA, that’s with a ±15V supply and under short-circuit conditions. In practice, due to supply droop and other factors, with our recommended 9V AC plugpack, we were unable to get our prototype to get anywhere near the limit. Having said that, we didn’t deliberately short-circuit the output, so we can’t promise it’s short-circuit proof. But we think, if you are careful not to abuse it, it should be OK. Construction The Headphone Amplifier is built on a double-sided PCB coded 01103241 that measures 148 × 82mm. The same PCB is used regardless of which version you are building. Fig.8 is the component overlay diagram that includes all components for building the full version of the Amplifier, with two sets of stereo inputs. Fig.9 shows the same arrangement as Fig.8 but without the two buffer op amps. If you’re building it from a kit, you might as well build the full version as they are included, but it is possible to leave those two op amps out and save a few dollars. There will be more interaction between the volume controls, though. Fig.10 shows the PCB with just the components needed for one stereo input. We’ve chosen to retain CON2, but you could keep CON3 instead and fit the resistors, capacitors and potentiometers in the positions to the right instead. Regardless of which version you’re building, start by fitting all the smaller (¼W and ½W) resistors. They have colour-coded stripes that you can decode with the aid of the table in the parts list. Still it’s safer to check each set’s value with a DMM set to measure ohms before installing them. All the smaller resistors are laid flat on the PCB, so bend their leads, insert them, solder them and trim the excess. For the four 100W resistors, slip a ferrite bead over one of the leads before inserting it into the board. Solder the shorter end, then pull the other lead with a pair of pliers so it’s tight before soldering it. That should stop the ferrite bead from rattling if you move the board. Next, solder the two diodes, which are the same type. Make sure that both have their cathode stripes facing up, towards Q2. If using IC sockets, solder them in place now, ensuring the notches all face up as shown on the overlay diagrams. Otherwise, solder the ICs directly to the PCB, again ensuring that their notch or pin 1 dot faces up. Fig.8: use this overlay diagram as a guide to where to mount each component. This shows the full version with two buffered stereo inputs. Don’t forget to add the ferrite beads to the 100W resistors before soldering them and watch the orientation of the diodes, ICs and electrolytic capacitors. 34 Silicon Chip Australia's electronics magazine siliconchip.com.au This is important as they won’t work if reversed! Now is a good time to fit CON4 if you are using it. Once its pins are lined up with the pads, it should slot right into place. Solder it flat on the PCB. Next mount transistors Q2, Q4 & Q6. These are all the TTA004B PNP type. Make sure the writing is on the top side, then bend the leads down a few millimetres from their bodies so that they fit through the PCB pads while the mounting hole on the tab lines up with the one on the board. Add a small amount of thermal paste to the underside of each transistor, then feed a 10mm M3 machine screw up from underneath and push the transistor body over its shaft. Add a flat washer and hex nut on top and tighten while stopping the transistor body from rotating. Check the body is aligned properly, then solder and trim the leads. Use the same procedure to fit Q1, which is a TTC004B. Leave the other transistors off for now. Next, mount the two trimpots. They are the same type and only fit one way. Then move on to the capacitors, starting with the ceramics, which are not polarised, so they can go in either way around. Two of the 100nF capacitors are recommended to be MKT types; fit them next. They are also unpolarised. The The output filter inductors are wound on the bodies of the 1W resistors they’re paralleled with. You could add heatshrink tubing on top if you want. other 100nF capacitors can be MKT, ceramic or multi-layer ceramic, none of which are polarised. Then move on to the electrolytic capacitors, all of which are polarised. In each case, the longer lead goes into the pad next to the + symbol, with the stripe on the can facing the opposite way. The only thing to watch out here, apart from the polarity and the values being correct, is that there are three different types of 100μF capacitors specified. The four or eight capacitors marked 50V (in the middle of the board) should ideally be 50V types, to make the inputs as robust as possible. They could be lower-rated (eg, 35V) if absolutely necessary. The two low-ESR 100μF capacitors in the power supply section and two more at upper-right must be rated at least 25V, although higher-voltage types are suitable if they will fit. The two or four 100μF capacitors near VR1/VR2 can be 16V types, Fig.9: here are the difference if you’re building the two-input version without the buffer op amps. Fit the four links instead of the ICs and leave off the four 100kW resistors. siliconchip.com.au Australia's electronics magazine January 2025  35 although a higher rating certainly won’t hurt, as long as they will fit. Now is a good time to solder the two-pin header for JP1 in place. After that, fit VR1 and/or VR2, making sure they are pushed fully down and their shafts are perpendicular to the edge of the PCB. Also fit the barrel socket, again making sure it is straight and flat before using generous amounts of solder to attach it. The RCA sockets need the projection on the top cut off. It’s easiest to do it before mounting them on the board. Use a hacksaw or rotary tool to cut them off in line with the top edge of the socket face, then file or sand off any burrs or projections. Snap them into the PCB and make sure they’re flat before soldering the pins. Similarly, mount the on/off switch next. The LED goes next to the switch, with its lens at the same height as the switch shaft. Bend its leads by 90° about 3mm from the band of the lens, ensuring that when it’s inserted into the PCB, its longer (anode) lead will be to the right, as shown in the overlay diagrams. Insert and solder it so that its lens is at the same height as the switch and pot shafts. M3 machine screws. This bit can get a little fiddly and messy, so keep a damp cloth on hand, along with needle-nose pliers and angled tweezers. The mounting arrangement is depicted in Fig.11. First, bend the leads of all four transistors down so that they will fit into the PCB pads with the tab mounting hole in the correct position and the writing on the top. Make sure they can be inserted easily and that the tab hole is properly aligned, as that will make the rest of the job much easier. Insert a machine screw up through the bottom side of the PCB, then add a thin layer of thermal paste on both sides of one of the transistors. This will be Q3. Insert its leads and push it most of the way down to the PCB, then add a heatsink over the top, with the longer section projecting to the right (over Q3’s leads). Next, add thermal paste to the bottom side (only) of another transistor and add it on top of the heatsink (Q7). Place a flat washer over the screw shaft, then do up a nut on top. Hold the transistor bodies steady as you tighten the nut, then solder and trim all six leads. Repeat for the other transistor pair. Heatsinks Winding the inductors All four remaining transistors are TTC004Bs, and they are held to the board using 15mm or 16mm long We used 0.4mm diameter enamelled copper wire (ECW) to wind the inductors, although you could use a smaller diameter (down to about 0.25mm) if you happen to already have it. Cut it into two 1m lengths, then use a sharp hobby knife or emery paper to strip the insulation off the ends by 2-3mm. The inductors are wound using the bodies of the 10W 1W resistors as formers. Clamp a resistor in some sort of holder (we used the type that has mini grabbers), then add some solder to the leads on either end of the body. Solder one end of the ECW to that point, with the rest going past the body, then start winding it around the body. Try to keep it neat and closely spaced at first, although it’s basically impossible to keep it neat after the first layer. The good news is that there aren’t a huge number of turns required, so it hopefully won’t end up a jumbled mess by the time you have finished. Keep it wound tightly around the body, then solder the remaining stub close to the other end of the resistor body. Use a DMM to measure the resistance across the resistor. It should have dropped to around 0.2W (depending on your DMM lead resistance). If it’s close to 10W, that suggests the solder joint at one end (or both) is bad, so fix it. Repeat for the other resistor, then bend the leads, insert them into the PCB and solder them at similar Fig.10: if building the single-channel version, you can leave off either channel; here we’re showing CON2 fitted and CON3 not. Only one IC needs to be linked out in this case. In place of the two 1MW resistors, use 100kW instead. 36 Silicon Chip Australia's electronics magazine siliconchip.com.au heights. There’s no significant dissipation in these devices, but it’s easier to solder them spaced off the PCB, so you might as well. Finally, if you’re using the 6.35mm jack socket, CON5, solder it now. It will need to be a low-profile version to fit in the case. Jaycar’s PS0190 is unfortunately too tall, but many others like it sit lower. There are several suitable parts available from Altronics. Push it down fully and solder it in place using generous amounts of solder for good mechanical retention. Testing Adjust VR3 & VR4 fully anti-clockwise and ensure switch S1 is in the up (off) position. Plug in the plugpack and switch it on at the mains. Nothing should happen since the switch is off. Set your DMM to alternating current (AC) measurement mode (not DC!) in the amps range and connect the probes appropriately. Hold one against switch S1’s pad that’s closest to the large capacitor (ie, the one at the back & top). While watching the multimeter, touch the other to the middle pad for S1 for a second or two. If you’ve used IC sockets and the chips are not inserted, you should see a current draw of only a few tens of milliamps at most, and LED1 should light up. If all three op amps are soldered to the board, the current draw will be closer to 150mA. If you have fewer op amps installed, it will be in between (~50mA for one and ~100mA for two). If the current draw is a lot higher than that, or LED1 doesn’t light up, you have a problem. Disconnect the power supply and check the board for faults like pads bridged with solder, incorrectly orientated components, components in the wrong location etc. If it seems OK, set your DMM to measure DC volts and hold the black probe to a convenient ground point, such as the left-most pin of JP1 or the bottom-most end (closest to the PCB edge) of one of the row of four 100kW resistors between VR1 & VR2. Hold the red probe on pin 8 of one of the ICs and switch the power back on. You should get a steady DC voltage reading of around 13V DC for a 9V plugpack or 17V for a 12V plugpack. Then touch the red probe to pin 4 of the same IC, and you should get a negative voltage of a similar magnitude. Next, check the AC voltages at those siliconchip.com.au two pins. The reading should be no more than about 10mV AC (our prototype measured almost exactly 10mV with the ICs in-circuit). If you are using IC sockets and haven’t inserted them yet, switch off the power and wait for LED1 to extinguish. Install all the ICs you require, ensuring that pin 1 goes towards the upper-left corner, near the notches on the sockets. Now measure the DC output voltages relative to ground. They are available at the bottom ends of the two 10W 1W resistors that have the ECW wrapped around them. Measure those points relative to ground with the power on (see earlier for convenient ground points) and confirm that the readings are under 50mV (with either polarity). Our prototype measured around -25mV on both channels. If they are much higher than that, something is wrong, so switch off and search for faults. Rectify any problems you find and re-check the output voltages to verify they are under ±50mV before proceeding. Adjustment Connect a DMM set to measure millivolts between TP1 and TP2. The reading should be close to zero initially. Slowly rotate VR3 clockwise and by the time it reaches its midpoint, the voltage reading should start to rise. Adjust it for a reading close to 25mV (meaning 25mA quiescent current). Move the probes to TP1 & TP3 and the reading should be similar. Now connect the probes between TP4 & TP5 and perform the same adjustment using VR4. You can then check that the reading is similar between TP4 & TP6. At this point, you are ready for a listening testing. Switch off the power, rotate VR1 and VR2 fully Fig.11: the mounting arrangements for the power transistors and heatsinks. anti-clockwise and plug headphones or earphones into one of the sockets. Don’t put them over or onto your ears yet. Connect a low-level stereo audio signal source to one of the inputs, cue it up and switch the amplifier back on. Slowly wind up the volume pot associated with the channel you’re using (VR1 for CON2 or VR2 for CON3) and check that you can hear audio by moving the headphones/earphones closer to your ears. If it sounds normal, try putting them over/into your ears and adjust the volume to a comfortable level. Verify that the audio sounds normal and undistorted, with similar levels for both channels. If it sounds strange, switch off and look for faults on the PCB. Jumper option Before assembling the case, decide if you want to put a jumper shunt on JP1. With it out, if you plug headphones into both sockets, audio will only come from CON5 (CON4 will be disconnected). With it in place, the headphones will be connected in parallel and both will get audio (but possibly not at the same volume!). A close-up photo of the way the heatsinks are fitted. This is from the opposite side to that shown in Fig.11. Australia's electronics magazine January 2025  37 If you’ve only fitted CON5, it doesn’t matter if you put a jumper on JP1. If you’ve only fitted CON4, you must add the jumper, or it won’t work. While CON4’s ground is disconnected without JP1 if a plug is inserted in CON5, due to the way headphones are wired, you might still get some sound out of headphones still plugged into CON4. It’s unlikely to be anywhere near full volume, though. If it bothers you, simply unplug the unused pair. Case preparation & installation Preparing the case is relatively straightforward: all the holes to be made are in the front and rear panels, and they are all round, so you can use a drill (a stepped drill bit makes it easier). The locations are shown in Fig.12. There are six holes to make in the front panel and five at the rear, from 3mm to 10mm in diameter. You can download a PDF of Fig.12 from siliconchip.au/Shop/19/7406, print it out at actual size, cut it out and stick it to the panels using weak glue or scotch tape. Drill small pilot holes as accurately as you can in the centre of each location, then remove the templates and drill them out to the sizes shown. Deburr the holes and check that the panels fit over the assembled PCB in the case. You may need to slightly enlarge some holes if their locations are not perfect. The bottom of the case can be identified as it has four small circular recesses for feet. Stick small rubber feet in or near those locations, then secure the PCB to the base using four small self-tapping screws. Remove the nuts from the jack sockets, slot the lid on top, then push the front and rear panels in place. After that, you can attach the knobs. Our initial prototype was designed with the potentiometer and socket shafts essentially being flush with the front panel, so we couldn’t reattach their nuts. We didn’t think that was a problem as it seemed robust enough without them. Still, we made some adjustments to the final PCB so that the on/off switch, volume control pots and 3.5mm jack socket are closer to the front. That means you should be able to get the nuts back on the pots, which will provide a bit of extra rigidity, and it will make plugging into the 3.5mm socket easier, although you probably won’t be able to get its nut on. We have kept the front of the 6.35mm socket close to being flush with the front panel as we think it’s neater, and it’s mechanically secure enough without it. Using it It’s generally a good idea to wind VR1 & VR2 back to zero (or close to it) before playing audio if you don’t know if the levels set previously are appropriate. Then slowly advance the volume control associated with the input (VR1 for CON2 & VR2 for CON3) until you reach a comfortable volume level. It’s best to avoid ‘live plugging’ headphones as they can short the outputs when doing so. It will probably be OK, but it’s safer to switch the device off before plugging or unplugging. We also suggest you remove the headphones/earphones when switching the amp on or off to avoid any painful clicks or pops that may occur. This will also protect you in case you switch it on and the volume level is set too high. The amp draws no power when switched off, although AC plugpack will draw some power from the mains even when it has no load. So if you want to minimise power consumption when the amp is off, switch off the plugpack at the wall or unplug it when not in use. If you ever have to get the case apart again, it’s a bit tricky but it can be done. Remove the knobs and nuts, then detach the front panel on the switch side. The rear panel is almost impossible to remove once assembled as the RCA sockets prevent you from flexing it in such a way to release the tabs, so don’t try. Once you have the front panel off on one side, pull at the bottom on the jack socket side and squeeze the main part of the case in, and it should pop off. You can then gently lever the top off and pull it forwards to release the SC rear panel. Fig.12: the front (top) and rear (bottom) panel drilling details. Depending on how accurately you drill the holes, you may need to enlarge some slightly before the panels will snap into place. It’s best to start them all small and then increase them by a couple of millimetres at a time until they’re at full size. If building a single-channel version, only drill the two 9mm holes corresponding to the RCA sockets you have fitted (and the same for the 7.5mm potentiometer holes at the front). 38 Silicon Chip Australia's electronics magazine siliconchip.com.au 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. Programmable current indicator add-on for USB power banks USB power banks are not just useful for charging mobile phones; they are a handy way to power all manner of 5V devices, including when prototyping. Many power banks have a few LEDs as a simple charging state indicator. Others contain small LCD screens that display the charging state as a percentage. However, very few include a voltage indicator, and even fewer offer a current indicator. For those who experiment with electronic circuits, adding a simple current indicator to the power bank can be very helpful. This circuit combines a pre-made 10-LED bar graph with an 8-pin PIC microcontroller to create a simple dot/bar graph current indicator. There are already some specialised ICs that do this, but most of them are now becoming difficult to find. The microcontroller is inexpensive and can do a similar job. It can be a more cost-effective solution that provides better power efficiency, as well as the ability to update or modify the software without changing the hardware. The PIC12F1822 8-bit microcontroller is powered by a 3V micropower low-dropout (LDO) linear voltage siliconchip.com.au regulator; the MCP1700-30 has a quiescent (idle) current of just 1.6μA. This circuit relies on the supply not exceeding 3V because in both dot and bar modes, only one LED is driven at a time, with a minimum amount of current in short bursts, for energy saving reasons. If the supply exceeded 3V, two LEDs could be lit simultaneously (most likely including one of the red LEDs, as they can have a forward voltage as low as 1.65V). To drive the LEDs, We use a method called Charlieplexing that we described in an article on that topic in the September 2010 issue (siliconchip. au/Article/287). It allows us to drive up to 12 LEDs with only four GPIOs pins; in this case, RA0, RA1, RA4 and RA5. Four 56W current-limiting resistors are used. Pushbutton S1 allows the user to switch between a dot and bar display. In dot mode, the PIC runs at just 31kHz for the lowest possible power consumption. In bar mode, it has to run faster, at 4MHz. That’s necessary for fast enough LED multiplexing to avoid noticeable flicker. To measure the load current, a simple low-value shunt resistor is Australia's electronics magazine connected in series with the load’s ground connection. As a result, 0-0.1V is fed to the AN2 analog input of IC1. It measures that voltage as a proportion of its internal 2.048V reference voltage. This measurement, performed by a 10-bit ADC with 1024 steps (210), gives a resolution of 2mV. That corresponds to a current resolution of 20mA. The software is written to measure currents up to 1A. An optional (but useful) piezo alarm circuit is also shown, which will alert the user when the load current exceeds 1A. If you don’t need it, omit Q1, PB1 and L1. The software is available to download from siliconchip.com.au/ Shop/6/603 Mohammed Salim Benabadji, Oran, Algeria. ($90) Circuit Ideas Wanted Got an interesting original circuit that you have cleverly devised? We can pay you by electronic funds transfer, credit or direct to your PayPal account. Or you can use the funds to purchase anything from the Silicon Chip Online Store. Email your circuit and descriptive text to editor<at>siliconchip.com.au January 2025  39 LED Voltmeter/Ammeter, measuring from 3.3V to 30V The heart of this voltmeter/ammeter is the STC15W408AS microcontroller. It has a 10-bit analog-to-digital converter (ADC) with eight channels. It does not need a crystal to run and can be programmed using the STC Windows application and a USB-to-serial bridge like the CP2102. The voltage is measured using a voltage divider. With an input of 30V, 40 Silicon Chip the output of the voltage divider is 5V. The current is measured using an Allegro ACS712 5A Hall effect current sensor. It produces a 2.5V output when there is no current flow, increasing by 185mV per amp. To get the current reading, after converting the 0-5V output to a value between 0 and 1023, 512 is subtracted to remove the 2.5V offset. Australia's electronics magazine IC2 shows the voltage and current readings on two four-digit, seven-­ segment displays. The voltage for the common anodes is applied using one 2N3906 PNP transistor for each with 1kW series resistors to limit the base current and 5.1kW resistors to switch off the transistor when the corresponding microcontroller output pin is high. The cathodes siliconchip.com.au are driven directly by microcontroller pins. An MC34063-based buck/boost converter is used to power the circuit. It can operate from 3.3V to 30V. The 1kW/330W divider configures it to produce a 5V DC output. The seven-­ segment display anodes need 2.5V, so VLED is derived using an LM317 adjustable linear regulator. The voltmeter/ammeter can measure down to 3.3V, but the buck/boost siliconchip.com.au regulator has the lowest efficiency at this voltage, so ideally, the lowest voltage should be 4V. To program the STC15W408AS, use the STC Windows program (version 6.88E recommended: www.stcmicro. com/rjxz.html). First, connect a USBto-serial bridge to the microcontroller’s serial port. Disconnect power to the board, click Check MCU, then connect the power. To download the code, click on File, then Open Code File. Navigate to the hex file and click on Open. Disconnect the power from the board, click on Download Program, then connect the power. The seven-segment LEDs connected to the serial port pins will not affect programming the microcontroller since the anodes of the seven-­ segment display will not be driven during that process. The bootloader inside the microcontroller checks for the proper data from the STC software. If there is no data from the serial port after some time, the microcontroller runs its firmware. I have produced a PCB design for this, available to download as Gerber files, along with the firmware, from siliconchip.au/ Shop/6/458 My PCB is quite big because it is a prototype; a smaller double-sided version could be designed. REG1’s internal reference could be 1.21.3V, so the two equal external resistors will give an output of 2.42.6V but the exact voltage is not critical. Noel A. Rios, Manila, Philippines ($90). Australia's electronics magazine Handbag light I was asked to design a light for a handbag that switches on when the bag is opened. They suggested that common magnetic press studs could function as a switch, although a magnetic reed switch could also be used. I came up with this circuit; the key is to minimise the standing current despite the switch being ‘backwards’, ie, closed when we want the light to be off and open when it should be on. A Darlington NPN transistor (KSP13) controls an ultra-bright white LED (Jaycar ZD0290). The published minimum current gain for the transistor at 10mA is 5000 but I have measured typical values of about 25,000. With a 10MW base resistor and a 9V battery, the standby current will be about 0.9μA. I found that sufficient to drive the transistor close to saturation, generating a maximum current of about 13mA through the LED, adequate to light up the interior of a handbag. When the magnetic press stud is closed, the quiescent current of 0.9μA will have a negligible effect on battery life. I built the circuit on the back of a pin-on brooch from an opportunity shop, using old-fashioned point-to-point wiring. However, a simple PCB could also be used. My female friends have received the prototype with enthusiasm. James Goding, Princes Hill, Vic. ($40) Editor’s note: a Mosfet like the 2N7000 could be used instead of the KSP13 to eliminate the base current and any concern about the transistor’s gain. However, it might be more easily damaged by static electricity. January 2025  41 Precision Electronics Part 3: difference & instrumentation amplifiers In this third article in this series, we will further develop our precision current measuring circuit. We will consider how to sense the current if the shunt was in the positive line instead of referenced to circuit ground. By Andrew Levido Y ou will recall that previously, we sensed a 0 to 1A current using a 100mW resistor, one side of which was connected to circuit ground. We amplified the resulting voltage by a factor of around 25 to get a ground-­ referenced signal of about 2.5V fullscale, which we could apply to an analog-­to-digital converter (ADC) to make the measurement. We are assuming there is a microcontroller in the circuit that can trim out much of the fixed offset and gain error, leaving us with a trimmed precision of around ±0.04% at 25°C with about ±0.075% additional error over the 0–50°C operating temperature range. We deemed this overall precision of just over 0.1% ‘good enough’ for our purposes. In practice, we often want to sense the current in the positive leg of the circuit, as shown in Fig.1. The reason is that it is possible (sometimes even unavoidable) that the grounds of both the source and the load are connected to a common potential, such as mains Earth. If this were to happen with the original circuit, the sense resistor would be shorted out, so measuring the current would be impossible. Moving the shunt resistor to the positive line solves that problem but introduces another. One terminal of the resistor is sitting at the load’s positive supply voltage (up to 20V in our example), while the other is up to 100mV higher, depending on the current through it. We are interested in amplifying only the difference in voltage between these two points, not the much larger voltage on which it is floating. We also want the resulting amplified signal to be referenced to circuit ground so it’s within the ADC’s range, and so we don’t need to use a differential ADC to measure it. Fig.1: to measure current with a sense resistor in the positive line, we need to extract the relatively small differential signal from the larger common-mode signal. Fig.2: to achieve what we need in Fig.1, the “Signal Conditioning” box needs to amplify Vdm with a high gain (Gdm) but minimise the contribution of Vcm, meaning Gcm should be kept low. 42 Silicon Chip Differential and common mode signals In cases like this where we have two sense terminals, we refer to the voltage between them as the differential-mode voltage (Vdm) and the voltage at the terminals with respect to ground as the common-mode voltage (Vcm). This is shown diagrammatically in Fig.2. The differential voltage of interest (Vdm) is ‘riding on’ the common-mode voltage (Vcm) that we want to ignore. For ground-referenced signals, the common-mode voltage is zero (in an ideal world, anyway). The output of the generalised conditioning circuit block will be a voltage that is the sum of the differential-­ mode input (Vdm) multiplied by a differential-­ mode gain (Gdm), along Australia's electronics magazine with the common-mode input (Vcm) multiplied by the common-mode gain (Gcm). We usually want Gcm to be zero (or as close to it as we can practically get) so that the unwanted common-­ mode voltage is rejected. We describe the degree to which a circuit like this rejects common mode signals as the common-mode rejection ratio (CMRR). This is the ratio of the differential-mode gain to the common-­ mode gain (Gdm ÷ Gcm) and is usually expressed in decibels, calculated as 20log10(Gdm ÷ Gcm). You have probably guessed by now that any hope of perfectly rejecting common-mode signals (ie, achieving an infinite CMRR) is just a pipe dream. The harsh reality of electronics design means we always have to put up with something less than perfection. Difference amplifiers One of the most common ways to amplify a small differential signal riding on a large common-mode voltage is to use a difference amplifier like that shown in Fig.3. Two pairs of matched resistors (R1a = R1b & R2a = R2b) and an op amp form an amplifier with some very interesting characteristics. This general form of difference amplifier (with separate sense and reference terminals) is a very flexible Fig.3: the classic difference amplifier using an op amp and four resistors is a very useful and flexible circuit. Usually, the value of R1a is the same as R1b and R2a the same as R2b. siliconchip.com.au circuit that can be used to implement a wide variety of functions, as shown in Fig.4. All those circuits use a difference amplifier with unity gain (R1a = R1b = R2a = R2b). The terms “difference amplifier” and “differential amplifier” are often used interchangeably. I am using the former term to describe any amplifier in which the output is proportional to the difference between the input voltages. Some sources use “differential amplifier” as the general term and “difference amplifier” to describe the specific configuration where the differential-­mode gain is equal to one (eg, siliconchip.au/link/ac1h). To add to the confusion, the terms “differential amplifier” and “fully differential amplifier” are both used to describe op amps with complimentary positive and negative outputs. These are specialised devices are normally used to drive high-speed twisted pairs or differential input ADCs Getting back to the device itself, connecting the sense terminal of a difference amplifier to the output and the reference terminal to ground produces the familiar configuration illustrated in Fig.5. This has a differential-mode gain of Gdm = R2 ÷ R1 (where R1 = R1a = R1b and R2 = R2a = R2b). The common-mode gain of the difference amplifier would be zero if the op amp was ideal and the resistor matching was perfect. If you build a difference amp with a typical op amp with an open loop gain of 100,000 and 1% resistors, the CMRR would be in the region of 34dB. This means you would see about 1/50th of the common-mode voltage at the output. That would equate to 400mV in our example, almost half of the differential-mode signal we are interested in! We can do better with matched resistors. For example, using the ACASA range of resistor arrays we used last time (matched to within 0.05%), we would have a CMRR in the order of 60dB. That still means we would see a common-mode voltage of up to 20mV at the output, which is clearly not good enough for our application. You can buy integrated difference amplifiers with on-board lasertrimmed resistors that have CMRR values in the 80-100dB range at modest cost. If we used one of these, say with a CMRR of 90dB, the common-­ mode voltage at the output would be just 632µV. That is pretty good, but it still represents a 0.025% error, which will have to be added to the other errors. There is a bigger problem, however. Off-the-shelf difference amplifiers are typically only available with gains up to about 10, with most having a gain of just one or two (we will see why a little later). Another limitation of difference amplifiers is their relatively low input impedance, typically in the range of 10-500kW. That is not much of a problem with a very low impedance source such as our 100mW shunt, but it becomes more of a concern as the source impedance rises. You can see from Fig.5 that any source impedance will be in series with the difference amplifier’s input resistors R1a and R1b, potentially impacting both the gain and CMRR. A good rule of thumb is to make sure the source impedance is lower than the input impedance of the difference amplifier by the same order of magnitude as the CMRR. So, for a difference amplifier with a 90dB CMRR and 10kW input resistors, the source impedance should be less than 316mW. Any higher than that and the CMRR will be adversely impacted. Fig.5: this configuration delivers a ground-referenced voltage proportional to the difference between the input voltages; Vout = (R2 ÷ R1) × (Vin+ – Vin–). In fact, the data sheets generally specify CMRR with an input source impedance of 0W. That is obviously a totally unrealistic scenario – yet another reason to be wary of data sheet claims. You might think that the CMRR would be maintained if you had equal source impedances on each input, since both input resistors would be increased by the same amount, but no such luck. The manufacturer’s laser-trimming matches the R1/R2 ratios in each divider, not necessarily their absolute values, which may be a bit different. Adding the same source resistance to both inputs will likely unbalance the ratios, making the CMRR worse. By now you might be asking why we should even bother with difference amplifiers if they have all these limitations. Apart from the flexibility we have already seen, and their role in instrumentation amplifiers that we will discuss soon, the difference amplifier excels in the area of input common-mode voltage range. With the right resistor values, the common-mode voltage can extend well beyond the op amp’s power supply rails. Off-the-shelf devices are readily available with common mode input ranges better than ±100V. I have built discrete difference amps with Fig.4: eight possible ways to use the Fig.3 circuit to achieve different gains, level-shift signals and even sum/average voltages. siliconchip.com.au Australia's electronics magazine January 2025  43 Table 1: error budget for Fig.8 using an INA821 At Nominal 25°C Abs. Error Error Nominal Value Shunt Resistor: RESI PCSR2512 (0.5%, 15ppm/˚C) 100mW Differential Voltage due to I × Rshunt 100mV 0.5mV InAmp: INA821 (Vos ±35µv, 5µV/˚C) 0mV 0.035mV InAmp Input Voltage total (Line 2 + Line 3) 100mV 0.535mV 0.54% 0.1625mV 0.163% InAmp Gain Resistor Rg: RN73C2A (0.1%, 10ppm/˚C) 2kW 2W 0.10% 0.5W 0.025% InAmp Gain (Line 5 × Line 6) 25.7 0.0296 0.12% 0.0289 0.113% Vout DM (Line 4 × Line 7) 2.57V 0.0167V 0.65% 0.0071V 0.275% Vout CM (20V, 120db, ±1.5db over 0-50˚C) 0V 0.02mV Vout (Line 8 + Line 9) 2.57V 0.0167V Instrumentation amplifiers One obvious solution to the difference amplifier’s input impedance problem is to add a pair of unity-gain input buffers in front of the input resistors, as shown in Fig.6. This solves the input impedance problem (at the expense of common mode voltage range), but does nothing to help us reach higher gains or achieve better CMRR. The classic three-op-amp instrumentation amplifier (or ‘inamp’) shown in Fig.7 is a neat solution to the problem. The two input op amps now work to maintain the differential-­ mode voltage across resistor Rg. With this understanding, it is pretty easy to show that this input stage has a differential mode gain of Gdm = 1 + 2 × (R3 ÷ Rg) and a common mode gain of Gcm = 1. We can see that with the right choice of resistor values, this input stage can improve the overall circuit’s differential gain but, given it has a common-­ mode gain of one, it may not be as obvious how this front-end can improve the overall CMRR. Consider a situation where we want an overall differential gain of 100 and the highest possible CMRR. Imagine the difference amplifier has a differential gain of 1 and a CMRR of 80dB. The input stage will have a differential gain of 100 and a common-mode gain of 1, giving a CMRR of 40dB. The second stage adds 80dB of additional CMRR for a total circuit CMRR of 120dB. The instrumentation amp is effectively a gain stage with a CMRR equal to the gain, followed by a common-­mode rejection stage with a differential gain Fig.7: the classic threeop amp instrumentation amplifier consists of a high impedance gain stage made up of two op amps followed by a difference amplifier. This can provide both higher gain and improved CMRR compared to difference amplifier alone. Silicon Chip 0.50% Australia's electronics magazine Rel. Error 0.038% 0.0375mV 0.038% 0.125mV 0.02% Fig.6: this circuit fixes the low input impedance exhibited by difference amplifiers but it limits the input voltage range and does not add gain. 44 Abs. Error 0.50% InAmp Gain Error (0.015% ±35ppm/˚C) common mode voltages up to ±300V without problems (but a lot of care). Rel. Error 0-50°C (Nominal ±25°C) 0.088% 0.0038mV 0.65% 0.0071V 0.275% of unity or thereabouts. You can now see why lots of the difference amps on the market favour CMRR over gain – they are intended for use in instrumentation amplifier applications. Another nice feature of the instrumentation amp is that the gain can be set by changing just one resistor, Rg. This means practical devices can have precision-trimmed matched resistors R1a/b, R2a/b and R3a/b, leaving the user to provide Rg externally to set the gain. You can even switch in different resistors to change the gain or use a potentiometer to trim it. A typical example of an off-theshelf instrumentation amplifier is the INA821 from Texas Instruments (TI). The data sheets show it has a CMRR of 112dB for Gdm = 10 and 132dB for Gdm = 100. This suggests they are getting 92dB of CMRR from the difference amp stage (and 20dB or 40dB from the input stage). The input impedance is 100GW, which should be high enough for pretty much any source impedance. The cost of the INA821 is about $8.60 in single quantities, which is much cheaper than anything you could build yourself, given the very tight-tolerance resistor matching required. Let’s go through the process of designing the circuit of Fig.8 to compare with the ground-referenced circuit we built last time. We will build up the error budget shown in Table 1 as we go. Fig.9 shows the internal configuration of the INA821. We need a gain of around 25, so we will choose Rg to be 2kW, giving a gain of 25.7. The tolerance of this resistor is not critical since we’ll trim the gain, but we do care about its tempco. For this reason, I chose the RN73C2A2K0BTD from TE siliconchip.com.au Table 2: error budget for Fig.8 using an LT1167A instead At Nominal 25°C Abs. Error Error Nominal Value Shunt Resistor: RESI PCSR2512 (0.5%, 15ppm/˚C) 100mW Differential Voltage due to I × Rshunt 100mV 0.5mV InAmp: LT1167A (Vos ±40µv, 0.2µV/˚C) 0mV 0.04mV InAmp Input Voltage total (Line 2 + Line 3) 100mV 0.54mV 0.54% 0.0425mV 0.043% InAmp Gain Resistor Rg: RN73C2A (0.1%, 10ppm/˚C) 2kW 2W 0.10% 0.5W 0.025% InAmp Gain (Line 5 × Line 6) 25.7 0.0308 0.12% 0.0129 0.050% Vout DM (Line 4 × Line 7) 2.57V 0.017V 0.66% 0.0024V 0.093% Vout CM (20V, 106db over 0-50˚C) 0V 0.1002mV Vout (Line 8 + Line 9) 2.57V 0.0171V Fig.8: an off-the shelf instrumentation amplifier (‘inamp’) can provide the necessary gain (about 25) with around 120dB of common-mode rejection. siliconchip.com.au Abs. Error 0.50% InAmp Gain Error (0.02% ±10ppm/˚C) Connectivity. It has a tolerance of 0.1% and a tempco of ±10ppm/°C. The INA821’s input common mode voltage range extends to within 2V of either supply rail, so we need a power supply of 22V or more on the positive side and -2V or more on the negative side. I am going to assume we have a +24V DC supply available, since this would be the sort of input the power supply’s series pass stage would need. I have already used ±5V power rails in my previous experiments, so I will power the instrumentation amplifier from +24V and –5V rails. The INA821 has a maximum power supply voltage of 36V, so this should be fine, with a total of 29V applied (24V + 5V). It is worth noting that it is quite OK to power op amps asymmetrically like this, as long as you understand that the input common mode range and output swing will likewise be asymmetrical. We can now complete the error budget table (Table 1). The first 8 lines of the table are similar to the previous examples, arriving at a cumulative error of 0.65% with an additional Rel. Error 0-50°C (Nominal ±25°C) 0.50% 0.038% 0.0375mV 0.038% 0.005mV 0.02% 0.275% error over the 0°C to 50°C temperature range. Unlike the previous circuit, we now need to add the error due to the common-mode signal making its way through to the output. With a gain of 25.7, we can estimate the CMRR to be 120dB based on 92dB for the difference amp stage plus 20log10(25.7) = 28dB for the input stage. With a common-mode voltage of 20V, we will therefore see 20µV at the output. That’s insignificant compared to the 16mV of error due to the differential-­mode stage. The change in CMRR with temperature is a bit harder to estimate. TI provides graphs that show the temperature variation of CMRR for five sample devices at gains of one and ten. From these, I have taken a value ±1.5dB over 0°C to 50°C. It is a bit of a guesstimation, but it does not matter since the overall level of common-mode feedthrough is so low as to make this figure irrelevant. The net result is shown therefore shown at the bottom of Table 1. The worst-case untrimmed error at 25°C is ±0.65%, just a little worse than the ±0.55% error for the ground-­ referenced circuit. In both cases, most of this error is the 0.5% shunt resistor tolerance. Rel. Error 0.025% 0V 0.66% 0.0024V 0.093% Unfortunately, the circuit’s performance over the temperature range is not great. We are seeing ±0.275% error, with two major contributors: the instrumentation amplifier’s input offset voltage drift and its gain drift. The LTC2057-based circuit was much better at 0.075%, as we would expect from an auto-zero op amp. Doing better – but at a price I wanted to see if we could improve on this, so I looked for a ‘better’ instrumentation amp. The LT1167A fits the bill. Its input offset voltage at 25°C is similar to the INA821, but its offset drift is 25 times better at 0.2µV/°C. Its gain drift with temperature is also better at ±10ppm°/C, compared with the ±35ppm/°C. Table 2 shows the error budget for this version of the circuit. As an aside, it’s a good idea to create these error budget tables in a program like Excel or LibreOffice Calc. I set up the formulas so that I can easily try new parts and have the whole table recalculate automatically. Compared to the INA821, the new circuit shows a similar error at nominal temperature of ±0.66%, but an error over the temperature range three times better at 0.093%. So, we should use this device, right? Well, the LT1167A costs $30 each in one-off Fig.9: the INA821 has six laser-trimmed precision resistors and three op amps. The user must provide an external resistor (Rg) to set the overall gain. Australia's electronics magazine January 2025  45 Fig.10: the measured untrimmed data for the INA821based circuit shows about 0.3% gain error; most of this is due to the shunt resistor tolerance. quantities, so we would want to be certain there was no alternative. It should however come as no surprise that precision components that are at the very extremes of performance will be costly. The manufacturers know full well that if there are no or few alternatives, you will have to pay up. Test results I spared no expense and tested both devices. I built the circuits and measured the input current vs output voltage characteristics with both zero and the full common-mode voltage of 20V. The results for VCM = 20V are shown in Tables 3 and 4, and plotted in Figs.10 & 11. For the INA821, the untrimmed errors range from 0.01% at zero current to around 0.33% at 1A. The results were a little better with zero common mode voltage. As expected, this is better than our error budget’s 0.65% worst-case estimate. The errors increase steadily with the magnitude Fig.11: the untrimmed data for the LT1167A-based circuit shows the same 0.3% gain error as Fig.10 but has more offset error. It should perform better over the temperature range. of current, suggesting a gain error is the main contributor. The graphed results and line of best fit shows this to be the case. The offset correction we need to apply is very low (around 250µV) and the gain error is about 0.3% (the measured gain is about 0.3% higher than we expect). Again, the shunt resistor with its 0.5% tolerance is likely to be the culprit. After correcting the results, we get a trimmed error of ±0.03%, very comparable with the ground-referenced circuit. However, our concern with the INA821 circuit is its performance over temperature. The measured CMRR of this circuit was 106dB – not as good as the estimates of 120dB, but nevertheless acceptable. It’s actually a bit difficult to measure CMRR, since things like op amp input offset voltage can also change over the common-mode range, and it’s impossible to isolate the causes with a simple output voltage measurement. The LT1167A circuit has worse Table 5: theoretical improvement to Table 1 with dynamic zero trim untrimmed accuracy, peaking at almost 0.47%, but again the graphs show it to be almost all gain error. After trimming, the error is reduced to ±0.025%, very similar to the INA821. The temperature coefficient is better, of course. Another solution Rather than commit to a $30 chip, I want to introduce another trick we can use to improve precision in this type of situation. So far, we have applied fixed offset and gain corrections to minimise the static errors in the circuit. In practice, this would be done for each sample in software, based on some one-off calibration performed at a standard temperature when we initially set up the instrument (and maybe when we periodically re-calibrated it). Another approach might be to try to obtain the corrections in real-time at the ambient operating temperature. High-end instruments, like the 6½ digit multimeters that I used to obtain At Nominal 25°C Abs. Error Rel. Error 0-50°C (Nominal ±25°C) Error Nominal Value Shunt Resistor: RESI PCSR2512 (0.5%, 15ppm/˚C) 100mW Abs. Error Differential Voltage due to I × Rshunt 100mV 0.5mV InAmp: INA821 (Vos ±35µv, 5µV/˚C) – zero trimmed 0mV 0.035mV InAmp Input Voltage total (Line 2 + Line 3) 100mV 0.535mV 0.54% 0.0375mV 0.038% InAmpGain Resistor Rg: RN73C2A (0.1%, 10ppm/˚C) 2kW 2W 0.10% 0.5W 0.025% 0.50% InAmp Gain Error (0.015% ±35ppm/˚C) 0.50% Rel. Error 0.038% 0.0375mV 0.038% 0mV 0.02% 0.088% InAmp Gain (Line 5 × Line 6) 25.7 0.0296 0.12% 0.0289 0.113% Vout DM (Line 4 × Line 7) 2.57V 0.0167V 0.65% 0.0039V 0.150% Vout CM (20V, 120db, ±1.5db over 0-50˚C) 0V 0.02mV Vout (Line 8 + Line 9) 2.57V 0.0167V 46 Silicon Chip Australia's electronics magazine 0.0038mV 0.65% 0.0039V 0.150% siliconchip.com.au Measured Data I (mA) Fig.12: we can improve the temperature-dependent error of the circuit by adding switches to dynamically measure the offset, like an auto-zero op amp. the results shown here, effectively perform a zero and full-scale calibration every 20ms measurement cycle. Any temperature drift is calibrated out more-or-less in real-time. We are not aiming for anything near that level of precision, but a simpler version can be a useful technique. It is pretty difficult to do a full-scale calibration of our test circuit, as we would need a precision 1A current source, but we could do a zero calibration fairly easily. This won’t let us trim out gain drift due to temperature but would let us calibrate out temperature-dependent offset errors in real-time – a bit like auto-zero op amps do. Let’s take a look at this approach using the INA821 example. Looking at the error budget table, we can see in line 3 that we have a possible ±125µV drift in offset voltage over the temperature range. If we could calibrate that out, as shown in Table 5, we would almost halve the temperature error from ±0.275% to ±0.15%. Fig.12 shows one way we could achieve this in practice. Normally, S1 would be closed and S2 open so that we could take current measurements as before. Opening S1 and closing S2 shorts the inputs of the instrumentation amplifier so that we can use the ADC to read the circuit’s offset voltage. We would still need a fixed gain correction as before, but we can use the zero-scale reading to create a dynamic offset correction that will eliminate some of the temperature drift error. Extending the range Let’s regroup and consider what we have achieved so far. siliconchip.com.au Untrimmed Error Vout (mV) Absolute (mV) Trimmed Error Relative Absolute (mV) Relative 0.000 0.233 0.23 0.01% 0.48 0.019% 99.726 256.654 0.36 0.01% -0.19 -0.007% 199.824 514.948 1.40 0.05% 0.05 0.002% 299.980 772.739 1.79 0.07% -0.36 -0.014% 400.008 1031.164 3.14 0.12% 0.19 0.008% 499.980 1289.040 4.09 0.16% 0.34 0.013% 600.007 1546.980 4.96 0.19% 0.41 0.016% 699.965 1804.750 5.84 0.23% 0.49 0.019% 800.024 2062.770 6.71 0.26% 0.56 0.022% 899.971 2320.490 7.56 0.29% 0.61 0.024% 999.866 2578.110 8.45 0.33% 0.70 0.027% Table 3 – untrimmed measured results from the INA821 circuit shown in Fig.8. Measured Data Untrimmed Error Trimmed Error I (mA) Vout (mV) Absolute (mV) Relative Absolute (mV) Relative 0.000 1.614 1.61 0.06% 0.46 0.018% 99.759 258.745 2.37 0.09% 0.14 0.005% 199.898 517.182 3.44 0.13% 0.14 0.005% 299.829 775.181 4.62 0.18% 0.24 0.009% 400.044 1033.716 5.60 0.22% 0.15 0.006% 500.013 1291.840 6.81 0.26% 0.28 0.011% 600.390 1549.970 6.97 0.27% -0.64 -0.025% 700.009 1807.980 8.96 0.35% 0.28 0.011% 800.060 2066.110 9.96 0.39% 0.20 0.008% 899.975 2323.980 11.04 0.43% 0.21 0.008% 999.872 2581.780 12.11 0.47% 0.20 0.008% Table 4 – untrimmed measured results from the INA821 circuit shown in Fig.8 when replaced with an LT1167A. We have shown that with the shunt in the positive supply, we can probably achieve a trimmed accuracy of around 0.03% at 25°C with an additional 0.15% error over the 0–50°C temperature range if we use the INA821 instrumentation amplifier and dynamic offset correction. Let’s call this 0.2% of total error. This suggests we will have an overall resolution of ±2mA in our 1A current (ignoring ADC precision for now). That is not good enough to measure the microamp resolution we would like to achieve. I hope it is clear by now that we are not going to get the required three orders of magnitude improvement in precision just by improving the signal conditioning circuit. Even if we could, we will run into ADC quantisation limits, which we will cover in a later article. Australia's electronics magazine The current circuit needs an ADC with at least 10 effective bits of resolution – three more orders of magnitude would require over 33 bits of effective resolution, which is pushing the limits of what is possible! There is another way. We could pretty easily scale the range of the circuit by using a different shunt resistor. For example, using a 10W resistor would give a range of 0 to 10mA with ±20µA resolution, while a 1kW resistor would yield a range of 100µA fullscale with ±200nA resolution. That will require some additional circuitry to switch the ranges. This, and the dynamic offset zeroing, will require us to add some switching elements to our signal path, which will themselves introduce some imprecision. We will look more deeply into signal switching in the next instalment of this series. SC January 2025  47 Mini Projects #018 – by Tim Blythman SILICON CHIP Gesture-Controlled USB Lamp We designed this circuit to work with a lamp, but it could control just about any USB-powered device (rated at 5V). You could add an IR receiver for IR remote control, or an LDR to make it an automatic night light. By waving your hand over the small purple module, you can switch power to the USB socket; perfect for controlling a USB lamp. B ack in March 2022, Jim Rowe wrote about gesture recognition modules such as Jaycar’s XC3742. These nifty little modules are capable of recognising about 10 different hand gestures using an integrated IR pixel array (siliconchip.au/Article/15247). Now, we’re using this module to control a USB lamp. Since this project switches power to a USB socket, it could be used to switch any number of devices that run from USB power. You can see a video of it working at siliconchip.au/Videos/Gesture+Lamp We built it on a prototyping shield that sits above a Leonardo main board. That makes it easy to tweak the circuit if you wanted to make modifications. Circuit Fig.1 shows the circuit. Apart from the Leonardo board, all the parts shown there are fitted to a prototyping shield. There are two sections; on the right is the gesture recognition module, while on the left we have the USB power switching circuit. The wiring to the gesture recognition module is simple enough. It just needs connections from 5V, GND, SDA and SCL to the module, which incorporates the I2C pullup resistors and a voltage regulator to power the onboard chip. We’re using a pair of transistors to switch power to the USB socket’s positive (5V) pin. The ground pin is permanently connected. Pin A0 (which can be used as an analog input) is configured as a digital output. This keeps the wiring on the prototyping shield neat. When A0 is pulled high, about Parts List – Gesture-based USB Lamp (JMP018) 1 Arduino Leonardo [Jaycar XC4430] 1 Arduino prototyping shield [Jaycar XC4482] 1 Hand Gesture Recognition Module [Jaycar XC3742] 1 1kW 5% (or better) axial ¼W (or more) resistor [Jaycar RR0572] 1 120W 5% (or better) axial ¼W (or more) resistor [Jaycar RR0550] 1 PCB-mounting USB Type-A socket [Jaycar PS0916] 1 TIP32 40V 3A PNP transistor, TO-220 [Jaycar ZT2290] 1 BC546, BC547, BC548 or BC549 100mA NPN transistor, TO-92 [Jaycar ZT2154] 1 30cm length of insulated wire in various colours [Jaycar WH3032] 1 USB-A to micro-USB cable to suit the Leonardo board 1 USB-powered light or similar device to control 48 Silicon Chip Australia's electronics magazine 4mA flows through the 1kW resistor and base-emitter junction of the NPN transistor, so it switches on and allows current to flow through its collector to its emitter. This, in turn, causes about 40mA to flow through the PNP transistor’s base via the 120W resistor, which switches it on as well. That means that 5V is available at the USB socket to power a connected device. When A0 is held low, both transistors are off and there is no voltage at the USB socket. It might seem unnecessary to have two transistors, but this arrangement provides enough drive to the PNP transistor to ensure it is switched fully on and does not drop any significant voltage. It also means that our input at A0 is intuitive; a high level swiches the output on and a low level swiches it off. The circuit operation depends on firmware loaded on the Leonardo, which we will discuss later. Construction We built everything on a prototyping shield to create something reasonably robust. You should be able to see how everything is wired up from the photos. We’ve used yellow wires for 5V connections, since red could be difficult to see against the red shield PCB. There is no significant wiring under the shield, so everything is visible from siliconchip.com.au Fig.1: all parts of this circuit apart from the Leonardo board are fitted to a prototyping shield. It supplies 5V to the USB socket on the left when the A0 pin is brought high. above. The only thing to note is that the four wires connecting to the gesture recognition module do so underneath the shield, connecting to the immediately adjacent wire in each case. We’ve positioned the USB socket to make use of the IC breakout pads on the shield. It also means that the USB input (to the Leonardo) and the output (on our shield) are at the same end, making external connections tidier. That puts the gesture sensor at the other end, where it can be accessed easily. Start by soldering the header onto the gesture module and then solder it to the shield. We lifted ours up slightly so it sits just below the top of the shield header sockets. Run the four connecting wires next. There is a yellow wire from 5V on the shield to Vcc on the module, as well as a single black wire for ground (GND) and two blue wires for SCL and SDA. Make sure those are routed as shown. Refer to the photo of the USB sockets that show how we’ve bent the two large tabs outwards. That allows them to be soldered to the top of the shield. The four smaller pins should slot into the pads with a bit of wiggling; the pad spacing is not quite the same as the pin spacing on the socket, but it is close enough. We have left a row of pads behind the USB socket so that wires can be attached there. Add a generous amount siliconchip.com.au of solder to the larger tabs to give them some mechanical strength, then solder the four smaller pins. Next, fit the two transistors, being careful with their orientation. In our photos, from left-to-right, the pins are (for the PNP transistor) emitter, collector, then base, followed by (for the NPN transistor) collector, base and emitter. The photo overleaf shows the wiring most clearly, also check that your wiring matches the circuit. Next, add the two resistors, with the 1kW resistor going from A0 to the NPN transistor’s base and the 120W resistor going between the PNP transistor’s base and the NPN transistor’s collector. Follow with the remaining wires as shown in the photos. Note that some 5V and GND connections are made on pads near the USB socket. They should be marked, but you can carefully follow the copper traces on the prototyping shield to be certain. Finally, plug the completed shield into the Leonardo and connect it to a computer for programming. Software The sketch is quite simple and just needs one external library for the gesture sensing; we used one of the same libraries Jim used back in 2022. It can be installed by searching for “RevEng_PAJ7620” in the library Bend the tabs on the USB socket as shown here (on the right) so they can be soldered to the top of the prototyping shield. This will provide the needed mechanical strength for devices being plugged and unplugged. Australia's electronics magazine January 2025  49 Silicon Chip PDFs on USB ¯ 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) WWW.SILICONCHIP.COM. AU/SHOP/DIGITAL_PDFS 50 Silicon Chip Follow the wiring to match our circuit (Fig.1). The wires for the gesture module connect to their adjacent pads under the PCB. That is the only wiring on the underside of the shield PCB. You can also see the generous solder blobs that we have added to the USB socket to secure it to the board. manager. Alternatively, you can install the zipped copy we are including with the software package, which can all be downloaded from siliconchip.au/ Shop/6/526 The sketch initialises the sensor. If that fails, the Leonardo’s onboard LED flashes. Otherwise, the software monitors the sensor, switching on the USB output with an ‘up’ gesture and off with a ‘down’ gesture. The Leonardo’s onboard LED also indicates the on/off state, while other debugging information is available on the serial terminal. Some of the values that correspond to other gestures are listed if you wish to change the default behaviour. To program the Leonardo board, open the sketch in the Arduino IDE, choose the correct board in the dropdown menus along with its serial port and then upload the sketch. The sketch and circuit should work without changes with the Uno Australia's electronics magazine R3 board instead of the Leonardo, although we have not tested that. Testing While connected to your computer, wave your hand above the module and verify that the Serial Monitor reports the gesture correctly. We found that moving our hand about 10cm above the sensor worked well. You will also see the Leonardo’s onboard LED switch on and off. Plug a USB device into the socket and confirm that it operates as expected. If all is well, you can plug the Leonardo into a USB power supply to untether it from your computer. The transistor and thus USB socket can deliver about 1A at most; of course, that will depend on your power supply being able to provide enough power. In our testing with a 500mA lamp, there was no noticeable temperature rise in any of the components. SC siliconchip.com.au Multi-coloured prints, uninterrupted Meet the K2 Plus CFS* Combo, the ultimate game-changer for 3D printing enthusiasts. 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It tells the time, is three feet wide and one foot tall (that sounds larger than 90 × 30cm). The BIG Clock has a simple circuit and we think our clever readers will come up with other ideas for using the BIG, bright display we have designed. W e’ve thought for a while that addressable RGB LED strips would be a good way to make a large, bright display. We thought of arranging the strips in rows to create a dot-­ matrix type display, but that would not have been as big as the BIG Clock. It is 90cm wide and 30cm tall, with the active area of the digit display being 65cm wide and 20cm tall. These strips have a connection for power, ground and data in at one end, with a matching connection at the other end for power, ground and data out. Multiple strips can be joined by simply connecting power to power, ground to ground and data out to data in. Many strips can also be cut to shorter lengths; the smaller strips can then be rejoined in the same way. We’re using Jaycar’s XC4390 WS2812B RGB LED strip. It is 2m long and contains 120 RGB LEDs. Thus, there is one LED every 16.6mm. If we had cut this into five strips of 24 LEDs (about the minimum number of rows needed to make a working dot-matrix display), it would be about 9cm tall and 40cm wide. Instead, we have arranged the LEDs as multiple 7-segment digits. If the segments have five LEDs, we can make digits that are each nearly 20cm tall Fig.1 (left): by arranging the strips in this fashion and wiring in this order, the length of wire between each segment is kept short. Photo 1 (right): here is what a single digit (showing a ‘0’) looks like up close. siliconchip.com.au Australia's electronics magazine January 2025  55 Fig.2: this segments order was arranged to simplify the wiring and matches the software mapping of the segments in the BIG Clock sketch. and 10cm wide. The 120 LEDs yield three 7-segment digits, with three segments to spare; enough to make a 12-hour clock display. You can see a video of it in operation at siliconchip.au/Videos/BIG+Clock Photo 1 and Fig.1 show the basic arrangement of a digit. Like smaller 7-segment displays, we have tilted the segments about 10° from vertical. The digits are around 9cm wide and 18cm tall, with the segments each about 8.3cm long. Fig.1 also shows the way we have wired the segments in each digit. You can see that this keeps the wiring quite short and tidy. Fig.2 shows the pattern we used to wire all the segments of the digits on our BIG Clock, with the output of one numbered segment going to the input of the segment numbered one higher. We used two spare segments to create a leading ‘1’ (#1 & #2) to show hours up to 12. This leaves a single segment spare, which we used as a dash (#10) to separate the hours and minutes. Circuit details The LEDs are controlled by an Arduino Uno WiFi R4 microcontroller board. Its inbuilt WiFi radio can be used to fetch the time using NTP (network time protocol) from the internet. To provide a discreet (and discrete) interface, we added a magnetic reed switch to allow daylight savings to be switched off and on. The circuit is pleasingly simple. All we need is a microcontroller to provide the necessary serial signal to produce the clock display. This digital signal comes from the A0 pin. Although it can be used for analog functions, we have used it as a digital output since it is close to the other (5V and GND) pins needed to drive the LEDs. The reed switch is connected between A2 and GND; an internal pullup means this pin is high unless a magnet is nearby, when the switch shorts the pin to ground. Fig.3 shows the circuit. Construction Laying out and connecting the segments is the most time-consuming part of the construction process. If you want to test the LEDs before or during assembly, jump forward to the Software section so that you can load up the libraries or a test sketch to do so. We used a 900mm x 600mm sheet of Corflute cut in half lengthwise, giving a panel 300mm tall and 900mm wide. Corflute is like corrugated cardboard but made from plastic. The corrugations run parallel to the long side, which is helpful when sketching out your plans. Fig.4 shows the critical dimensions of a single digit and its relation to adjacent digits. Using a plastic substrate means that you can use an erasable Fig.3: the circuit of the BIG Clock is simple; the microcontroller board provides power and data to a series of addressable LEDs. A magnetic reed switch provides a digital input that can be used to toggle the daylight saving mode. 56 Silicon Chip Australia's electronics magazine siliconchip.com.au Photo 2: much of the wiring is hidden at the back of the panel. If you have kept the other half of the Corflute sheet, it could be used to make a rear panel to hide the wiring. marker to demarcate the locations and remove them later. Find the centre of the panel and mark the corresponding horizontal and vertical lines. Add horizontal lines 9cm below and above the centreline. Similarly, add vertical lines 15cm and 30cm to the left and right of the centre. These will allow you to use Fig.4 to sketch the outline of each digit. Next, mark out the holes needed for each digit (the green dots in Fig.4). Note that not all locations require holes! The centre position, for example, will only need holes for the central segment, as it only shows a dash. Double-check the segments against the photos as you go. Cut the LED strip into sections with five LEDs. Be careful to leave some visible copper on each side to allow soldering. The first and last segments come with fixed wiring attached, so they will have to go in the locations marked 1 and 24 in Fig.2. We found that attaching the wires was a bit tricky, since the conformal coating applied to the board inhibits soldering, although it can be soldered through with patience and ventilation. We suggest wiring the segments in groups of seven (for the three full digits) using short pieces of wire (about 5cm). For each connection, join 5V to 5V, DO to DIN and GND to GND. If you cut narrow slots in the Corflute as marked by the cyan lines in Fig.4, you can slot these short wires in from the front, so they are hidden. Then, you only need to make the longer joins between the segments; the longer lengths will allow a bit of room to manoeuvre the strips into a position to allow wiring. Take your time, ensure that the strips go in the correct locations and that wiring flows in the direction of the arrows marked on the strip and in the siliconchip.com.au order shown in Photo 1. Don’t be afraid to hook it up to test that the segments wired so far are working correctly. Photo 2 shows our layout from the rear of the panel. Once everything is roughly in place, remove the backing paper from the adhesive on the strips and press them against the Corflute. There are extra red and white (power) wires at each end. We connected these with insulated wire and heatshrink tubing to provide an extra power feed and to terminate the loose ends. We cut the extra set of three wires short so that they would not get in the way or contact anything else. Uno WiFi R4 wiring We plugged an 8-way header into the headers on the Uno WiFi R4 as shown in Fig.3 and Photo 3. Remove the middle two pins by pulling them out with pliers. This will prevent an inadvertent connection to VIN, since the LEDs only work with 5V supplies. When bending the leads of the reed relay, avoid straining them where they enter the glass envelope, or it can break. We suggest you grasp the lead with pliers close to the body, then bend it, so that no bending force reaches the glass. Solder the wires as shown in Fig.3 and plug the header into the main board. The wire colours on the LED strip might be different; ours had red for 5V, white for ground and green for data. Photo 3 shows that detail on our build. Fig.4: use these dimensions to sketch out the segments on your Corflute before making the holes marked in green. Their size is not critical; about 5mm should work well. The cyan lines indicate slits that can be used to feed the wires through from the front of the panel. Australia's electronics magazine January 2025  57 Parts List – Big Clock (JMP019) 1 Arduino Uno WiFi R4 microcontroller board [Jaycar XC9211] 1 120 RGB LED addressable strip [Jaycar XC4390] 1 magnetic reed switch [Jaycar SM1002] 1 8-pin header, 2.54mm pitch [cut from Jaycar HM3211] 3 1.5m lengths of insulated wire in different colours [Jaycar WH3032] 1 600mm x 900mm sheet of Corflute or similar [Bunnings 0390160] 1 short length of double-sided tape to secure the Uno WiFi R4 [Jaycar NM2821] 1 reel of electrical tape to secure loose wires 1 10cm length of 3mm diameter heatshrink tubing 1 magnet to operate the reed switch 1 USB-C cable to suit the Uno WiFi R4 Secure the Uno WiFi R4 to the Corflute using double-sided tape, and secure loose wires with the electrical tape. Software Arduino IDE needs to be installed and the Arduino R4 board profile selected. This can be installed by searching for “R4” in the Boards Manager and then clicking install. The Adafruit Neopixel library is also required; we’ve included it in the software download package (siliconchip. au/Shop/6/530), along with the sketch file. You can also search for “neopixel” in the Library Manager to find it. There are several sketches under the Neopixel examples (and one in the Jaycar XC4390 data sheet); you just need to change the LED_PIN to A0 and the LED_COUNT to 120. These are a quick way to test that the display is functional. Open the BIG_CLOCK_UNO_R4_ WIFI sketch and change the WiFi credentials at the top of the sketch. There are other parameters that can be changed, but that should be enough to check that all features are functional. Upload the sketch after selecting the correct board and serial port from the menus. The Serial Monitor will report the Clock’s status (115,200 baud); a typical boot sequence is shown in Screen 1. The LEDs should all switch on for two seconds, then normal operation will start. If you see an E0 message on the LEDs, the WiFi connection has failed. E1 indicates that the time has not been updated. Table 1 also lists some commands that can be entered at the serial monitor. Customisation There are a few things that can be changed in the sketch code. The standard time zone offset (in minutes) is set by STD_TZ_OFFSET. The daylight saving adjustment (effected by using a magnet on the reed switch) is one hour. The colour of the lit LEDs is set by Connected IP address: 192.168.0.15 Checking UTP on connection Starting NTP check UDP packet sent 25ms round trip. Packet received Time OK UTC is 2024-09-11T04:39:20 Time is 2024-09-11T14:39:20 Time is 2024-09-11T14:40:00 Time is 2024-09-11T14:41:00 Time is 2024-09-11T14:42:00 Screen 1: this shows a normal startup on the Serial Monitor. The IP address and round trip time are not important. Other messages may appear automatically or if the commands from Table 1 are run. the CLOCK_COLOUR #define; you can use the values given or provide RGB triples. The brightness dictates the current draw, which peaked at 700mA for a BRIGHTNESS setting of 70 on our prototype. Check your supply capabilities and adjust this to suit. If you have Arduino experience, you can also modify the LED layout or mapping. The digits array is a C++ struct (with type seg7_t) for each digit. The struct’s first item is the number of pixels for each segment, followed by the first pixel of each segment in order from ‘a’ to ‘g’. For example, with just four LEDs per segment, you could have 30 segments, giving a full four 7-segment digits plus a couple of segments to spare; enough to count up to 19999. We think the white Corflute looks smart but it doesn’t give a lot of contrast against the LEDs. When we get the chance, we plan to paint the Clock’s background a flat grey to make the SC numbers stand out more. Photo 3: an 8-pin header with the two central pins removed is used to connect the wiring to the Uno WiFi R4. The green wire at right carries data to the DIN pad of the first LED pixel. When completed, the Big Clock is three feet (900mm) wide and lights up the room. We think it will give our readers some great ideas for creating other large displays. Table 1: serial commands 58 Silicon Chip Australia's electronics magazine Command Action u Force NTP time refresh r Reboot processor t Make time invalid 0 Turn off daylight saving 1 Turn on daylight saving siliconchip.com.au Subscribe to DECEMBER 2024 ISSN 1030-2662 12 The VERY BEST DIY Projects ! 9 771030 266001 $13 00* NZ $13 90 INC GST INC GST CompaCt HiFi hea dpHone�� ampliFier �� �� THE PICO COMPUTER 1W into 16Ω 3.5mm & 6.5mm headphone jack Class-AB operating mode 9-12V AC plugpack using a Raspberry Pi Pico Undersea Communications The vast underwater fibre-optic cable network. Capacitor Discharger A great piece of gear for safely discharging small & large capacitor s. Raspberry Pi Pico 2 We review the newest Raspberr y Pi Pico 2 which is priced at around $8 each. Win a DHO-924S Oscilloscope Australia’s top electronics magazine See page 9 for details and enter before December 15th for a chance to win! ...and even more inside this issue 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 * does not include the cost of postage The Pico Computer; December 2024 Rest of World New Zealand Australia If you have an active subscription you receive 10% OFF orders from our Online Shop (siliconchip.com.au/Shop/)* 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! Capacitor Discharger; December 2024 FlexiDice; November 2024 Surf Sound Simulator; November 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 5MHz 40A Current Probe A current probe is an incredibly useful tool for development, testing and debugging but is usually quite expensive. This DIY version performs well compared to many commercial offerings but at a fraction of the price! Project by Andrew Levido Levido U sing an oscilloscope to monitor the current in a circuit can be challenging. Oscilloscopes are made to measure voltages, so if you can find or add a suitable shunt resistor in the current path, you can measure the current indirectly. However, this is usually only practical at relatively low current levels and only if you can safely connect your ‘scope probe ground to one side of the shunt (which is often not the case). Say there is no suitable shunt resistor, the current runs to several amps or the circuit is not conducive to the safe connection of a grounded probe. You will probably have to use an isolated current probe in those cases. If your circuit operates at mains potential, an isolated probe is mandatory. You can certainly buy such current probes. The problem is getting good performance at a reasonable price. You can opt to spend thousands of dollars on a high-end 50-100MHz probe from one of the big names in test equipment, or you can spend $100 or less on AliExpress for a no-name probe with a bandwidth of just a few hundred hertz. I wanted an inexpensive, high-­ performance current probe, so I built my own. The resulting probe, described here, can measure current up to ±40A, with a bandwidth from DC to 5MHz. Its output is fully isolated from the measurement terminals, so you can safely measure the current of mains-powered devices. The output, available on a BNC connector, is scaled to 100mV per amp, so it is in the range ±4V. The device is powered by an internal rechargeable lithium-ion cell. Charging is via a power-only USB-C socket. Current Probe Features & Specifications » » » » » » » » » » 60 Current measurement: bi-directional Output scaling: 0.1V/A (±40A translates to ±4V) Input/output isolation: 420V RMS (600V peak) ‘reinforced’ Maximum current: 40A peak (35A continuous) Bandwidth: DC-5MHz Power supply: onboard Li-ion cell Battery life: approximately 30 hours Charging: USB Type-C socket (5V DC) Charging time: approximately 3 hours from flat Charging current: 300mA (optionally 100mA) Silicon Chip Australia's electronics magazine Scope 1 shows a typical ‘scope capture made using the probe. This is the mains inrush current of a variable-­ frequency motor drive unit, which has a large capacitor bank charged via a bridge rectifier from the mains. A softstart circuit limits the inrush current at power-on. The vertical scale of the scope capture is 2V per division, corresponding to 20A per division. You can see that the peak charging current is about 34A in the first half-­ cycle, with a reduction in the current each cycle after that as the capacitors charge. Another example capture is shown in Scope 2. Here, a short is applied across a bench power supply set to a 6A current limit. The peak current spikes to almost 50A (showing some headroom in the current probe’s design) but rapidly drops as the power supply current regulation circuit begins to operate. Within a millisecond, the current is brought under control and limited to 6A. Design The heart of the current probe is the ACS37030 chip from Allegro Microsystems. Like many similar devices, this uses a Hall effect sensor to measure a current indirectly by measuring the magnitude of the magnetic field it produces. See the separate panel for some background on siliconchip.com.au how the Hall effect works and how it is used in this application. Hall sensors are useful in these applications since they work with DC; however, their frequency response is typically limited to a few hundred kilohertz. The ACS37030 family of sensors is particularly interesting because it pairs a Hall effect sensor for DC and low-frequency signals with inductive sense coils for high-­frequency signals. They are available with full-scale current ratings of ±20A, ±40A or ±65A and come in a 6-pin SOIC (small-outline integrated circuit) package with 3500V RMS isolation. All for less than $8 in low quantities. The block diagram of the ACS37030 is shown in Fig.1. You can see the Hall current sensor at lower left (“Dynamic Offset Cancellation”), with the inductive sensor just above it. The output from the transducers is conditioned by two separate signal chains, which come together at a summing junction. The resulting signal is buffered and offset to produce the output signal. An advanced digital subsystem uses calibration data stored in non-volatile memory to manage the gain of the two signal paths, providing an accurate output over the whole frequency and operating temperature range. Since the sensor uses a single 3.3V supply, the output signal swings around a zero-current level of 1.65V, provided by an internal bandgap reference. For the 40A device used here, this output voltage is 1.65V±33.3mV/A for a maximum output swing of just under 0.3V to 3.0V. Notably, the 1.65V reference is available on one of the pins. Some chips lack such a facility, and it is very difficult to zero them. Even if you have an external trimmable reference voltage that you adjust to get 0V output at 0A current, any differential drift between the two reference voltages will cause the output accuracy to deteriorate significantly. To display the output voltage conveniently on an oscilloscope, we must remove the offset and amplify the signal to get a ±100mV/A signal based around zero volts. The most straightforward way to achieve this is to use an instrumentation amplifier. An instrumentation amplifier is a high-­ precision differential amplifier based on op amps that usually uses a single resistor to set its overall gain. siliconchip.com.au Fig.1: the ACS37030 current sensor features a Hall sensor for DC and lowfrequency measurements, plus an inductive sensor for higher frequencies. These are combined by some clever circuity to provide a flat response from DC to 5MHz. Scope 1: the inrush current for a variable-frequency motor drive as measured by the current probe. The scale is 20A per division. Scope 2: This scope grab, made using the current probe, shows the current supplied by a short-circuited bench power supply, at 10A per division. The current peaks at almost 50A before being rapidly brought under control and limited to 6A. Australia's electronics magazine January 2025  61 precision that would be difficult (read expensive) to emulate with discrete components. Fig.2: the INA849 instrumentation amplifier uses the classic three-opamp topology with six lasertrimmed matched resistors. One external resistor, Rg, sets the overall gain. The instrumentation amplifier used in this project is the INA849. It is a classic three-op-amp configuration, as shown in Fig.2. The input stage consists of two non-inverting amplifiers with internal 3kW feedback resistors. A single external resistor, Rg, sets the gain of this stage according to the formula G = 1 + 6kW ÷ Rg. The second stage is a differential amplifier. As the name suggests, it amplifies the difference between two voltages but strongly attenuates any common-mode signal. In the case of the INA849, the differential gain is Circuit details unity. Therefore, the output voltage is given by the formula Vout = Vref + (Vin+ – Vin-) × (1 + 6kW ÷ Rg). The REF terminal is often connected directly to ground, as per the figure, but you can use it to add an offset to the output if required. We could build our own instrumentation amplifier from discrete op amps, but it’s convenient to use an integrated package like this because the common-mode rejection depends on the close matching of the resistors. Packages like this use internal laser-trimmed resistors matched to a Now we can turn to the complete circuit diagram (Fig.3) to see how it all works. The ACS37030 (IC1) is powered by a 3.3V rail supplied by low-dropout (LDO) 3.3V linear regulator REG2. The sensor output voltage and the 1.65V reference are applied to the instrumentation amplifier’s non-inverting & inverting inputs, respectively. Achieving a differential gain of three requires a nominal Rg value of 3kW. While 3kW resistors are available (it’s a common E24 value for 1% resistors), I used a combination of fixed resistors plus a trimpot to allow an adjustment range of about ±3% around this figure. This allows the user to trim out any gain error in the sensor, which could be as much as ±2%. The trimmer has the added advantage of obviating the need for high-­ precision resistors here. Fig.3: the current probe circuit reveals that the signal path is very simple, consisting of just the instrumentation amplifier with its associated gain and offset trimming. The balance of the circuit is the power supply and battery charger. 62 Silicon Chip Australia's electronics magazine siliconchip.com.au The ACS37030 data suggests that along with the ±2% gain error, there could also be a potential offset error of up to ±10mV. That would translate to ±30mV at the output after amplification. This is why I am driving the instrumentation amplifier’s REF pin with an adjustable offset trim voltage of ±50mV derived from the divider that includes trimpot VR2. The offset trim voltage is buffered by op amp IC4a since the input impedance of the REF pin is relatively low. The instrumentation amplifier's output goes to the output BNC connector (CON3) via a 100W resistor to protect the amplifier IC from short circuits at the output. The rest of the circuit is the power supply. We require a ±6V split supply for the amplifiers. This was chosen because the common-mode input voltage of the instrumentation amplifier can’t be any closer than 2.5V from either supply rail. Since our maximum input voltage extends to 3V, we need supply rails of at least ±5.5V. I chose ±6V to provide a bit of headroom. These rails are derived from a single Li-ion cell via REG6, which contains a boost converter to create the positive rail and an inverting converter to provide the negative rail. The switching Mosfets are internal to the package, but the inductors and rectifier diodes are external: L1/D2 for the positive rail and L2/D3 for the negative. The output is regulated by providing voltage feedback via two 100kW/20kW resistive voltage dividers, which reduce the ±6V outputs to ±1V, matching REG6’s internal feedback target voltages. The R1283K regulator can operate with an input voltage of 2.5-5.5V, which is ideal for a single Li-ion cell. When power switch S1 is on, the cell is connected to the DC-to-DC converter. If it is off, the cell is instead connected to IC5, a MAX1555 dual-­ input Li-ion battery charger. This linear device charges the cell at 100mA if powered via the USB input or 300mA if powered from the DC input. The USB input is useful for charging from legacy USB hubs, which may not be able to supply more than 100mA. However, this current probe is designed for a USB type-C power source that can supply at least 500mA, so the higher charge current is used to minimise the changing time. The PCB has provision for either siliconchip.com.au configuration. LED1 will be on while the battery is charging and will switch off once full charge is reached. The USB-C connector is a power-­ only type with just a subset of the normal 24 pins. These include the power pins and the control channel (CC) pins, which are terminated with 5.1kW resistors to ground. This tells the source that it should supply 5V. The power input is protected from overvoltage by a TVS diode (TVS1) and a series PTC resettable fuse, PTC1. Construction All components are mounted on a double-sided PCB coded 9049-01 that measures 56.5 × 76.5mm. The component overlay diagram (Fig.4) shows where everything goes. To keep it compact, this project uses almost entirely surface-mounted parts. I managed to mostly avoid any difficult-­to-handle parts; all passives are M2012/0805 size (2.0 × 1.2mm) or larger and all but one of the semiconductors are in easy-to-solder SOIC-8, SOT-223 or SOT-23 packages. Unfortunately, the DC-DC converter (REG6) is not available in anything other than a DFN (dual flat no-lead) package, so that is where I suggest you start the assembly process. It really helps if you have a hot air reflow station. These stations are useful for soldering chips like this one and also make desoldering many SMDs straightforward. They are not terribly expensive if purchased online. The easiest way to get REG6 soldered down is to use a soldering iron to lightly tin the pads, including the thermal pad in the centre. The solder should just cover the pads and not be too lumpy. If you put down too much, you can use solder wick to remove the excess. Next, apply a generous amount of flux paste, position the chip carefully (making sure its pin 1 mark is oriented correctly) and hold it in place with tweezers while you use hot air to gently reflow the solder. Once the solder melts, surface tension should pull the chip neatly into place. Any visible solder balls or bridges can be removed with solder wicking braid and a hot iron. After cleaning the area with isopropyl alcohol or similar, you have completed the hardest part. Mount the rest of the surface mount parts using your preferred method. I apply a dab of solder to one pad first, then position the component with tweezers and reflow that pin with the soldering iron. With just one pin soldered, I can tweak the location if necessary to get the other pin(s) into a place I am happy with. Finally, I solder the remaining pin(s). Fig.4: all components are easy-tosolder surface mount or through-hole types, with the exception of REG6. It requires a little more care but is easily achievable for the hobbyist. January 2025  63 Parts List – 5MHz 40A Current Probe 1 ABS handheld instrument case, 92 × 66 × 28mm [Hammond 1593LBK] 1 double-sided PCB coded 9049-01, 56.5 × 76.5mm 1 14500 (AA-size) Li-ion battery with PCB pins (BAT1) [Altronics S4981] 2 6.8μH 1A SMD inductors, M3225/1210 size (L1, L2) [Murata 1276AS-H-6R8M=P2] 1 0.75A 24V PTC fuse, SMD M3225 size (PTC1) [Littelfuse 1210L075/24PR] 1 100W 4.9 × 3.9mm SMD trimpot (VR1) [SM-42TW101] 1 1kW 4.9 × 3.9mm SMD trimpot (VR2) [SM-42TW102] 1 PCB-mounting sub-miniature DPDT toggle switch (S1) [E-Switch 200MDP1T2B2M6RE] 1 red panel-mounting binding post (CON1) [Cal Test Electronics CT2232-2] 1 black panel-mounting binding post (CON2) [Cal Test Electronics CT2232-0] 1 right-angle PCB-mount 50W BNC socket (CON3) [Molex 0731000105] 1 USB type-C power-only socket with through-hole mounting pins (CON4) [Molex 217175-0001 or equivalent] 2 panel-mount 3mm light pipes, 15mm long (for LED1 & LED2) [Dialight 51513020600F] 2 Koa RCUCTE SMD test points (TP0, TP1; optional) 4 No.4 × 6mm self-tapping screws 1 100mm length of 1.0-1.5mm diameter tinned copper wire Semiconductors 1 ACS37030LLZATR-040B3 5MHz 40A current sensor, SOIC-6 (IC1) 1 LD1117S33 or equivalent 3.3V 800mA LDO regulator, SOT-223 (REG2) 1 INA849DR 28MHz instrumentation amplifier, SOIC-8 (IC3) 1 LM358 dual single-supply op amp, SOIC-8 (IC4) 1 MAX1555EZK-T Li-ion battery charger, SOT-23-5 (IC5) 1 R1283K001B-TR buck/boost switching regulator, UFDFN-14 (REG6) 1 yellow SMD LED, M2012/0805 size (LED1) 1 green SMD LED, M2012/0805 size (LED2) 1 SMBJ5.0CA 5.0V TVS diode, DO-214AA (TVS1) 2 30V 1A schottky diodes, DO-214AC/SMA (D2, D3) [MBRA130LT3G, SS14] Capacitors (all SMD M2012/0805 50V X7R unless noted) 1 100μF 25V tantalum, SME case [Kyocera TAJE107K025RNJ] 7 10μF 16V 3 100nF 1 220pF C0G Resistors (all SMD M2012/0805 ⅛W 1% unless noted) 2 100kW 2 20kW 1 2.7kW 1 510W 1 240W 2 56kW 2 5.1kW 1 1.8kW 1 100W 1 0W I find this works for two-pin devices like resistors and capacitors, as well as for the ICs. The current sensor chip, IC1, straddles an unplated slot cut into the board. This slot is to provide plenty of creepage distance between the current being measured and the rest of the circuit. However, it makes the board quite flexible in this area, so handle it carefully after soldering IC2. If the board is flexed too much, it is possible to overstress the IC’s pins and break them – as I unfortunately discovered! Solder the USB connector’s throughhole tabs first to locate it, then the six smaller surface-mounting pins. Case preparation Before you fit the BNC connector, binding posts or battery (well, cell), it’s a good idea to prepare the case. That will allow you to align those larger connectors properly. The cell should be left off until the testing described below is completed. Drill the enclosure's cover and end plates as shown in Fig.5. The USB slot can be made by drilling two 2.8-3.0mm holes at either end and then filing out the plastic between them. Now drop the PCB into the case, resting on its mounting bosses, insert the BNC connector through the hole in the end plate and drop it into its mounting holes on the PCB. If everything lines up, you can tack-solder the BNC connector in place from the top, then remove the whole assembly from the case and solder it properly, taking care that it doesn’t bend as you do so. If it doesn’t fit perfectly, you will need to enlarge the panel hole slightly and try again. Now attach the input terminals (binding posts) to the appropriate end plate and tighten the nuts. Connect the input terminals to the PCB using a few lengths of tinned copper wire bent over the terminal studs, through the PCB slots, and soldered in place. Remember that this connection could carry up to 40A, so ensure it is solid. Remove the assembly from the case and trim off any excess wire. Make sure that there cannot be any shorts between the terminals! Testing With the switch to the right, the probe is powered; to the left, it can be charged via USB. 64 Silicon Chip Australia's electronics magazine If you have a current-limited bench supply, it’s a good idea to test the circuit before soldering the battery to the board. Set the onboard switch to the siliconchip.com.au on position, towards the BNC connector. Set the bench supply to 4V with a limit of around 100mA. Then, taking care to connect it with the correct polarity, hold its output leads to the two battery pads and monitor the current draw. The power supply should not go into current limiting; the circuit should only draw about 20-30mA. LED2 (green) should light. If all seems well, you can disconnect the power supply, switch off the onboard power switch and solder the battery in place. If something is wrong, check all your soldering carefully and verify that all components are installed correctly. Before soldering the battery to the PCB, make sure the power switch is in the off (charging) position, with the toggle switch away from the BNC connector. Once the battery is installed, treat the board with care. Inadvertently shorting things now could be catastrophic, as Li-ion batteries can source a lot of current. Next, check that the battery voltage is between 3.0V and 4.2V. Switch the unit on, and LED2 should light again. Check for 6V across the 10μF capacitor immediately to the left of L2 and the similar capacitor immediately to the left of D3. That will verify that both supply rails are correct. If the readings are wrong, switch it off immediately; you most likely have a problem with the power supply section. Check REG6 and its surrounding components, especially L1, L2, D2 and D3. Once the power supply is working correctly, you can check the battery charging circuit by switching the unit off and connecting a suitable USB supply. The yellow charge LED (LED1) should light, and the voltage across the battery should begin to slowly rise. When the battery voltage reaches about 4.2V, the charge LED should go out, indicating that the battery is fully charged. Depending on the battery’s initial state of charge, that could take a few hours. Final assembly You can now apply the front panel label, shown in Fig.6 (download from siliconchip.au/Shop/11/490). Once it is in place, carefully cut out the LED holes and insert the light pipes from the outside. They can be secured with a drop of cyanoacrylate (super) glue on the inside of the case. Fig.5: drilling the case is straightforward. The USB slot is best made by drilling two 2.8-3.0mm holes at the ends and joining them with a file. You can see how the finished case looks at left, and how the PCB slots into the case shown enlarged above. Slip the end plate over the BNC connector, switch and USB socket and screw the whole assembly into the base of the enclosure with 6mm self-­ tapping screws. Calibration To calibrate the Probe, you will need a current-limited bench power supply capable of sourcing a few amps and a multimeter. If you have two meters, so much the better. First, set the offset trim. Connect the meter, switched Fig.6: the label artwork for the front of the enclosure. Print it on sticky-backed paper, cut out the outline and apply it (or laminate it, or use your preferred label-making method). Use the case as a template to cut the holes for the light pipes. The Hall Effect Edwin Hall first described the Hall effect in 1879, just a decade after Maxwell published his seminal work on the interaction of electric and magnetic fields. The lower left diagram shows how it works. A current (green arrow) flows through the long axis of a conductor that is subject to a magnetic field perpendicular to the direction of current flow (lavender arrow). Hall discovered that under these circumstances, the electrons making up this current – which flow in the opposite direction to the current – would experience a Lorentz force pushing them towards one side of the conductor, as shown by the curved blue arrow. As a reminder, Lorentz’s law states that a charged particle, such as an electron, moving in a magnetic field will experience a force at right angles to both the direction of the field and its velocity. This is the basic principle by which electric motors and generators work. The build-up of negative charge on one side of the conductor (and the corresponding positive charge on the other side, where there will be a dearth of electrons) produces an electric potential across the conductor. This Hall voltage is proportional to both the conductor current and the strength of the magnetic field. The Hall effect also works in semiconductors, although the polarity of the Hall voltage may be different in some semiconductors where ‘holes’, rather than electrons, are responsible for current flow. In practical Hall effect sensors, the current to be measured passes through a conductor surrounded by a magnetically permeable core. The Hall sensor is positioned in a narrow gap in this core, so the magnetic field produced by the current in the conductor passes through the element perpendicular to both the excitation current and the Hall voltage measurement terminals. Since the excitation current is fixed, the Hall voltage is proportional to the magnetic field strength, which is, in turn, proportional to the conductor current. Magnetic Core Fixed Current Conductor Hall Sensor Hall Voltage Sensing 66 Silicon Chip Australia's electronics magazine to a low voltage range, between the probe output test point (TP1) and ground (TP0). With the unit switched on and nothing connected to the input terminals, adjust the offset pot (VR2) for a meter reading close to 0V. You should be able to get it to less than ±1mV. To trim the gain, configure the power supply to deliver a few volts and set the current limit to 3A or whatever maximum your power supply will deliver. Switch your meter to read current (remember to swap the probes to the correct jacks), select the appropriate range, and connect it across the power supply. The supply should go into current limiting and regulate the current somewhere near the setpoint. Record the current value displayed on the meter. Now switch the meter back to volts and connect it back to TP0 and TP1 as before. Connect the current probe inputs across the power supply without changing any of the settings. The output voltage should read close to one-tenth of the current reading you noted earlier. For example, if you measured the current to be 3.02A, you should see something like 0.302V on the meter. If the reading is a bit off, adjust the gain pot VR1 to get it as close as possible. If you have two meters, you can measure the input current and output voltage at the same time (the current meter goes in series with the probe across the power supply). That will be a bit more accurate (and easier) than switching the meter around. Using it Due to the high currents that the probe can handle, probes (alligator clip wires etc) should not be used unless both the voltage and current are low (under 50V DC/AC & 5A). For higher voltages/currents, you can cover the exposed wires that are attached to the binding posts with heatshrink. As there is exposed metal on the binding posts, if any voltage above 50V is applied to the Probe, that end of the device must be considered live. Position the Probe so that nobody can come in contact with that end, and also to keep the isolated measurement end away from any high-voltage wiring. The Probe itself has a high isolation, but you must ensure that isn’t degraded by any external shorting hazards. SC siliconchip.com.au SOnline ilicon Chip Shop Kits, parts and much more www.siliconchip.com.au/Shop/ Compact OLED Clock & Timer September 2024 Short-Form Kit SC6979: $45 siliconchip.au/Article/16570 This kit includes everything needed to build the OLED clock, except the UB5 Jiffy box and Li-ion cell. Dual Mini LED Dice August 2024 Micromite-Explore 40 October 2024 Complete Kit SC6991: $35 SMD LED Complete Kit SC6961: $17.50 TH LED Complete Kit SC6849: $17.50 siliconchip.au/Article/16418 siliconchip.au/Article/16677 Includes either 3mm through-hole or 1206sized SMD LEDs. Choice of either white or black PCB. CR2032 coin cell not included. Includes the PCB and all onboard parts. Audio Breakout board and Pico BackPack are sold separately. ESR Test Tweezers Mains Power-Up Sequencer Complete Kit SC6952: $50 February-March 2024 June 2024 siliconchip.au/Article/16289 This kit includes everything needed to build the ESR Test Tweezers. Does not include the CR2032 (or CR2025) coin cell or optional 5-pin header CON1. USB-C Serial Adaptor Complete Kit SC6652: $20.00 June 2024 siliconchip.au/Article/16291 Includes the PCB, programmed microcontroller and all other parts required to build the Adaptor. Hard-To-Get Parts SC6871: $95 siliconchip.au/Series/412 The critical components required to build the Sequencer such as the PCB, micro etc. Other components need to be sourced separately. → Subscribers receive a 10% discount on all purchases, except for subscriptions (postage is not discounted). → Prices listed do not include postage. Postage rates within Australia start at $12, rates are calculated at the checkout. Battery-Po Battery-P owered Mode dell Tra Traiin BY LES KERR This modification eliminates the need to keep model railway tracks clean. If you let them oxidise, power won’t get to the trains, causing all sorts of problems. By making the train battery powered, it no longer needs to draw power from the tracks, making it much more reliable! M y grandson was visiting and he was at me all the time to let him drive trains on my OO gauge railway. As it hadn’t been used for quite a time, there was quite a build-up of debris on the track and the engine pickups that resulted in the first train running erratically. After laboriously cleaning the track, the trains ran smoothly. Most of my newer power tools are battery powered, so I wondered if I could power the train from onboard rechargeable AAA cells. These could be mounted in the carriage behind the engine, and the speed and direction could be controlled by a simple 433.9MHz link. I calculated that four fully-charged 900mAh NiMH cells in series could run the train for more than five hours on a charge. With most model train layouts, the 433.9MHz transmitter will only be a few metres from the train at any time, so there is little chance of interference. To ensure the train doesn’t go haywire, check bytes are sent so that the Receiver can verify the speed and 68 Silicon Chip direction data it is getting are correct. This virtually eliminates the possibility that erroneous signals will result in incorrect operation. The NiMH battery voltage of around 4.8V is too low to run the motor, so I selected a small step-up converter module that produces 15V DC from the battery voltage to power the motor. It operates at about 1MHz with an efficiency approaching 90%. Another small boost converter generates a steady 5V rail to run the control circuitry. To drive the engine, I looked at all the standard H-bridges on the market and selected the DRV8871 IC that is mounted on a 24.5 × 20.5mm PCB. It runs from 5-37V at up to 2A, driving a single motor bidirectionally. Over-temperature and over-current protection is built in. It is a bit overkill for a 12V train that takes about 250mA maximum, but it could be used with higher power engines too. The motor speed and direction are controlled by a microcontroller on Australia's electronics magazine the same PCB as the motor driver that mounts in the carriage, behind the engine. This PCB also has a 433.9MHz receiver to allow remote control. To cater for various size carriages, I designed two Receiver PCBs, a small one using SMD components (carriage length 185mm) and a larger one with through-hole (TH) components. The handheld controller (Photo 1) has a potentiometer that controls the speed of the train and a toggle switch to select forward or reverse. The Transmitter has a PIC12F617 microcontroller that monitors those controls and sends signals via a 433.9MHz transmitter within the handheld controller. A 3mm red LED on the carriage lights when the battery needs charging. The fourth PCB I designed is a trickle Charger (Photo 2) that connects to a socket on the battery carriage using a 2.5mm jack plug. This system of three modules – Transmitter, Receiver and Charger – provides everything you need to run siliconchip.com.au Photos 1-3: the transmitter (left & right), and the charger (centre) box. a model locomotive without requiring an electrical connection (for either power or communications) through the track. You can see a video of it in operation at siliconchip.au/Videos/ Battery+model+train Transmitter circuit details The Transmitter circuit is shown in Fig.1. It is powered by a 9V battery via on/off toggle switch S1 and a 1N5819 schottky diode. The diode prevents accidental battery polarity reversals from destroying the circuit. A schottky diode is used as its forward voltage drop is a lot less than a standard silicon diode, so the battery lasts longer. A 78L05 regulator provides +5V for the microcontroller. The 100μF capacitors connected to its input and output reduce any ripple to a negligible level, while the 100nF ceramic or MKT capacitors reduce any high-frequency noise that may be present. So that potentiometer VR1 varies the train speed, microcontroller IC1 measures the voltage at its wiper using its internal analog-to-digital converter (ADC) via analog input AN3. It converts the 0-5V on its wiper to an 8-bit number between 0 and 255. That value is sent out as pulses via digital output GP0 (pin 7), to the transmitter module, to be picked up by the Receiver on the train. Digital input GP5 (pin 2) is pulled Fig.1: the Transmitter circuit. It runs from a 9V battery; microcontroller IC1 and transmitter MOD1 convert the position of speed potentiometer VR1 and forward/reverse switch S2 into a 433.9MHz-modulated ASK serial data stream for the Receiver. siliconchip.com.au Australia's electronics magazine January 2025  69 high by the 10kW resistor when S2 is in the forward direction or low, to ground, by S2 when it is in the reverse direction. The microcontroller senses this level using its GP5 digital input and sends different numbers via the 433.9MHz transmitter depending on the switch state. The 100nF ceramic capacitors at those two inputs prevent noise from affecting the readings taken. The signal sent to the transmitter module via the GP0 output is serial data at 1200 baud that contains the speed and direction variables, along with preamble and check bytes. This 433.9MHz module transmits this using amplitude-shift keying (ASK) via a quarter-wavelength (173mm long) wire antenna. Receiver circuit The Receiver circuit is shown in Fig.2. Signals from the Transmitter are received by the 433.9MHz receiver module, and the demodulated serial data is applied to the RC2 digital input (pin 8) of the PIC16F1455 microcontroller (IC2). The 8-bit train speed data and the direction data are extracted and stored in memory, then used to generate the pulse-width modulated speed signal and the direction signal. Two logic inputs, IN1 and IN2, control the H-bridge driver (IC3). To turn Fig.3: pulse-width modulation (PWM) involves setting the output high at a fixed interval, then leaving it high for a period ranging up to that interval. The result is a varying average voltage, even though the output only switches between two levels. the motor in one direction, we apply a pulse-width modulated (PWM) signal to vary the speed to IN1 while holding IN2 high. If the train is to run in reverse, the PWM signal is applied to instead IN2 while IN1 is held high. To stop the train, both input are kept at the same level (both low or both high). Fig.3 shows the signals for driving the motor at various speeds. The battery supply voltage is halved by the two 10kW resistors and the resultant ~2.4V is monitored by analog input RA4 (pin 3) of IC2 using its internal ADC. If the voltage at that pin falls below 2V (ie, the battery is below 4V), digital output RC4 (pin 6) is taken low, switching on red LED2 to alert you that the battery needs charging. The micro also provides signals to drive the DRV8871 H-bridge IC. To turn the motor in one direction, the PWM signal is applied to digital output RC3 (pin 7), while RC5 is taken high (+5V). To reverse the motor direction, the PWM signal is applied to RC5 and RC3 is taken high. The higher the speed value, the faster the motor turns. When the speed control is near its minimum position, both RC5 and RC3 are taken low (to 0V), causing the PWM module to go into sleep mode, reducing the current drawn from the battery. The +5V supply for the receiver and micro is provided by the S7V7F5 high-frequency voltage up/down converter (MOD4) that takes the 4-6V battery voltage and provides a regulated +5V output. If the battery has been recently charged (it could be as high as about 6V), MOD4 steps down the voltage Fig.2: MOD2 picks up the data from the Transmitter and feeds it to microcontroller IC2, which decodes it and produces PWM waveforms for H-bridge motor driver IC3 on MOD5. MOD3 boosts the battery voltage to 15V to run the motor. IC2 also monitors the battery voltage and lights LED2 if it is low. 70 Silicon Chip Australia's electronics magazine siliconchip.com.au to +5V; if it is discharged below 5V, it steps it up. The 100μF electrolytic capacitor and 100nF ceramic capacitor reduce any noise or ripple on the supply. Similarly, the U3V16F15 (MOD3) provides the +15V DC supply for the motor. We use 15V instead of 12V to overcome any voltage drop in the tiny cables connecting the carriage to the train motor. Polulu recommend in their data sheet that you add a 47μF capacitor across the battery input when using these inverters, which I have done. Both these modules are available locally for around $9 each. There is a 2.5mm switched jack socket (CON1) so the battery can be charged. It also allows the battery power to the Receiver to be switched off simply by inserting a jack plug. With the jack plug in the socket, the battery is connected to the Charger and disconnected from the Receiver as its positive side is disconnected. Charger circuit Looking at Fig.4, the battery is trickle charged at C/10 (90mA) for 16 hours unless its output voltage exceeds 6V, indicating the battery is fully charged. In that case, the charge current is switched off. When the power pack is switched on, 9V is applied to the 78L05 voltage regulator (REG2), which reduces the voltage to +5V to Photo 4: the 433.9MHz receiver (above) and transmitter (below) modules. They are sold under various model numbers, but this particular set is very common to find online. As long as yours look like these, and don’t have low-quality soldering, they should work (avoid the cheapest ones!). siliconchip.com.au power the PIC12F617 microcontroller, IC4. The two 100μF capacitors smooth out any residual ripple, while the two 100nF capacitors provide high-­ frequency bypassing. On powering up, digital output GP4 (pin 3) of IC4 pulses the green LED at 200ms intervals, indicating it is in standby mode. Pressing the Start button (S3) pulls the GP2 digital input low (pin 5), causing an interrupt routine to be triggered that takes the Charger out of standby mode and puts it into charge mode. The 100nF capacitor eliminates any contact bounce from the pushbutton. This results in the green LED switching off and the red Charge LED flashing at 500ms intervals. Mosfet Q1 (IRL540N) is switched on by digital output GP5 going high, and the 16-hour countdown timer starts. When on, the drain of the Mosfet goes low, connecting the 90mA constant current source to the battery. The current source comprises the BD136 transistor (Q2), an LM285 2.5V reference diode and a 220W resistor in parallel with a 22W resistor. It works by holding the PNP base 2.5V below the +9V supply. This sets the emitter at 1.8V (2.5V – 0.7V), which matches the voltage across the parallel resistors. They have a resistance of 20W (220W || 22W). With 1.8V across 20W, Ohm’s law (I = V ÷ R) tells us the current must be 90mA (1.8V ÷ 20W). The battery voltage is halved by the two 10kW resistors and applied to analog input GP0 (pin 7) of IC4. Once per second, it measures the voltage and if it is above 3V (battery fully charged), charging stops and the Charger goes back into standby mode, shown by the green LED flashing. If the battery voltage doesn’t exceed 6V, the charging stops after 16 hours. The 1N4004 diode (D2) prevents the battery from discharging if it is left connected when the charger is not powered. The 1N4148 diode (D3) prevents the ADC input from rising above 5.6V, although that is unlikely because the battery would have to be charged to over 11V. Still, it’s possible CON2 could accidentally be connected to a voltage source, so it’s better to be safe. Sourcing parts The receiver and transmitter modules are available from several suppliers under different part numbers. Fig.4: this NiMH battery trickle charger will stop charging when the battery voltage reaches 6V (1.2V per cell) or after 16 hours of charging. Q2, REF1 and the surrounding components form a 90mA constant current source while Mosfet Q1 controls whether charging is active. Australia's electronics magazine January 2025  71 Programming a microcontroller in-circuit To program the micro with it in the circuit, you will need to solder wires to the +5V and 0V rails as well as pin 4 (MCLR), and the pads on the ICSPCLK and ICSPDAT pins. Those are pins 9 & 10 respectively for the PIC16F1455, or pins 6 & 7 respectively for the PIC12F617. Connect those wires to your programmer, referring to its manual to see which wire goes to which pin. For the PICkit 3, the pins are (starting from pin 1) MCLR, VCC, GND, ICSPDAT and ICSPCLK. You can download and install the free MPLAB IPE software from the Microchip website and then use the included MPLAB IPE software to open the appropriate HEX file (which you can download from siliconchip.au/Shop/6/508) and flash it onto the target chip via your programming hardware. I have given a couple of examples in the parts list, but there are many others. Sometimes the part number is for a transmitter/receiver pair and the individual parts don’t have individual codes (or they are not specified). The main thing is to check that what you are buying looks like the modules shown in Photo 4. If you type “433MHz modules” in a search engine, you will find plenty of suppliers of modules that look identical or nearly so. Be careful, though, as I found that one of the very cheapest suppliers’ modules were poorly soldered and were unusable. Construction Let’s start by building the Transmitter. It is assembled on a single or double-­ sided PCB coded 09110241 that measures 49 × 36mm. During assembly, refer to the PCB overlay diagram, Fig.5. Fig.5 shows the off-board components wired directly to the PCB. You can do it that way, but it’s easier to instead solder pin headers in those positions and then cut pairs of female-female DuPont jumper wires in half. That way, you can plug them into the headers and solder the bare ends to the other components. You can see from the photos that I soldered wires to header sockets instead of using DuPont wires; either approach can work, but it’s easier and slightly neater to cut the jumper wires in half. You can often get them joined together in a ribbon, making it easy to split off pairs or sets so they stay together (like a figure-8 cable). Start the PCB assembly by fitting the headers, 8-pin IC socket and the capacitors. The IC socket makes it easier to remove the microcontroller and reprogram it later if necessary. Take care to orientate the socket and electrolytic capacitors correctly. For the electros, the longer positive lead goes into the pad nearest the + symbol, with the stripe on the can indicating the negative end opposite that. Now add the resistors, which are mounted vertically, then the 78L05 voltage regulator, 1N5819 diode (with its cathode stripe facing as shown) and the 433.9MHz transmitter module. As the clearance inside the Hammond box is less than the height of the 433.9MHz module, the module should be mounted 20° from vertical towards the edge of the board (it’s shown as if it’s laid flat in Fig.5 for clarity). Make sure all the semiconductors and the transmitter are correctly orientated. Don’t fit the PIC12F617 microcontroller yet. If you have purchased it from the Silicon Chip Online Shop, it will already have the firmware loaded. If you wish to program it yourself, you can download the firmware from: siliconchip.au/Shop/6/508 To load the firmware onto the chip, you will need a suitable programmer and an adaptor socket. For the former, Fig.5: this shows where components mount on the Transmitter board and how to wire it up. While wires are shown soldered straight to the PCB, we recommend using headers and wires with DuPont plugs to make assembly and disassembly easier. MOD1 is mounted about 20° off vertical so it fits in the case; it is shown horizontally here for clarity. Fig.6: this view from the inside of the case front shows where to drill the holes. The large one is for the pot shaft, the 5mm holes are for the two switches and LED, while the M3-tapped holes are for mounting the board. If you would rather not tap them, drill them to 3mm and use extra machine screws (ideally countersunk) from the outside to fix the tapped spacers. 72 Silicon Chip Australia's electronics magazine siliconchip.com.au you can use a PICkit or Snap programmer (or similar); for the latter, see our PIC Programming Adaptor (September 2023; siliconchip.au/Article/15943). Finally, check for any dry solder joints or solder bridges. plug the DuPont connectors into the headers on the board using Fig.5 as a reference. Make sure everything goes to the right location, or it won’t work properly. Case preparation With the microcontroller (IC1) out of its socket, check the orientation of the battery connector, 78L05 voltage regulator and the 433.9MHz transmitter module. Connect the 9V battery and switch it on. The LED on the front panel should glow. Connect a voltmeter with its red probe to pin 1 on the IC socket and the black lead to pin 8. The measured voltage should be very close to +5V DC. If not, verify that the 5V regulator is the correct way round and there aren’t any solder bridges shorting any tracks or pins. Assuming it’s OK, switch off the power and insert the microcontroller. If you have an oscilloscope, connect it to pin 7 of the IC, with the Earth connector to 0V. Switch on and you should should be able to capture a serial data waveform at 1200 baud similar to that in Scope 1. If all is good, attach the back of the case using the supplied screws and you are ready to move on to the Receiver. Drill and tap the Hammond 1593Y case as shown in Fig.6. That shows a view from the inside of the front part of the case. The large (9.5mm) hole is for the shaft of VR1, the three 5mm holes are for the two switches and LED, and the four M3-tapped holes are for mounting the PCB. If you would rather not tap the holes, you can simply drill 3mm holes and use screws from both sides (which is accounted for in the parts list), but it will look worse and the extra screws will protrude outside the case unless you countersink them. Now refer to Fig.5 and Photo 3 to see how everything goes together. Fit the LED, PCB, potentiometer, knob and toggle switches as shown. Split off the DuPont cables into sets, cut them in half, then solder them to the chassis-­ mounted parts and battery clip, using 1.5mm diameter heatshrink tubing to insulate the joints where necessary. Solder a 173mm length of wire to the aerial pad on the transmitter module and insulate the other end. Then Photo 5: these are the Adafruit DRV8871 (top), Polulu U3V16F15 (lower left) and S7V7F5 (lower right) modules. We recommend you solder the right-angle headers so that they are parallel with the board (see Fig.2 and Photo 7). siliconchip.com.au Testing the Transmitter Receiver construction First you must decide which version of the Receiver you want to build. The all through-hole version is larger at 74 × 23mm and uses a PCB coded 09110242, while the mixed SMD/TH version measures just 23 × 30mm with a PCB coded 09110243. Both versions share many parts (all the modules are the same). The main difference is that the smaller version uses an SMD microcontroller and mostly SMD passives. The smallest parts are 2.0 × 1.2mm, so they are not terribly difficult to handle, and the IC has a fairly generous 1.27mm lead pitch. The surface-mount PCB is the one I used to fit in my 85mm-long OO gauge carriage. You will need to use the SMD version if the TH board won’t fit in yours; otherwise, the choice is yours. The first task for both types of PCBs is to solder the supplied header pins to both of the Polulu DC/DC converter modules. Assemble them as shown in Fig.2, Fig.7 and Photo 6, making sure that the pins are parallel with the module PCBs. For the DVR8871 Australia's electronics magazine Scope 1: this shows the serial data that’s transmitted via a 433.9MHz wireless link with the switch in the forward position and the speed control at about halfway. module, you have to add a four-pin right-angled header; again, make sure that the pins are parallel with the DVR8871 PCB. SMD PCB assembly Since I etched mine myself, it is a single-sided design, although you can get the double-sided version from Silicon Chip, which avoids the need to fit the two wire links. The surface-mount components go on the copper side of the board, while the though-hole components and modules are inserted from the opposite side. The overlay diagram (Fig.7) shows both sides. This is a good project if you are interested in improving your SMD soldering skills, since it has a few different types and sizes of components. I am 79 and can still manage these parts. The SOIC-package PIC16F1455 will need to be programmed at some point. The easiest way is to purchase a pre-programmed PIC, although it is possible to program it in-circuit. See the panel for details if you wish to do that. Use a flux pen or a syringe of flux paste to coat the PIC16F1455 IC’s leads and its associated pads. Hold the PIC in place (eg, using tweezers) with the correct orientation and use your soldering iron to tack solder one lead in place, then check that it is positioned correctly. If so, solder the remaining leads. Clean off the flux residue and inspect the leads under magnification to ensure that all the solder joints have formed correctly. If you are not sure about any of them, add more flux and apply heat (and possibly more solder) to reflow the joint. If you have bridged any pins, use more flux and some solder wick to remove the excess solder. January 2025  73 Fig.7: the SMDs are soldered to the underside of the small Receiver PCB, as shown at right, while the through-hole parts mount on the top. MOD2 & MOD5 are shown on their sides for clarity but actually mount vertically. You can solder terminal blocks to MOD5 for the outputs, or just solder wires directly. Now use a similar procedure to fit the remaining SMDs. They are all the same size except the 47μF capacitor, which is a bit larger. The 1kW resistors will have a code like 102 or 1001 printed on top, while the code for 10kW is 103 or 1002. The capacitors will not be labelled. Finally, using an ohmmeter on its lowest range, check each passive SMD component across its terminals to make sure you haven’t accidentally created any short circuits. Turn the board over and solder in the links (if you are using a single-sided board), the two electrolytic capacitors, and the four modules. Make sure all the components are the right way around. The four modules are mounted at right-angles to the main board, although some are shown horizontally in Fig.7 for clarity. The final task is to attach the headers and connect the wires to the red LED and train motor. Disconnect the wires that connect the train wheels to the motor because we don’t want the rails to act as aerials to radiate interference from the motor brushes. For my 85mm carriage, the motor wires are 12cm long, the wires from the PCB to the connector are 7cm long, the wires from the jack plug to the PCB connector are 6cm long, the wires from the jack plug to the battery connector are 6cm long and the battery connector wire is 4cm long. All connections are insulated using heatshrink tubing. Inside the train engine, the manufacturer should have fitted two inductors in series with the motor wires (typically around 30μH) and a 100nF capacitor across the motor terminals to suppress radiation from the motor brushes often on a small PCB. 74 Silicon Chip It is important to have such a circuit, as without it, the radiated signal can be picked up by the receiver, causing potentially erratic operation. If it is missing, the train’s manufacturer should be able to supply a new one. The wires to the wheels should be disconnected from the two series inductors. The engine is powered from the carriage by a twisted pair of thin cable that connects from a two-pin male header to the two series inductors inside the engine. Finally, connect a 173mm length of multi-stranded wire to the antenna terminal of the receiver module. All connections should be insulated using heatshrink tubing. SMD version testing Connect a voltmeter between the LED anode (red) wire and the 0V battery input, and a dual-trace oscilloscope to IN1, IN2 with the Earth connected to the 0V input. Connect a variable power supply to the 4.8V battery input, with the red wire going to the positive terminal and the black to the ground terminal. Slowly increase the voltage to about 5V; the meter should read 5V. Switch on the Transmitter with the speed control set about halfway. The oscilloscope should show a 5V peakto-peak 7kHz waveform with about a 50% duty cycle on either IN1 or IN2 (depending on the position of the forward/reverse switch). Increase the speed to maximum, and the display should change to a continuous +5V DC. On reducing it to minimum, you should see a 6% duty cycle square wave. If IN1 shows the 7kHz waveform then IN2 should be at +5V, while if IN2 shows the 7kHz waveform, IN1 should be at +5V. Reduce the input voltage to less than 4V and you should see the red LED switch on. If you don’t have an oscilloscope, you can instead connect a DVM to either IN1 or IN2 (with the black probe to ground) and vary the speed potentiometer. The DVM should read the average voltage of the PWM signal, meaning it should increase smoothly as you advance the speed control clockwise. If it’s stuck at 5V, switch the DVM probe to the other terminal (IN1 or IN2). Through-hole version If you have long OO gauge carriages Fig.8: the larger Receiver board uses all through-hole parts that mount on the top. You only need to fit the three wire links if you have a single-sided board. All modules mount vertically; MOD5’s component side is towards the bottom of the PCB as shown, while MOD2 has the majority of its components near the top edge. Fig.9 (far right): the 3mm hole is for the LED, while the 4mm hole is for the jack socket. The slot is for the wires to exit the carriage and go to the engine. These are suggestions only; you can customise them for your carriage configuration. Australia's electronics magazine siliconchip.com.au or a train that will take the board and batteries, you might find building this one a bit easier. Since I etched mine myself, it is a single-sided design, although you can get the double-­ sided version from S ilicon C hip , which avoids the need to fit the wire links. Refer to the PCB overlay diagram, Fig.8. Solder in the links (if you are using a single sided board), the three electrolytic capacitors, the 14-pin IC socket and DC/DC converter modules, making sure they are orientated correctly. Then add the headers, MKT/ceramic capacitors and resistors. Wire up the red LED and train motor as shown. The length of the wires will depend on the size of the carriage you are using. All connections should be insulated by using heatshrink tubing. Through-hole version testing Check that the components are the correct way round and there are no solder bridges on the PCB. Connect the battery red wire to the positive terminal of a 5V power supply and the black wire to the 0V terminal, switch it on and use a DVM to measure the voltage between pin 1 and pin 14 of the IC socket. It should be very close to 5V. Also check the 15V supply by measure the voltage between the Vout and 0V terminals of the U3V16F15 module. The result should be very close to 15V. If all is well, fit the DVR8871 H-bridge module, 433.9MHz receiver and insert the PIC16F1455 chip into its socket, making sure they are all Fig.10: the ground wires from the battery pack and PCB are joined at the ground tab for the jack socket, while the red wires go to different pins so that the Receiver PCB is switched off when the jack plug is inserted (for charging, or just to cut the power). orientated correctly. Connect a 173mm length of multi-stranded wire to the antenna terminal of the receiver. The rest of the testing is the same as that for the surface-mount version of the PCB, so refer to that section above. The wiring lengths are different for this version, as is the position of the red LED and jack plug socket. These will depend on your train’s dimensions. Mounting the Receiver The 3mm LED and 2.5mm jack socket need to be mounted on the carriage, along with an access groove for the cable connecting to the engine. Fig.9 shows the suggested carriage cover modifications to achieve this. Fit the jack plug socket into the 4mm hole so that pin 1 is as close as possible to the side of the carriage cover. Once they are mounted, wire up the jack socket and battery as shown in Fig.10. Insulate any exposed connections with 1.5mm diameter heatshrink tubing. Next, load the battery holder with fully charged cells and connect the battery to the jack socket. Connect the black lead of a DVM to the negative wire that will go to the Receiver PCB in the engine, and the red lead to the positive wire. You should get a reading close to 4.8V (the charged battery voltage). Now plug a jack plug into the socket and check again; the voltmeter should read 0V. Next, measure the voltage across the jack plug terminals and it should be once again be close to 4.8V. Insert the red LED into the 3mm hole. Fit the PCB and battery holder as shown in Photo 9. Connect the power wires to the Receiver PCB, tucking them and the excess wire down the side of the battery holder. Coil the antenna cable and tuck it down between the PCB and the carriage end that holds the jack socket. Leave the jack plug in, as this stops power from the batteries flowing into the Receiver. Cover the wheel assembly with a strip of insulating tape where the bottom of the PCB may contact it. You can then fit the wheel assembly to the carriage cover. Final testing Switch on the Transmitter and set the speed control to its minimum position. With the engine laying on its back, connect it to the carriage. Switch on the Receiver by removing the jack plug. Rotate the speed control on the Transmitter and the engine wheels should start to move, gaining speed as the control is rotated further until maximum speed is reached. Photos 6 & 7: the top and bottom sides of the prototype SMD version of the Battery-Powered Model Train Receiver PCB. siliconchip.com.au Australia's electronics magazine January 2025  75 Photo 8: the through-hole version of the Receiver PCB is much larger than the SMD version (about twice as wide), but it is easier to assemble due to using through-hole components. Photo 9: the SMD Receiver PCB and four AAA cells just fit into a OO-gauge train carriage. Turn the control back down and the speed should decrease to zero just before minimum rotation. Repeat with the forward/reverse switch in the other position. Switch off the Transmitter and insert the jack plug to switch off the Receiver. the same speed. Switch the Transmitter on again, rotate the potentiometer fully anti-clockwise and the train should stop. Insert the jack plug to switch the Receiver off. If the red LED is lit, plug in the Charger until the batteries are charged. Always stop the train before operating the forward/reverse switch; failure to do so may destroy the motor. Always switch the Transmitter on before switching the train on, and always switch off the train off before the Transmitter. This avoids the train running by itself if in the unlikely event of an interfering signal that’s interpreted as valid by the Receiver. Running the train Place the engine and assembled carriage onto the tracks and connect the motor lead and socket. Switch on the Transmitter and turn the speed control to minimum and the forward reverse switch to forward. Remove the jack plug from the carriage (power on). Rotate the potentiometer clockwise and the train should move forward; its speed should increase with the advancement of the control. If it goes in reverse, unplug the motor leads from the train and reverse the connections. It should now run forwards. Repeat the test with the switch in the reverse position. With the train running, switch off the Transmitter; the train should continue running at Charger construction 76 Silicon Chip Fig.11: fit the parts to the Charger PCB as shown here. This also shows how to wire the off-board parts. While wires are shown soldered straight to the PCB, we recommend using headers and wires with DuPont plugs. The Charger is built on a single- or double-sided PCB coded 09110244 that measures 63 × 32mm. Its overlay diagram is shown in Fig.11. Once again, headers are not shown in the wiring but it’s easiest to use headers and plugs. Start by fitting the headers, IC socket, wire link (if needed) and the capacitors. Take care to orientate the socket and electrolytic capacitors correctly. ...continued on page 78 Australia's electronics magazine siliconchip.com.au ONLINESHOP SILICON CHIP .com.au/shop PCBs, CASE PIECES AND PANELS 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.) JUL24 JUL24 JUL24 AUG24 AUG24 AUG24 AUG24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 OCT24 OCT24 CSE240203A CSE240204A 11104241 23106241 23106242 08103241 08103242 23109241 23109242 23109243 23109244 19101231 04109241 18108241 18108242 07106241 07101222 Subscribers get a 10% discount on all orders for parts $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 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) COMPACT HIFI HEADPHONE AMP (BLUE) CAPACITOR DISCHARGER PICO COMPUTER ↳ FRONT PANEL (BLACK) ↳ PWM AUDIO MODULE DIGITAL CAPACITANCE METER BATTERY MODEL RAILWAY TRANSMITTER ↳ THROUGH-HOLE (TH) RECEIVER ↳ SMD RECEIVER ↳ CHARGER 5MHZ 40A CURRENT PROBE (BLACK) OCT24 OCT24 OCT24 NOV24 NOV24 NOV24 DEC24 DEC24 DEC24 DEC24 DEC24 JAN25 JAN25 JAN25 JAN25 JAN25 JAN25 15108241 28110241 18109241 11111241 08107241/2 01111241 01103241 9047-01 07112234 07112235 07112238 04111241 09110241 09110242 09110243 09110244 9049-01 $7.50 $7.50 $5.00 $15.00 $5.00 $10.00 $7.50 $5.00 $5.00 $2.50 $2.50 $5.00 $2.50 $2.50 $2.50 $2.50 $5.00 PRE-PROGRAMMED MICROS As a service to readers, Silicon Chip Online Shop stocks microcontrollers and microprocessors used in new projects (from 2012 on) and some selected older projects – pre-programmed and ready to fly! Some micros from copyrighted and/or contributed projects may not be available. $10 MICROS $15 MICROS ATmega328P PIC10LF322-I/OT PIC12F617-I/P 110dB RF Attenuator (Jul22), Basic RF Signal Generator (Jun23) Range Extender IR-to-UHF (Jan22) Active Mains Soft Starter (Feb23), Model Railway Uncoupler (Jul23) Battery-Powered Model Railway Transmitter (Jan25) PIC16F1455-I/P Railway Points Controller Transmitter / Receiver (2 versions; Feb24) Battery-Powered Model Railway TH Receiver (Jan25) PIC16F1455-I/SL Battery Multi Logger (Feb21), USB-C Serial Adaptor (Jun24) Battery-Powered Model Railway SMD Receiver (Jan25) PIC16F1459-I/P Mains Power-Up Sequencer (Feb24 | repurposed firmware Jul24) 8-Channel Learning IR Remote (Oct24) PIC16F15214-I/SN Digital Volume Control Pot (SMD; Mar23), Silicon Chirp Cricket (Apr23) PIC16F15214-I/P Digital Volume Control Pot (TH; Mar23), Filament Dryer (Oct24) PIC16F15224-I/SL Multi-Channel Volume Control (OLED Module; Dec23) PIC16F18146-I/SO Volume Control (Control Module, Dec23), Coin Cell Emulator (Dec23) Compact OLED Clock & Timer (Sep24), Flexidice (Nov24) STM32G030K6T6 Variable Speed Drive Mk2 (Nov24) W27C020 Noughts & Crosses Computer (Jan23) PIC16F1847-I/P PIC16F18877-I/PT PIC24FJ256GA702-I/SS PIC32MX170F256B-I/SO Digital Capacitance Meter (Jan25) Wideband Fuel Mixture Display (WFMD; Apr23) Ohmmeter (Aug22), Advanced SMD Test Tweezers (Feb23) ESR Test Tweezers (Jun24) Battery Multi Logger (Feb21), Battery Manager BackPack (Aug21) ATmega32U4 ATmega644PA-AU Wii Nunchuk RGB Light Driver (Mar24) AM-FM DDS Signal Generator (May22) $20 MICROS $25 MICROS PIC32MX170F256B-50I/SO + PIC16F1455-I/SL Micromite Explore-40 (SC5157, Oct24) PIC32MX470F512H-120/PT Micromite Explore 64 (Aug 16), Micromite Plus (Nov16) PIC32MX470F512L-120/PT Micromite Explore 100 (Sep16) $30 MICROS PIC32MX695F512H-80I/PT Touchscreen Audio Recorder (Jun14) PIC32MZ2048EFH064-I/PT DSP Crossover/Equaliser (May19), Low-Distortion DDS (Feb20) DIY Reflow Oven Controller (Apr20), Dual Hybrid Supply (Feb22) KITS & SPECIALISED COMPONENTS - 1.3in OLED for Digital Capacitance Meter (Blue – SC5026; white – SC6511) - ESP-PSRAM64H 64Mb SPI PSRAM chip (SC7377) - DS3231 real-time clock SOIC-16 IC (SC5103) - DS3231MZ real-time clock SOIC-8 IC (SC5779) COMPACT HIFI HEADPHONE AMP (SC6885) (DEC 24) CAPACITOR DISCHARGER KIT (SC7404) (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) PICO COMPUTER $15.00 $5.00 $7.50 $10.00 $70.00 $30.00 (DEC 24) 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 - PWM Audio Module kit (SC7376) $10.00 FLEXIDICE COMPLETE KIT (SC7361) (NOV 24) MICROMITE EXPLORE-40 KIT (SC6991) (OCT 24) Includes all required parts except the coin cell (see p71, Nov24) Includes all required parts (see p83, Oct24) $30.00 $35.00 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) DUAL MINI LED DICE (AUG 24) AUTOMATIC LQ METER KIT (SC6939) (JUL 24) 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) $35.00 $50.00 $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 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) $100.00 $3.00 $12 flat rate for postage within Australia. Overseas? Place an order via our website for a quote. All items subect to availability. Prices valid for month of magazine issue only. All prices in Australian dollars & include GST where applicable. 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. 01/25 Parts List – Battery-powered Model Train 1 500mm length of 1.5mm diameter black or clear heatshrink tubing various lengths & colours of light-duty hookup wire (wire for the power to the engine can be from old USB and mouse cables) Charger Fig.12: the Jiffy box needs holes at each end for the power input and charging output, plus four countersunk holes for mounting the PCB, plus three more for the pushbutton and two LEDs. Now add the resistors, which are mounted vertically, the BD136 transistor, IRL540N Mosfet, LM285-2.5V voltage reference diode, 78L05 voltage regulator, plus the 1N4148 and 1N4004 diodes. Make sure all the semiconductors are correctly orientated and in the right places. Don’t fit the PIC microcontroller yet. If you purchased the micro from the Silicon Chip Shop, it will already have the firmware loaded. If you wish to do this yourself, the files can be downloaded from siliconchip.au/ Shop/6/508 and we had some comments earlier about ways to program the chip. Once the PCB is fully assembled, check for any dry solder joints or solder bridges. It mounts in a UB3 Jiffy box that has to be drilled for the LEDs, 78 Silicon Chip 1 single- or double-sided PCB coded 09110244, 63 × 32mm 1 UB3 Jiffy box 1 9V DC 150mA+ plugpack 1 2.5mm mono jack plug (CON2) [Jaycar PP0100] 1 chassis-mount DC socket to suit plugpack (CON3) 1 chassis-mount SPST miniature momentary pushbutton (S3) 1 8-pin DIL IC socket 5 2-way pin headers, 2.54mm pitch 6 female-female DuPont jumper wires, ideally joined in a ribbon 4 M3 × 8mm countersunk head machine screws 8 M3 hex nuts 1 500mm length of single-core screened microphone cable 1 PIC12F617-I/P 8-bit microcontroller programmed with 0911024C.HEX, DIP-8 (IC4) 1 LM285-2.5 voltage reference diode, TO-92 (REF1) 1 78L05 5V 100mA linear regulator, TO-92 (REG2) 1 IRL540N 100V 36A Mosfet, TO-220 (Q1) 1 BD136/138/140 45/60/80V 1.5A PNP transistor, TO-126 (Q2) 1 5mm green LED (LED3) 1 5mm red LED (LED4) 1 1N4004 400V 1A diode (D2) 1 1N4148 75V 200mA diode (D3) 2 100μF 16V low-ESR radial electrolytic capacitors 3 100nF 50V ceramic, multi-layer ceramic or MKT capacitors 4 10kW ¼W 1% axial resistors 3 2.2kW ¼W 1% axial resistors 2 220W ¼W 1% axial resistors 1 39W 1W 1% axial resistor (for testing) 1 22W ¼W 1% axial resistor Transmitter 1 single- or double-sided PCB coded 09110241, 49 × 36mm 1 Hammond 1593Y plastic case [DigiKey, Mouser, RS] 1 3-pin 433.9MHz transmitter module, WRF43301R or XLC-RF5 (MOD1) [Little Bird, AliExpress, eBay] 1 9V battery snap 1 9V battery 1 8-pin DIL IC socket 1 3-way pin header, 2.54mm pitch 4 2-way pin headers, 2.54mm pitch 7 female-female DuPont jumper wires, ideally joined in a ribbon pushbutton, PCB mounting screws and power input socket. The drilling details are shown in Fig.12. Once the box has been drilled, attach the red and green LEDs, start pushbutton and the barrel socket as shown in the photos. The PCB is held in place by four 8mm-long countersunk head M3 machine screws and eight M3 hex nuts. The four extra nuts are used to space the PCB off the case. Use DuPont wires to make the Australia's electronics magazine connections between the PCB and the offboard components, as shown in Fig.11. Insulate all exposed connectors and the wire connections to the LEDs with 1.5mm diameter heatshrink tubing. Finish the Charger off by preparing the box, as shown in Fig.12, then mounting the PCB and all the chassis-­ mounting parts to it. Testing the Charger Make sure that the microcontroller siliconchip.com.au 2 SPDT subminiature toggle switches (S1, S2) 1 10kW 16mm linear potentiometer with large knob (VR1) 8 M3 × 6mm panhead machine screws 4 M3 × 6mm tapped hex spacers 1 PIC12F617-I/P 8-bit micro programmed with 0911024T.HEX, DIP-8 (IC1) 1 78L05 5V 100mA linear regulator, TO-92 (REG1) 1 high-intensity 5mm LED, white recommended (LED1) 1 1N5819 40V 1A schottky diode (D1) 2 100μF 16V low-ESR radial electrolytic capacitors 4 100nF 50V ceramic, multi-layer ceramic or MKT capacitors 3 10kW ¼W 1% axial resistors Receiver (common to both versions) 1 4-pin 433.9MHz receiver module, WRF43301R or XLC-RF5 (MOD2) [Little Bird, AliExpress, eBay] 1 Polulu U3V16F15 15V output step-up DC/DC converter (MOD3) 1 Polulu S7V7F5 5V output step-up/down DC/DC converter (MOD4) 1 Adafruit DRV8871 motor driver module (MOD5) 4 1.2V 900mAh NiMH AAA cells [Jaycar SB1739] 1 2×2 AAA battery holder with flying leads 1 2.5mm mono switched chassis-mounting jack socket (CON1) [Jaycar PS0105] 2 4-way right-angle pin header, 2.54mm pitch (for MOD2 & MOD5) 2 female-female DuPont jumper wires, ideally joined together 1 red 3mm LED (LED2) available from Core Electronics 🔹 🔹 🔹 🔹 Receiver (TH version only) 1 single- or double-sided PCB coded 09110242, 74 × 23mm 1 PIC16F1455-I/P 8-bit microcontroller programmed with 0911024R.HEX, DIP-14 (IC2) 1 14-pin DIL IC socket 3 100μF 16V low-ESR radial electrolytic capacitors 2 100nF 50V ceramic, multi-layer ceramic or MKT capacitors 3 10kW ¼W 1% axial resistors 1 1kW ¼W 1% axial resistor Receiver (SMD version only) 1 single- or double-sided PCB coded 09110243, 23 × 30mm 1 PIC16F1455-I/SL 8-bit microcontroller programmed with 0911024R. HEX, SOIC-14 (IC2) 1 100μF 16V low-ESR radial electrolytic capacitor 1 100μF 6.3V radial electrolytic capacitor 1 47μF 16V X5R M3216/1206 SMD ceramic capacitor 2 100nF 50V X7R M2012/0805 SMD ceramic capacitors 3 10kW ⅛W 1% M2012/0805 SMD resistors 1 1kW ¼W 1% M2012/0805 SMD resistor is not in its socket; at the same time, check the orientation of all the semiconductors and electrolytic capacitors. Connect the power supply and switch it on. Take a voltmeter and connect the red lead connected to pin 1 of the empty IC socket, and the black lead to pin 8. You should measure very close to +5V DC. If not, check that the 5V regulator is the correct way round and there aren’t any solder bridges shorting the tracks. siliconchip.com.au Assuming it’s OK, switch off the power, insert the microcontroller and connect a 39W 1W resistor between the battery terminals (eg, using clip leads). Apply power again and the green LED should flash. Press the Start button; the green LED should extinguish and the red LED should flash, indicating ‘charging’. There should be about 3.5V across the 39W resistor, indicating 90mA of current flow. To simulate a fully charged battery, Australia's electronics magazine Photo 10: the Charger board easily fits inside a UB3 Jiffy box (or a smaller case) as shown here and in Photo 3. disconnect the 39W resistor. The green LED should then flash, and the red LED will extinguish. If you want to check that the timer is working, reconnect the 39W resistor, press the Start button again and wait for 16 hours. The red LED should extinguish and the green LED will flash. Using the Charger When the battery voltage in the carriage falls below 4V, the 3mm LED in the rear of the carriage glows, alerting you that the battery needs charging. Connect the Charger to the carriage via the 2.5mm jack plug. Switch on the Charger and press the Start button to begin charging. The Charger will revert to standby mode (with the green LED flashing) when the battery is fully charged. SC January 2025  79 Using Electronic Modules with Jim Rowe TCS230-based Colour Sensor Module This interesting module can sense the colour components of any object or light source in front of it. It does this using an array of 64 tiny photodiodes, and it has four white LEDs that can illuminate a surface or object. It is compatible with almost any microcontroller, including Arduinos. T hose 64 photodiodes are split into four groups of 16: one group to detect red light, one for green, a third to detect blue, and the fourth to detect white light. As you can see from the photos, it is pretty tiny at just 33 × 33 × 30mm. That last depth dimension includes the four LEDs at the front and the two 5-pin headers at the rear. The array of 64 photodiodes it uses to detect colours are all extremely small, all inside a single SOIC-8 SMD device with a transparent top. It is mounted in the centre of the module’s PCB and surrounded by a small black plastic ‘shroud’. The SOIC-8 device concerned is the TCS230, made by US firm Texas Advanced Optoelectronic Solutions Inc (aka TAOS). They describe it as a “programmable colour light-to-­ frequency converter”. To give you a better idea of the size of those 64 photodiodes, the TAOS data sheet says that they are each only 120μm x 120μm (micrometres) in size and arranged on 144μm centres. So the total array of 8×8 photodiodes measures only about 1.3mm square. That’s pretty impressive, considering that 48 of the diodes have their own colour filter above them! Inside the TCS230 Fig.1 shows what is inside the TCS230 sensor chip. On the left, you can see the 8×8 array of photodiodes, with the 16 diodes for each colour arranged in four rows of four and the four ‘banks’ intertwined so they each get a ‘fair share’ of the light reaching the array. Note that the 16 photodiodes in each bank are all connected in parallel. The logic block shown to the right of the array allows you to select which colour bank you want using the control inputs S2 and S3 (pins 7 and 8). The logic levels used to do this are shown in the table at upper right; for example, with S2 and S3 both low, the red photodiode bank is selected, while if they are both high, that selects the green bank. The bank select block feeds the output from the selected photodiode bank into the current-to-frequency converter block to its right. It converts the current from the selected photodiode bank into a square wave with a frequency directly proportional to the current level. The current-to-frequency scaling is programmable using control inputs S0 and S1 (pins 1 and 2). These work as shown in the table at the lower right of Fig.1. If S0 and S1 are both high, the scaling is 100%, but if S1 is taken low while S0 remains high, the scaling drops to 20% and so on. If they are both taken low, the chip is powered down and there is no output. Fig.1: a block diagram of the TCS230 sensor chip. Inputs S2 & S3 can be driven low (“L”) or high (“H”) to select the a subset of the photodiodes (which selects what colour to detect), while S0 & S1 change the current-to-frequency scaling. 80 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.2: the spectral response curves for each of the photodiode colours from the TCS230. All of the curves have been normalised such that the ‘clear’ bank of photodiodes has an output frequency scaling of 100%. The ‘full-scale’ frequency with S0 and S1 both high is around 500600kHz, while if S0 is high but S1 is low, the full-scale frequency drops to 100-120kHz. If S0 is low while S1 is high, the full-scale frequency drops to 10-12kHz. The scaled-down frequency ranges allow the device to be used with lower-cost microcontrollers or applications where period measurement is more appropriate. It’s also possible to disable the output from the TCS230 device using the OE input (pin 3). If pulled high, this pin turns off the chip’s output at pin 6, while if it’s taken low (to ground), the chip works normally. Response curves The spectral response curves of the TCS230 are shown in Fig.2. All four curves are ‘normalised’ to a scaling where the response of the ‘clear’ bank of photodiodes is set to 1.0 (or 100%) at a wavelength of 680nm (nanometres). The clear bank (black plot) has a broad response curve covering the full range of wavelengths from 300nm to 1100nm, while the red bank (red plot) is similar but narrower, mainly covering the range from 570nm to 1100nm. The plots for the green bank (green plot) and blue bank (blue plot) are a bit different, consisting of ‘twin peaks’ above and below the 680nm wavelength of the clear and red bank peaks. Their peaks are also significantly lower than the clear and red bank peaks. Visible light is generally considered to cover wavelengths from 380nm to 700nm. Ultraviolet light is below 380nm, while infrared is above 700nm. As you can see, the sensor responds quite strongly to near-infrared light on all four banks. Therefore, for the best accuracy with visible light wavelengths, an infrared filter should be placed in front of it. That would also cut out the secondary blue peak entirely, and most of the secondary green peak, so they would only respond to the ‘wanted’ ranges of 380-570nm and 450-620nm, respectively. The shroud around the sensor on the module is threaded; one possible Fig.3: the TCS230 module is a simple design with few components. Transistor Q1 controls four white LEDs, which are used to illuminate the object being measured. siliconchip.com.au Australia's electronics magazine reason for that is to allow an IR filter (and/or a lens) to be screwed in. The full module circuit The full circuit of the TCS230-based colour sensing module is shown in Fig.3. As you can see, there’s not much in it apart from the TCS230 chip and the four white LEDs (LED1-LED4) that can be used to illuminate objects that do not produce light themselves. Connections to the module are via two 5-pin SIL headers, CON1 and CON2. Both headers provide pins for supply voltage Vcc (nominally +5V) and ground, making it easy to connect more than one module to a microcontroller. CON1 provides pins for connections to programming inputs S0 and S1, plus another pin to allow control of LEDs 1-4. On the other side, CON2 provides pins for controlling inputs S2 and S3, plus the frequency output from the TCS230. Programming inputs S0 and S1 are provided with 10kW pullup resistors to the Vcc line, so if no external connections are made to these pins, the TCS230 will operate at the 100% frequency scaling level by default. The S2 and S3 inputs (via CON2) have no pullup resistors because these inputs must always be driven to select a photo­diode bank. January 2025  81 Transistor Q1 controls the four white LEDs (LED1-4) connected between its collector and the Vcc line with series 330W resistors. The base of Q1 is connected to the LED input pin of CON1 and the Vcc line via another 330W resistor, so the transistor will power the LEDs by default, unless the LED pin of CON1 is pulled to ground. That gives you the option of leaving the LED pin unconnected for the LEDs to be permanently lit, connecting it permanently to a GND pin to disable them entirely, connecting a switch between the LED pin and GND to control them manually, or driving the LED pin from the digital output of a microcontroller, where a high level will switch them on and a low level will switch them off. The only other things to note about the module circuit are the 330W resistor in series with the OUT pin of CON2, presumably to protect the TCS230 from damage due to excessive load current, and the 10μF and 100nF bypass capacitors between the Vcc and ground lines to stabilise the supply voltage. Connecting it to an Arduino Fig.4 shows how easily the module can be connected to an Arduino Uno. It should be just as straightforward to connect it to any other versions of the Arduino, including the new Uno R4 Minima we reviewed recently, or to many other microcontrollers such as the Micromite or Maximite. All you need to do is connect the module’s Vcc and GND pins to the +5V and GND pins of the MCU (microcontroller unit), connect its S0-S3 programming inputs to four of the MCU’s digital outputs (IO4-IO7 here) and connect its OUT pin to one of the MCU’s digital inputs (IO8 here). Then, if you want to turn the LEDs on and off, you can connect a switch as shown. It will leave the module’s LED pin at ~0.6V when the switch is open (LEDs on) or pull it to GND when closed (LEDs off). What about software? Regarding the software needed to use the TCS230 module with an Arduino or any other MCU, Jaycar provides a listing of a simple sketch to put their XC3708 module through its paces with an Arduino Uno or similar. It is worth a try, but note that their sketch expects different connections between the module and the Arduino than those shown in Fig.4. It also does not drive the module’s S0, S1 or LED pins, so the sketch allows the TCS230 to run at 100% frequency scaling and assumes that you will have the LEDs permanently on/off or controlled manually with a switch. I found a couple of informative tutorials on the internet on using a TCS230 module with an Arduino, and both provided suitable sketches: • How To Mechatronics – https:// siliconchip.au/link/abre • Random Nerd Tutorials – http:// siliconchip.au/link/abrf The second of these sites provided the listing of a simple sketch to put the TCS230 module through its paces, written by a chap called Rui Santos. After checking that it expected the The TCS230 is primarily used to detect colours in the RGB spectrum, there’s also the similar TCS3200 which works over a wider range. module connections shown in Fig.4, I copied and pasted that into the Arduino IDE, verified and compiled the sketch and finally uploaded it to my Arduino Uno. I then held pieces of red, green, blue and white card in front of the module and checked the results in the IDE’s Serial Monitor window. I found the output a bit puzzling, so I decided to analyse what was going on in Mr Santos’s sketch. I found that in the sketch, he was using the Arduino language function pulseIn() to measure the frequency of the TCS230’s output. When I looked up that function, I discovered it actually measures the duration (length) of pulses in microseconds, not their frequency. After this discovery, I decided to adapt Mr Santos’s sketch so that it would produce the TCS230 output Fig.4: the wiring diagram for the TCS230 module to an Arduino Uno or similar. A switch can be connected to the circuit to allow the four LEDs on the module to be switched on or off. 82 Silicon Chip Australia's electronics magazine siliconchip.com.au A close-up of the TCS230 colour sensor. While not very apparent on this photo, if you look at the sensor with a microscope, you should be able to see the photodiode array. A better photo can be seen at: siliconchip.au/ link/abrg frequency rather than the pulse duration. And after a bit of trial and error, I came up with a sketch which did just that. A screen grab of the SerialMonitor window when this sketch was running is shown in Screen 1, with annotations indicating which card was in front of the TCS230 module when the measurements were taken. As you can see, the colour frequencies that match the card are generally higher than the others. With the green card, the blue values were almost as high as green, suggesting it was more of an aquamarine (blue-green) colour than a pure green. When any of the red, green or blue cards were sensed, the clear figure was roughly equal to the sum of the other three figures. Of course, this sketch is pretty basic. If you want to use the TCS230 module for some serious work – identifying specific colours, for example – you would need to improve on it considerably. But you should find this sketch a good place to start. My sketch is called “TCS230_coloursensormodule_checking_sketch.ino”, and you can download it from siliconchip.com. au/Shop/6/324 Where to buy it The TCS230-based colour sensing module shown in the photos is currently available from several suppliers, including Jaycar Electronics (Cat XC3708), for $19.95 plus delivery. A very similar module, the DFRobot SEN0101, is also available from suppliers such as DigiKey, Mouser, element14 and RS at prices ranging from $13.56 to $14.19. But note that the SEN0101 module lacks the cylindrical black plastic ‘shroud’ around the SC TCS230 sensing device. Output from our sketch adapted from the one by Rui Santos RED GREEN BLUE WHITE 15:33:39.996 -> Red = 1228 Green = 377 Blue = 484 Clear = 1945 15:33:40.371 -> 15:33:50.353 -> Red = 1213 Green = 377 Blue = 485 Clear = 1901 15:33:50.681 -> 15:34:00.710 -> Red = 447 Green = 729 Blue = 606 Clear = 1736 15:34:01.038 -> 15:34:11.020 -> Red = 447 Green = 729 Blue = 609 Clear = 1773 15:34:11.395 -> 15:34:21.377 -> Red = 437 Green = 1002 Blue = 1683 Clear = 3086 15:34:21.705 -> 15:34:31.687 -> Red = 436 Green = 1002 Blue = 1683 Clear = 3086 15:34:32.062 -> 15:34:42.044 -> Red = 2074 Green = 2192 Blue = 2762 Clear = 6944 15:34:42.372 -> 15:34:52.354 -> Red = 2074 Green = 2192 Blue = 2762 Clear = 6944 15:34:52.682 -> Screen 1: the sketch produces counts for each photodiode bank that are proportional to the frequency and thus light intensity. siliconchip.com.au Australia's electronics magazine Ideal Bridge Rectifiers Choose from six Ideal Diode Bridge Rectifier kits to build: siliconchip. com.au/Shop/?article=16043 28mm spade (SC6850, $30) Compatible with KBPC3504 10A continuous (20A peak), 72V Connectors: 6.3mm spade lugs, 18mm tall IC1 package: MSOP-12 (SMD) Mosfets: TK6R9P08QM,RQ (DPAK) 21mm square pin (SC6851, $30) Compatible with PB1004 10A continuous (20A peak), 72V Connectors: solder pins on a 14mm grid (can be bent to a 13mm grid) IC1 package: MSOP-12 Mosfets: TK6R9P08QM,RQ 5mm pitch SIL (SC6852, $30) Compatible with KBL604 10A continuous (20A peak), 72V Connectors: solder pins at 5mm pitch IC1 package: MSOP-12 Mosfets: TK6R9P08QM,RQ mini SOT-23 (SC6853, $25) Width of W02/W04 2A continuous, 40V Connectors: solder pins 5mm apart at either end IC1 package: MSOP-12 Mosfets: SI2318DS-GE3 (SOT-23) D2PAK standalone (SC6854, $35) 20A continuous, 72V Connectors: 5mm screw terminals at each end IC1 package: MSOP-12 Mosfets: IPB057N06NATMA1 (D2PAK) TO-220 standalone (SC6855, $45) 40A continuous, 72V Connectors: 6.3mm spade lugs, 18mm tall IC1 package: DIP-8 Mosfets: TK5R3E08QM,S1X (TO-220) See our article in the December 2023 issue for more details: siliconchip.au/Article/16043 January 2025  83 Extracting ROM data from old microcontrollers Chips like microcontrollers that require programming are a significant problem when repairing equipment. If that chip fails, can you get a replacement? Does the required code even still exist outside the chips in service? This article describes how such critical data can be preserved for future repairs. By Dr Hugo Holden This CRT-based video display unit is driven by the MC1468705G2 microcontroller. I wanted to extract the program data from it so I could clone it in case the original failed. B y the early 1980s, some manufacturers, like Motorola, decided to make a processor with internal RAM & ROM. It was a good decision, as that became the ultimate format for future microcontrollers. Before that, CPUs almost universally required peripheral external RAM and ROM chips to operate. The separate architecture is greatly preferred for equipment longevity and future repairs and restorations. That’s because ROM ICs can be removed from motherboards and their contents dumped at will, preserving the code. Faulty RAM is easily replaced, too. As most vintage computer restorers know, CPUs are generally fairly reliable, more so than RAM and ROM of a similar vintage. The popular ROMs at the time were various types of UV-erased EPROM. They have an attractive clear quartz window to allow for UV erasure, and you can see the IC die inside. Early types often had very large dies and are quite beautiful to examine. However, there usually isn’t any provision to get the data out of a microcontroller. So unless you can somehow get a hold of the original files used to 84 Silicon Chip program them, there is no way to generate replacements. Or is there? In the past, when I tried getting the original ROM file from the manufacturer, I had no luck. The original data files probably went into the bin decades earlier, like the Apollo 11 computers did. Generally, microcontrollers like the MC1468705G2 are used in systems controls, especially designs with touch buttons, key scanning and memory for settings. The microcontroller that’s the subject of this article was deployed in a 1980s vintage high-quality colour VDU (video display unit) made by Conrac. Conrac, based in California, was America’s premier CRT-based VDU maker. CRT-based VDUs are no longer manufactured. If this microcontroller failed, the Conrac VDU would be completely useless. Hence the desire to back up its contents. This sort of problem will be encountered more and more as time passes. Many modern appliances and equipment run internal firmware, which is usually not made publicly available. How will these devices be repaired Australia's electronics magazine without factory support when they get older? Clearly, if we want to repair and keep many vintage appliances (as they will be in the future) working, we will have to put our thinking caps on to figure out ways of extracting program data from many modern processors. In some cases, security locks make it even harder. Therefore, the purpose is not to copy manufacturers’ firmware to make rival equipment and sales but to keep vintage apparatus running. This helps prevent piles of e-waste from discarded products when they can be repaired and kept in use. It is unlikely that preserving this data from machinery that is decades old would greatly impact sales of new goods. From this point, I will refer to the original Conrac-programmed MC1468705G2 in the VDU I wanted to clone as the MCUC and blank MC1468705G2 chips I bought to clone the program onto as MCUTs. The MC1468705G2 Motorola described this device as a high-performance CMOS silicon gate technology 8-bit EPROM siliconchip.com.au microcomputer. It contains user-­ programmable, UV-erasable EPROM non-volatile memory, an oscillator, CPU and RAM. It also has an internal boot-loader program so that the micro can program itself without requiring an external programmer device, aside from some simple hardware. The MC1468705G2 has 112 bytes of onboard RAM, 2096 bytes (2KiB) of user-programmable ROM at locations 0x080 to 0x8AF, plus 11 extra bytes for programming the MOR (Mask Option Register and Vectors) at memory locations 0x1FF5 to 0x1FFF. This IC has 32 bi-directional I/O lines. The CPU was internally similar to the ubiquitous Motorola 6800. The manufacturers set it up so the user program bytes were placed in an external 8KiB ROM (such as an MCM68764 or MCM68766) in a ZIF socket on the programmer board. When the MC1468705G2’s bootloader was activated, it transferred the bytes in the external ROM into the micro’s internal ROM, a one-way trip for the data file. This programming event occurred when the reset switch on the programmer board was released. The self-­ programming was said (in the data sheet) to take 200 seconds or just over three minutes, ie, 100ms per byte. The verify program was then said to run for eight seconds. I have yet to determine why this was stated, as it is certainly not the case for the micros I have. Possibly those remarks only relate to the early version MJ3 mask variants. With the micro clocked by its usual 1MHz crystal, it takes only about one second to program the device, and the verify protocol takes another second to check all 2107 bytes. Fortunately, the verify program can be made to run separately, as this program is the key to extracting the micro’s ROM data. It might have been possible to do what I did even if I had to run the programming and verify sequence together, by disabling the micro’s Vpp programming voltage; however, that would have doubled the processing time. It appears that no program (firmware) was placed in the micro to export the ROM data from it, or at least nothing was documented by Motorola. There have been suggestions that this micro contains an “undocumented protocol” to export the files. However, without details on that, the bytes remain trapped within. The programmer board I realised that I would have to become familiar with programming this micro to have any hope of extracting the internal ROM data. If I was going to be 100% certain that I had extracted an accurate byte file from inside the IC, I would have to know what to expect. To this end, I bought three newold-stock MC1468705G2 micros and erased them, ready for programming. The next step was to build the Motorola programmer board, described in Motorola’s Application Note AN907A. It includes the circuit diagram and PCB patterns. Photo 1 shows the board I had made. I altered the PCB design a little by adding extra jumpers, for example, to run it from an external clock, to experiment with the IRQ line and to ground the Vpp line to disable micro programming while conducting some experiments. I also added headers to monitor the address lines, PA7 to PA0 and PD4 to PD0, corresponding to address lines A0 to A12. These were useful to monitor with a logic probe in the experimental stages of the project. In the Program & Verify mode, the microcontroller is held in reset by S2 being closed. +5V and -18V power is then applied via switch S1, then S2 is opened, releasing it from reset. Then the internal boot loader code runs, transferring the programming bytes from the 8KiB external ROM in the 24-pin socket into the micro. The Verify protocol runs after that. DIP switches S3, S4 & S5 are all closed in Program & Verify mode. To run just the Verify mode, S3 is left open. The board can be powered from +5V DC and -18V DC external power supplies or a 24V centre-tapped mains transformer. If a transformer is used, it is necessary to link out the -18V regulator IC. Because I ultimately used this board to power a “Data Extraction Machine” (DEM) via the 24-pin ZIF socket, I had to increase the 100μF filter capacitors to 1000μF. Extracting the ROM data In the Verify mode only, after release from reset, the CPU marches through all of the addresses in a sequence starting at 0x080 (decimal 128) for 2096 bytes, then it skips to address 0x0FF5 to verify the data in the MOR (mask option register) and then 0x0FF6 to 0x0FFF to verify the Vector locations. This adds 11 bytes, the total being 2107 bytes. Photo 1: I made this programmer board using the circuit from the Motorola data sheet. I added a few jumpers for flexibility. Photo 2: this Motorola MC1468705G2 microcontroller was one of the early examples with onboard RAM and nonvolatile storage; in this case, UVerasable EPROM. siliconchip.com.au Australia's electronics magazine January 2025  85 If all the bytes loaded to the micro’s internal ROM match the external one, the micro switches on the verify LED and the address lines go to zero and stay there. However, if a byte does not match between the micro’s internal byte file and the byte in the external ROM, the verify LED never lights. That is because the micro stops (stalls) the verification process. The program stops executing. Mercifully, the micro’s address lines stay on the exact address of the defective or mismatched byte. It does not keep incrementing the address or reset the address lines to zero in this condition. It stays there until the micro is reset. This creates the opportunity for data extraction by an external device in place of the external ROM. The idea is to keep changing the byte in the external ‘ROM’ while repeatedly cycling the verify program until the byte matches and the verify program moves on to the next byte (at the next higher address) and so on, until the entire ROM passes verification. How long might this data extraction Photo 3: my Data Extraction Machine. It plugs into the programmer board and continuously verifies the contents of the microcontroller against data in the onboard NVRAM, adjusting the data in the NVRAM one byte at a time until it matches the microcontroller code. Photo 4: the DEM plugged into the programmer board and well into the process of extracting the data, as you can see from the high address it has reached. 86 Silicon Chip Australia's electronics magazine process take? It appears to take around one second from when the micro is released from reset to verify the entire byte file. As it’s quicker if the verification fails earlier, across the entire ROM, that means an average of about half a second per attempt. In the best-case scenario, the trial byte would match on the first attempt. In the worst case, it could require 256 counts. So we can guess that, on average, it will take 128 attempts to extract one byte; about one minute at two per second. With a little over 2000 bytes to extract, that’s 2000 minutes or about 36 hours (one-and-a-half days). This system works by initiating verification, then monitoring the lower address lines, A0 and A1, for activity. If the activity stops for slightly longer than it takes to skip over the address, when the byte at the address verifies normally, that means the verification has failed. The emulator increments the trial byte to the next value, then pulses reset to re-run the Verify process. Conrac did not use all the user-­ programmable address space in the micro. The usable range is 0x080 to 0x08AF, but they actually used 0x100 to 0x7F9 and left the rest of the bytes as 0x00. There were also some zones in the used address range that were not programmed (all 0x00). If we set the trial bytes to start at zero, that will significantly accelerate the process. Having built and programmed the DEM, I found that it matched around 200 bytes per hour in the early phase of the file, slowing to around 28 bytes per hour towards the end. It took 26.2 hours to complete the process. At the end of the process, a Dallas DS1225 NVRAM IC holds a record of what is in the micro’s internal ROM. This is exactly the same byte file that was used to program the micro in the first instance. Essentially, this tool reverses the function of the Motorola programmer board and ultimately puts the micro’s byte file back into the external NVRAM. After the process is complete, the contents of the DS1225 can be extracted using a GQ-4x ROM reader, or similar, and the file saved. As the DS1225 now contains the correct data, it can also be used to program a fresh, blank micro. While I probably could have shortened that by reducing the time siliconchip.com.au Fig.1: this ‘cycle generator’ is responsible for determining when to trigger the next verification process. It waits until it’s sure the last verification step has failed before resetting the microcontroller and initiating the next one. constants on the DEM, ultimately, it was the speed that the Verify firmware runs that was the primary determinant of the total time taken. I was much more interested in reliable byte recovery than a quicker extraction. The Data Extraction Machine I designed the DEM board, shown in Photo 3, to plug into the 24-pin ZIF socket on the Motorola programmer board. Because any intermittent connection could ruin the long timeframe process, I didn’t use a breadboard or regular protoboard, but built it on a plated-through PCB with each connection carefully soldered. I covered the wiring side of the board with a styrene sheet to prevent accidental short circuits to the programmer board below. This board also obtains 5V power from the ZIF socket. Photo 4 shows the DEM plugged onto the ZIF socket on the programmer board. The siliconchip.com.au rectangular hole in the DEM is for access to the ZIF socket’s release arm. Most of the DS1225 NVRAMs in my stock now have discharged internal lithium batteries. Therefore, the DS1225 I have plugged into the DEM has an external lithium ‘support’. This is connected by milling the plastic casing down to the + battery terminal in the module, which, in this variant, is on the module’s top. Many other DS1225s have the battery at the bottom, making it more awkward to gain a connection to the battery’s + terminal. The negative terminal is pin 14 of the IC. With the additional current consumption on the 5V rail, it is worthwhile adding a flag heatsink to the 7805 voltage regulator on the programmer board (this can be seen in Photo 4). Also, as mentioned earlier, the two 100μF supply filter capacitors were increased to 1000μF. There are three plug-on wire links Australia's electronics magazine between the two boards. One is from the reset switch on the programmer board (it was isolated from the micro reset line by cutting the link track); the second is a feed to the micro’s reset pin. The third is to transistor Q6’s collector on the programmer board. This line goes low when the green verify LED activates, inhibiting the machine cycle of the monostable IC8 pin 3 on the DEM. Capacitor C1 is removed from the programmer board. Fig.1 shows the machine cycle (MC) generator on the DEM. Its job is to identify when the verify protocol has stalled due to a mismatched byte. The DS1225 on the DEM acts as a stand-in for the usual ROM with the source data that would have been in the 24-pin ZIF socket for programming the micro. Delay timers, based on BS270 Mosfets, give reliable detection of the absence of address activity but with a slow enough response time January 2025  87 that correct sequential bytes are prevented from being stepped over as the verification process proceeds. The system starts with the DS1225 blanked (all bytes = 00). If the file does verify, no MC pulses are generated. One of the problems with detecting dynamic address changes is that initially, the verify cycle starts at the address decimal 128 (0x080), with address lines A0 and A1 low. They remain low if there is a failure to verify. This means the circuit must be designed to create a machine cycle if the address is stuck at 128 for long enough. After that, though, the value 128 can be ignored. That could have been done by detecting the value of 128 across all the address lines. I decided to do it a different way, by detecting address 129 and using it to set a flip-flop. That was because this is no longer a concern once address 128 has been passed, which happens in the first minute or so. So, I can inhibit the 128 signal from that point using IC6 pin 12 and use the flip-flop output to light an indicator LED from its Q output terminal. This LED shows that everything is working, and the first byte was successfully matched. Because the Conrac micro was programmed for all zeros between addresses 128 and 255, the dynamic address detector, monitoring activity on address lines A0 and A1, was active right from the start. So it turns out I did not actually need the 128/129 fixed address detectors. But of course, I would have needed a crystal ball to know that in advance! The MC signal drives the memory controller circuit shown in Fig.2. Once a byte mismatch is detected and address activity stalls, the trial byte counter is incremented and written to the current address (that the verify program stalled on) in the NVRAM. Then the system reset is auto-pulsed by monostable IC10’s pin 12. This allows the micro to have another go at matching the new byte at that address in the NVRAM. Monostables IC8 & IC10 generate the required pulses to increment the trial byte counter (based on a 74LS393), write the new value to the Dallas NVRAM and provide the reset pulse after the MC is complete. With the bytes in the Dallas NVRAM being all initially 00, the initial block of 00s verifies very quickly, as does a block at the trailing end of the user program area. The MC signal is used to tristate (make high-impedance) the DS1225’s data pins since they are used as outputs when the programmer board is doing its verification but as inputs when incrementing the trial byte values. The MC pulse could be shortened to 5ms, but calculations suggest that would shave less than one hour off the extraction time. Most of the delays are in the time it takes the micro to run the verify program after resets, especially later in the process. I used relatively long monostable pulses in the 1.5-millisecond range to be 100% Fig.2: the microcontroller on the programmer board sets up the address lines for the NVRAM (IC14). If IC13 is active, data from the NVRAM can be read back via D0-D7 on the programming socket. When the data in the NVRAM needs to be updated, IC13 is deactivated and IC12 is activated to drive the NVRAM data lines with the next value from counter IC9. 88 Silicon Chip Australia's electronics magazine siliconchip.com.au sure that everything was stable for the NVRAM writes. Address displays Since acquiring the bytes from the micro takes a moderate amount of time, a progress indicator of some kind is desirable. I decided to use hexadecimal LED displays to show the address of the byte undergoing the matching process. The display is stable because the display modules have internal data latching. I used the Q2 output of monostable IC8 (at pin 12) to latch the current address undergoing matching into the displays. The Innocor INL0397 display modules I used are low-current CMOS versions equivalents to the famous Texas Instruments TIL311. I noticed that when the verification process has not started or is complete, address lines A0 through A12 are tristated by the micro, and sit at a high impedance. Because of that, I added 10kW pulldown resistors on these address lines to make sure they were not susceptible to noise pickup in that state, and to be sure that after power up, the displays showed 0x0000. To reduce the display brightness and current consumption, the display modules are blanked 50% of the time using a square wave oscillator, provided by two spare gates in IC4. The circuit of the address display part of the module is shown in Fig.3. Unexpected findings The MCUT devices, having similar markings to the MCUC and similar date codes (none of them being the alternative early MJ3 mask set versions), would all initiate the verify protocol when released from reset and in the verify mode, without -18V applied to the programmer board. This supply is used to apply a zener-­regulated -14V to the micro’s IRQ pin (pin 2). I preferred to keep that -18V supply disconnected. Also, I permanently grounded the Vpp pin (pin 3) of the micro with jumper J1 and had DIP switch S3 open for “Verify Only” mode. I didn’t want to accidentally damage the file by unintentionally programming the original Conrac MCUC! When I put the precious Conrac MCUC into the programmer board siliconchip.com.au Fig.3: the four displays directly read the binary address data and display it in hexadecimal (0-9 and A-F). The oscillator built from IC4b and IC4c halves their brightness using PWM control with a 50% duty cycle. and released it from reset (as I had done for all the other MCUTs I had been using), it sat there doing nothing, and the verify program did not execute. Gulp! This was a somewhat horrifying moment after all the work I had put into designing & building the data extraction system! After some experimentation, I found that the MCUC requires the -14V applied to the IRQ pin, or the verification process will not start. The MCUTs all started the process with that pin at 0V until I programmed them with the recovered Conrac byte file, and it became clear what was going on. When programmed with the extracted Conrac data, the MCUTs also required the -14V supply on their IRQ pin, behaving as the original Conrac MCUC does. The reason was that the MCUTs had been programmed with random byte data for my experiments, including the MOR register, and that affects how the micro responds when released from reset. Conclusion and outcome I have been able to program three blank micros with the original ConAustralia's electronics magazine rac file, and they all work perfectly in the Conrac VDU. My Data Extraction Machine is possibly the only one currently in existence for the MC1468705G2 microcontroller. The method outlined here may work for other similar microcontrollers as long as they have some sort of verification feature. Enough detail is presented in this article for anybody to build their own Data Extraction Machine. My design also requires the Motorola programming PCB, although the entire device could be built onto one PCB. To come full circle, I programmed the data file retrieved from the Conrac-­ programmed micro into a pair of vintage MC68766 UVEPROMs (as recommended by Motorola) as the external ROM device on their programmer board. To do this, I had to deploy my vintage BP Micro-Systems 1400 programmer, as new-generation programmers do not commonly support this UVEPROM. This programmed ROM represents what Conrac would have had in their factory in the 1980s. It verifies with the original micro from the VDU and the three replica micros I made as spare parts. SC January 2025  89 SERVICEMAN’S LOG Relating a range of rambling repairs Dave Thompson Dave has been recruited by a shadowy organisation currently attempting to master the art of underwater sheep herding. While he is on an intensive four-week course learning to speak dolphin, we have a few stories from readers. Regular service resumes next month. My work laptop is connected via gigabit LAN. Unfortunately, there is only one spare LAN port in the rumpus room, so if I need to use my private laptop, it has to rely on WiFi. We have two access points that are reasonably centrally located on the ceilings of both floors of the house. When all is well, we get usable transfer rates of 300Mb/s. Recently, I was using my personal laptop to run a Microsoft Teams session to communicate with my coworkers on Brisbane’s cross-river rail project, located in the Brisbane CBD. I found that my laptop could not connect to the WiFi, so I had to resort to using a mobile phone instead. After the session finished, I set about determining the cause of the problem. Initially, I suspected the laptop because the WiFi driver had been reinstalled recently, but I noticed that my phone was not connected to WiFi either. I went to the downstairs access point and saw that none of its three indication LEDs were lit. The hardwood floor and a few plasterboard walls do a really good job of blocking both the 2.4-2.5GHz and 5.25.9GHz WiFi signals. Our network switch powers the access point 90 Silicon Chip via power-over-Ethernet (POE). Disconnecting and reconnecting the network cable to force a reboot did not produce any joy. The network switch is a second-hand enterprise-grade item (Cisco C3560X-24P), capable of supplying 30W from all 24 ports simultaneously. I tried another port on the switch, in case its POE hardware had failed on that port, but that also failed to make a difference. A final check was to put a basic continuity tester on the ends of the patch lead to the switch and the patch lead to the access point. This proved the patch leads and the house’s fixed wiring were good. Alexandra Hills is less than 4km from Moreton Bay as the crow flies, and we are on reasonably high ground, which results in salt corrosion. We had to replace some of the RJ45 sockets that were installed in the early 2000s, before WiFi was affordable. By now, it was reasonably clear that the access point had failed. The access points require 15W (17W peak from Cisco’s data sheet), so they run reasonably hot. The oncewhite plastic housing is now very yellowed and, in places, verging on brown. My initial thoughts were that I might get lucky and that failed electrolytic capacitors could be the cause of the problem. I opened the case by removing the four Phillips head screws concealed by rubber feet. This revealed a roughly square printed circuit board with five pressed metal antennas attached to the case. There were four aluminium electrolytic capacitors, with at least one showing signs of distress (a slightly convex end). The access points can be powered from a 12V DC adaptor, which had to be purchased separately. Because it was intended to use POE, no approved adaptor was available. After a quick look around the house, I found a potentially suitable adaptor. Using the adaptor with the access point connected to a switch without POE capability, it booted up displaying an amber power LED and two flashing green LEDs (LAN and WiFi). Checking the installation guide confirmed that the power LED is supposed to be green for POE and amber for 12V DC power. I forced my phone to connect to the access point by turning its WiFi feature off and on again while in close proximity to it. Using the access point’s web interface, I verified that the phone was connected to that access point. Now there was a realistic prospect that the access point was repairable. Australia's electronics magazine siliconchip.com.au Items Covered This Month • Repairing a Cisco WAP371 access point • A recurring fault • Failures in bench grinders • ... and another problem to grind • Fixing a Ryobi electric lawn mower 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 The electrolytic capacitors are all through-hole components, so I needed access to the other side of the PCB. There were no retaining screws for the PCB. It was located in the dished top of the case (when ceiling mounted) by bosses that prevented contact with the screws retaining the relatively flat lid. The holes in the PCB were visually larger than the bosses, so it was reasonable to expect that the board could be removed without any significant force. Before proceeding, I disconnected the three black coax cables to the antennas from the PCB. The remaining two grey coax cables were directly soldered to the board, but it looked like it would be possible to flip the PCB over without disconnecting the cables; this was a big mistake. The board proved to be a very tight fit on the bosses and required some leverage to release it. It came free with a jerk that broke one of the antennas off the tiny plastic spigots retaining it. I desoldered the other antenna, freeing the board from the case. The next mistake I made was not immediately desoldering the antenna that was still attached to the board. The coax braid was severed during subsequent testing, and the repair required the cable to be shortened and stripped for re-termination. Given that the coax has an outer diameter of less than 3mm, it was a challenging task. Examining the board, the circuitry associated with the LAN side of the power and communication circuitry could be clearly identified due to a several-­ millimetre-wide band siliconchip.com.au of translucent board substrate separating it from the rest of the circuitry. The band was bridged by the switch-mode power supply transformer, an opto-isolator (for voltage regulation feedback), the LAN transformer and several very chunky surface-mounted ceramic capacitors. Only the electrolytic capacitor associated with the LAN side of the power supply tested good in-circuit. The capacitor that looked likely to be the filter capacitor on the secondary side of the power transformer (CP9) measured as a short circuit. I recorded the capacity and voltage ratings of the capacitors in preparation for their removal. Removing the three suspect capacitors was not particularly easy, even with a professional vacuum desoldering tool. The use of lead-free solder, large ground planes and possibly a multi-layer board meant that a lot of heat and time was required to melt the solder. The process was aided by applying some additional lead/tin solder to improve the heat transfer. After removal, all the capacitors failed outof-circuit testing. The bad news was that there was still a short across CP9’s pads, even without the capacitor fitted. There were several reasonably large surface-mounting diodes near the secondary side of the transformer, all of which passed basic diode tests. Closer still to the transformer was an 8-pin package (QP3) labelled 9476GM, which looked like it should be an IC. A web search found a data sheet for a 60V 7.8A Mosfet in an 8-pin SOIC package. There was a very low resistance between its source and drain connections and the pads of CP9. At this point, the penny dropped; the power supply was using synchronous rectification to improve efficiency. Removing QP3 using a hot air rework tool eliminated the short across CP9’s pads. Out-of-circuit testing of the Mosfet indicated a high-quality source-to-drain short circuit. An internet search for a supplier of a direct replacement proved fruitless but a filtered search on element14’s website for the package and Vds rating came up with the SQ4850CEY as a potential substitute (rated at 60V, 12A). Additional checks on its Vgs threshold, on-resistance and maximum permissible gate-source voltage confirmed it as a viable substitute. I ordered that Mosfet plus some replacement capacitors, all low-ESR, 105°C rated parts from the Panasonic FN series. The rest of the repair was reasonably painless. I used hot-melt glue to retain the antenna that had broken free during dismantling. The repaired access point appeared to work normally. The only peculiarity was that when the access point was returned to its normal location, it would not work. The switch diagnostics claimed that the switch was working normally. However, the switch’s log file revealed that the relevant port had detected a current overload on many occasions prior to the access point being removed for repair. After rebooting the switch, the access point worked on the port to which it was originally connected. It is possible that the switch has an undocumented feature that causes it to give up trying to supply power after a large number of overcurrent events. Australia's electronics magazine January 2025  91 As a precaution, I have replaced the capacitors in the upstairs access point. This was an interesting learning experience and helped justify acquiring quality soldering tools when I retired. Replacing them with comparable WiFi 6 access points would cost around $600. We don’t currently have any devices that would benefit from WiFi 6 (802.11ax). D. H., Alexandra Hills, Qld. Intercom woes and a recurring test equipment fault I used to work as an RF technician for a commercial TV station in Brisbane, before and during the transition from analog to digital terrestrial TV. One day, the chief engineer asked me to fix the intercom on the transmission tower. It was an Aiphone brand installed by a separate company several years ago, before I commenced working there. There were handsets in master control, the base of the tower and several platforms up the tower. Even though we had VHF radios, and ‘phones for comms, it was needed as a backup. Since its installation, it had been very noisy and basically unusable. That was put down to the interference from all the RF floating around on the tower. There was the main VHF TV transmitter, various radio base stations, microwave links etc. Intermodulation products could also be present from various RF sources mixing together on the large metal tower. There was no documentation available for the installation, just a basic Aiphone user manual that was a couple of pages, with some basic wiring, showing connection with an AC adaptor for power. I just had my trusty Fluke multimeter, so I thought I would start at the handset in Master Control, as it was inside, out of the weather. As with other fault-finding, I decided to check the power supply first. When I opened the cover, there was a terminal strip with several unlabelled white wires. There were also two white wires connected to the only marked terminals, identified as + and −. When checking power supplies, it’s good to take a reading with both the DC and AC ranges to see what is going on. The result was a surprise; I measured 13V AC and basically no DC, when it was clearly labelled DC! I was expecting DC with maybe some AC ripple. Now the problem was: where was the power supply? Luckily, the station electrician remembered that it might be in the switchboard at the base of the tower. With his help, we removed the cover panel and found a Bell transformer that was the power supply we were chasing. The wiring matched, and it was definitely putting out 13V Servicing Stories Wanted Do you have any good servicing stories that you would like to share in The Serviceman column in SILICON CHIP? If so, why not send those stories in to us? It doesn’t matter what the story is about as long as it’s in some way related to the electronics or electrical industries, to computers or even to cars and similar. We pay for all contributions published but please note that your material must be original. Send your contribution by email to: editor<at>siliconchip.com.au Please be sure to include your full name and address details. 92 Silicon Chip AC – which did seem to match that Aiphone diagram, but I think there was confusion about the designation. Because of its location, rather than replace the transformer, I installed a bridge rectifier and a couple of big electros in a Jiffy box. The system performed perfectly now on a DC supply – there was no RF interference! The chief engineer was happy, and I earned a pay rise over it! I have a second repair story. Silicon Chip or EA published a couple of component checker adaptors for CROs. They were basically a low-voltage AC plugpack with a resistive voltage divider to deliver 1V AC to the probes of the oscilloscope in X/Y mode. They were really useful for testing components in unpowered equipment without removing them. It would quickly show on the screen if a component was OK. For example, a diode would give a hockey stick shape, capacitors would be ovals, resistors a diagonal line etc. That was great if you had a known-good board and a bad board; you could quickly compare the waveforms between the same component on the two boards. We used a Hewlett Packard 5342A microwave frequency counter for testing TV microwave links. It was used in conjunction with N-Type 20W dummy loads, N-Type pads and a DC blocker. Every couple of years, it would stop working and have to be sent out for repairs. The boss blamed us for not being careful enough when using it. When it died the last time, the boss decided the company had spent enough money on repairs and ordered a new model. As the older unit was now destined to gather dust on a shelf, I thought I might as well have a look at it. I got the repair information from the last time it was sent away – it stated: replaced 12V regulator and performed calibration – $5,000! It had several power supplies, including 12V DC and 5V DC outputs. I removed the 12V regulator and it was indeed dead. Before replacing it with a new one, I checked the load with an ohmmeter with the unit unpowered. The 12V rail seemed to have a very low resistance to Earth. The unit had about eight PCBs plugged into a motherboard, with a diagonal black line across the top so you can see instantly if they are in the correct order. I unseated them one by one and found one board that was the culprit. I found the track leading from the 12V rail on the edge connector. It branched off into several directions, and I was without any circuit diagram etc. I wouldn’t recommend it, but I made one or two cuts in the track with a Stanley knife to isolate the problem. There was a tantalum capacitor not far from the edge connector. Upon removing it, I found it was nearly a short circuit. I replaced with a good one, replaced the regulator and repaired the tracks. The unit powered up fine; I didn’t do a “calibration”, but I compared with the new machine, and it measured within a fraction of a dB. It was still very useful for vehicles and choppers etc in the field. I’m retired now, but the unit was still going when I left. I don’t know if that capacitor was the problem all along; I have found faulty tantalum capacitors before in other equipment, especially after a power surge. I would always be wary of using them, especially when they are close to a power input. A. G., Jindalee, Qld. Australia's electronics magazine siliconchip.com.au Component lead failures in bench grinders I recently received two small, identical 3-inch (76mm) bench grinders after they stopped working. They were a generic brand out of China meant for hobby use, for grinding and polishing. Someone with a mechanical bent had opened them up and pronounced they had black spots on the circuit boards; something that was beyond his skills to repair. They were passed to me as someone who knows about such things! Inspection of the internals showed them to be quite wellmade with appropriately wired and insulated connections. Apart from an On/Off switch, there was a starter capacitor and a small speed controller PCB. The board had a few components around a variable resistor and a three-legged semiconductor. Fortunately, no attempt had been made to remove component numbers during assembly, as is often the case, so I could see the three-pin device was a BT137 Triac. All fairly standard stuff, I thought. Close inspection of the boards showed the central or anode pin of the Triac was damaged on both boards. In the first instance, the lead was open-circuit where the right-­ angle bend had been made in the lead to allow the Triac to lie flat on the board after soldering. No obvious cause for this was evident, other than perhaps damage caused when it was bent. I soldered a short piece of wire from the stump of the lead to the board. On application of power, the grinder worked again. The second grinder had a slightly different fault. The anode lead was intact at the bend, but where it entered the board, the hole was blackened and the lead had broken where it made contact with the solder in the hole. The broken lead was still embedded in the solder on the reverse of the board, with no sign of any soldering defects, such as a dry joint. The blackening was probably the result of arcing after the lead had broken. I cleaned up the solder, remade the connection and tested it out. This time, the grinder ran, but there was no speed control. siliconchip.com.au I replaced the Triac and, to be safe, I also replaced the Diac. The grinder then successfully ran with speed control. Why these leads broke is a mystery. My thought is that they may have fatigued due to vibration. The grinders are not that well-balanced and, at high speed, they vibrate noticeably. The Triac is not secured to the board other than by the leads, which may have put stress on the shortest lead as it shook while operating. Unfortunately, I could not drill the board and secure the Triac by its mounting hole, as there were tracks on the reverse side of the board in that area. Time will tell if my theory on vibration is correct. N. D., Ocean Beach, WA. Another problematic grinder! I was using my Ferrex 125mm angle grinder with a 1mm cutting disc to cut some roofing sheets when it suddenly stopped. I’d had this grinder for a year and I hadn’t had any problems with it until then. Had the power or extension lead failed? I plugged the grinder directly into a working power point, but it still didn’t work. I removed the brush cover and then the side cover. I could see the brushes were still in good order with plenty left on them. I used my multimeter to test and there was no problem there. Next, I tested the switch. While holding the power button in, I checked for continuity between both sides of the switch and there was none, so that was the problem, the switch had failed. This was an Australia's electronics magazine January 2025  93 unusual type of switch, an SPST momentary rocker. I did not think I would have one in stock, but I checked anyway. I went through my box of switches and I had many different types, but nothing remotely resembling this one. I wondered if I could repair the switch, so I took it apart. The fault was obvious. The tiny contact had burnt. This switch is rated at 30A 32V DC and 16(14)A 250V DC. That rating is a figment of someone’s imagination because there is no way that tiny contact could carry that much current. No wonder it had failed after just a year of occasional use. A fine file quickly restored the contact, but there was no way I could reassemble the switch. It was obviously assembled by a robot because there is no way a human could put it together with all the small parts in it. So it was time to find a replacement switch. I thought I would ring the service centre number listed on the grinder. The person I spoke to said he doubted they would have internal parts for the grinder, but he would check and get back to me, so I left my email address for him to contact me. He said that they don’t have the switch. No surprise there, as so many things these days are designed to be thrown away and not repaired when they break. While searching with Google, I spotted the exact same switch from Altronics for $3.35. That was better, but the postage was between $10 and $13, so that killed that idea. However, my son mentioned that he would be going to Brisbane that day, and it just happened that he would be driving right past the Virginia store. He said he could pick up my order, so I ordered three switches (so I would have two spares). The way it was originally put together, it had crimp 94 Silicon Chip terminals to join the Active wire from the switch to the power cable, the switch to the motor and another crimp terminal from the Neutral wire to the motor. I didn’t like the idea of these crimp terminals. As the cable had some minor damage (not affecting its safety), I decided to replace it along with the switch. There is no actual cable clamp, as the cable is held in place by the moulded cable flexible strain relief. I started by pulling out the wires, then I used a drill bit (by hand) to remove the outer section of the original cable. I forced the strain relief over the new cable and used superglue to secure it. That works really well. Next, I checked if the replacement switch would fit. Luckily, it fitted easily with no modification needed. I connected terminals to the wires to avoid soldering the switch, as I was not sure if the plastic would melt if I soldered the wires to it. I did away with the crimp wire joiners and instead soldered the wires and covered the joints with heatshrink tubing. This is the only power tool I have come across with this type of joiner, and it’s a reflection of the quality of the grinder. All my other power tools have wires long enough to connect directly to the switch, and I think they all use DPST switches as well. The accompanying photo shows the inside of the switch area of the angle grinder after replacing the faulty switch. The broken switch and the crimp connectors can be seen above the motor. Like many power tools these days, this grinder is double-­ insulated and so has no Earth wire connection. The replacement cable I used was three-core flex rated at 10A, the same as the original cable. As the Earth wire was not used, I cut it off. This was a spare cable I had saved from something no longer in service. I reassembled the grinder and tested it, and it was once again working. I put it back into service and I’ve been using it for several days now. Even though this was just a cheap angle grinder, it was worth repairing it, as it was only the switch that needed replacing. There is some degree of satisfaction in being able to repair something that is unrepairable because spare parts are not available for it. Of course, a balance has to be struck in that it can’t cost more to repair something than what it’s worth. Otherwise, it’s better to just replace it. In this case, I spent $3.35, a bit of time and a bit of heatshrink tubing to repair a $30 tool. This is not the first time I’ve repaired a power tool when spare parts were not available for it. I have an XU1 angle grinder that wore out a brush in the motor and I could not get a spare part for it. However, I managed to track down a replacement brush on eBay in England and repaired the grinder and after several years; it’s still being used. I also find replacement brushes on eBay when spare brushes are not available. I have lost count of the number of devices I’ve been able to repair and get back into working order at minimal cost. B. P., Dundathu, Qld. Ryobi lawn mower repair I’ve fixed a lot of petrol-driven garden products in my time. When petrol engines are running, they’re great. However, they can be painfully difficult to start, especially if you don’t run them often. Australia's electronics magazine siliconchip.com.au Years ago, I had a petrol chainsaw that I rarely used and it would always take ages to get going. For that reason, I used it less and less, so in the end I never really used it even when I really needed it. On impulse one day I bought a mains-powered chainsaw at auction and have never regretted it – you take it out of the cupboard, make sure there’s chain oil in it, plug it in and start sawing. Electric lawn mowers are nothing new – those Flymo mains powered mowers were around when I was a kid, but I always wondered how many minutes it would be before I ran over the power cord. Battery mowers have come a long way. A friend raved about his 36V Ryobi mower when it came up in conversation, so when I drove past a Ryobi battery mower in a council clean-up, I immediately pulled over and had a look to see if was worth taking. It all looked pretty complete except for the key, so I threw it in the back of the car and took it home. I wanted to try it out. I own a few Ryobi 18V power tools; being able to swap batteries between many different tools makes battery management much easier. This mower turned out to be an 18V product, which was perfect for me, even if it wasn’t a 36V one like my friend’s. The first thing I did was bypass the ‘key’. Battery mowers all seem to have a removable key that allows you to disable the motor – I expect this it so that toddlers can’t put anyone in danger, including themselves. Luckily for me, there are no toddlers living at my house, just the cat, and he doesn’t like mowers at all. The key just consists of a removable short circuit on a couple of 6.3mm QC spades – it’s probably a blade fuse in a special moulding. I used a fuel pump relay bypass switch I made when I was trying to get my classic car engine to start. With my switch on and a battery in the socket, it was no surprise that the motor wouldn’t run when I pulled the run lever on the end of the handle bar. The lever felt pretty floppy, and I didn’t think it was doing anything. However, I decided to open the motor section and have a look at what was under the cover. It wasn’t too hard to open; just half a dozen or so Torx screws, all the same size. It took a few minutes to find the two underneath. Once I had them all out, the lid came off and I could see a motor and a separate electronic controller. The wiring was pretty straightforward, with a pair of small gauge wires from the controller running up to the switch. Apart from a few blades of grass and some dirt, it all looked good. I disconnected the plug to the run switch and was very happy that when I shorted out the connector pins on the controller with a piece of wire, the motor started. So the problem was in the handlebar switch, or the wires to it. The wires looked OK and, as I mentioned before, the switch lever felt a bit floppy, so I took to the switch mechanism with the same Torx driver. Like the base, it came apart pretty easily and I could see how it worked. Two hands are required to operate the switch – there is a switch plunger pushed by the lever, plus a button you have to press to enable the plunger to move. I found I could run the motor by manually activating the button and plunger directly on the switch. So what was wrong? The button was releasing the plunger to move OK, but for some reason the lever wasn’t pushing on the plunger. You have to be a bit patient with these mechanisms because you can never see them operating when they’re assembled. I thought perhaps the switch mount had broken and the switch had moved back, or something had broken off the lever. It took a bit of looking, but eventually, I found a threaded hole in the end of the lever. I think there had originally been an adjustment screw that has fallen out at some stage. I found a self-tapping round-headed screw about the same size in my scratch box, and without much trouble, soon had the mechanism operating properly. That was it. After reassembling the switch and putting the cover back on the motor, I mowed until the battery went flat, with no problems at all. Going forward, I just need to figure out a better key/fuse arrangement. It works really well. The battery doesn’t last too long, and at 37cm, the cut is a bit narrower than the 46cm cut my petrol mower has, so it takes a few more passes. However, it’s really quick to get out and start mowing, and lightweight, so easy to push around. It would be great on a yard about half the size of mine. I think a 46cm/36V version might be the go, if I can find one... SC D. T., Sylvania, NSW. Left: the workaround to the missing ‘key’. Right: the internals of the Ryobi lawn mower. siliconchip.com.au Australia's electronics magazine January 2025  95 Vintage Radio The Monarch “All-American Five” Wedge Radio This “All American Five” design appeared in the late 1930s as demand for cheap domestic radios took off. Accepting five valves as necessary for a well-performing superhet radio, the “AA5” design aimed to simplify the circuit as much as possible. By Ian Batty T he most obvious first step was to eliminate the power transformer. That would make the radio lighter and smaller. Being made for the common US 110~117V AC supply, designers chose to run the valve heaters in series across the mains supply. Astor’s Mickey OZ1 (up to Serial No 460) adopted such a design from one intended for the US mains supply. As the 12V series had not been released at that date, the OZ used valves with 300mA heaters (6A7, 6D6, 6B6, 43, 25Z5). The 43 and 25Z5 worked with 25V heaters (to give a 50V drop in series), 96 Silicon Chip but the remaining three only added some 19V. At around 69V in total, operating from a 110V supply would demand a series resistor to drop around 40V – wasteful, but probably not unreasonable. Australian releases ran on 240V and needed a series resistor to soak up a massive 170V. So Astor just popped in a 580W dropping resistor, with a power dissipation of over 50W! Some US manufacturers, needing to add voltage drops to meet their 110V mains, took the ingenious idea of making the mains cord resistive. Australia's electronics magazine While it did work, it meant that to replace the cord, either a cord with identical resistance was required, or the fitting of an actual resistor inside the chassis. It also earned these cords the nickname ‘curtain burner’ – hardly ideal! One solution to the problem was to use valves with 150mA heaters and double the heater voltage to compensate. Many of the 6xxn series (6SA7, 6SK7 etc) were released in 12V versions by 1939. This change simply required a different heater wire resistance: the rest of the valve was identical. siliconchip.com.au B7G miniatures (6BE6, 6BA6, 6AV6 etc) were also re-engineered. While 12V signal valves could give sufficient emission (as the heater power was still around 1.9W), this would not give sufficient emission for 12V/150mA output valves or rectifiers. But since the heater string needed to add up to the mains voltage, why not design output valves and rectifier heaters for higher voltages? This idea resulted in the 35V 35W4 with 5.25W of heater power and the 50C5 with 7.5W (in comparison, the 6X4 had 3.8W and the 6AQ5 2.8W). The extra emission would help give better performance at the low anode voltages found in these radios. Philco’s PT44 used Loctal 7-series valves with 6.3V, 150mA heaters for the converter, IF amplifier and audio preamplification, but glass octal valves for the audio output (50L6GT) and rectifier (35Z3). The total heater voltage only added up to 103V, so the dial lamp, shunted by a resistor, made up the rest. Putting a dial lamp in series with the heaters sounds like economy, but a blown dial light would stop the radio dead. Valves such as the 35W4 rectifier were designed with a heater tap that created a suitable supply for the dial lamp and used different resistance values for the two ‘halves’ of the total heater circuit. While this worked, a blown dial lamp would allow excessive voltage across its heater section, leading to heater burnouts. If you are working on any All American Five, check whether it has a dial light in the rectifier’s heater circuit and, if so, that the lamp has the correct rating and is working. Having no mains transformer meant half-wave rectification, with a resulting low HT supply; only 95V in the PT44. Signal valves would work satisfactorily at such low supply, with the 6BE6/12BE6 specified for 100V operation with little reduction in performance. But the lowest HT specified for the venerable 6V6 and its miniature equivalents was 180V. Valve designers, needing to produce high heater voltage types such as the 50L6, took the opportunity to redesign siliconchip.com.au The Masonite rear panel has plenty of ventilation and a stuck-on circuit diagram; not something you see these days, sadly. the electrode structure, allowing the 50L6 to be fully specified with an HT requirement of only 110V. While this only offered some 2W of output, they were used in economy mantel sets, where this lower output power would be acceptable. Valves such as the 25Z5/Z6 were designed with two completely isolated rectifier diodes and were used as voltage doublers in some sets. This would easily give the more usual HT of 250V, but the extra complexity was against the design concept of the AA5, and was rarely used. The direct-from-mains transformerless design meant that such sets would run from either AC or DC supplies, and were often branded as AC/ DC sets. They would, confusingly, sometimes not work on a DC supply until the mains plug was removed and flipped over. That is, until the positive side of the mains connected to the rectifier’s anode! DC operation often gave worse performance, as the filter circuit was not being charged to the peak value of the AC mains, around 150V, giving about 125V at the filter output for approximately 50mA of HT current. The rectifier’s forward voltage drop was only about 5V at the expected current of 50mA. Starting with, say, a 110V supply, the set would only be getting some 105V of HT on DC mains. Australia's electronics magazine The Monarch AA5 The Monacor 5-1H shown on Radiomuseum bears serial number 319387. Mine has no number, but if the serial numbers for this basic design started at one, there must have been around 400,000 made! It’s a minimalist set. The combination of the low US mains voltage and 150mA heater currents allowed the mains transformer to be eliminated. However, making it work with an HT as low as 100V would have been a challenge. Either the designers would need to put in effort to deliver acceptable performance, or buyers would need to accept this was a ‘kitchen radio’ and not expect outstanding performance. It is compact – I have any number of transistor radios that considerably exceed its volume of a bit over 1600cc (1.6L), and its weight of just under 1kg. The chassis weighs just 420g! Its transformerless design makes it economical in use, consuming only 23W when running. The chassis underside photo is not distorted; the chassis front is angled to match the central depression in the cabinet face. Circuit description The circuit is shown in Fig.1; it’s a conventional five-valve superhet using a pentagrid converter and simple automatic gain control (AGC). January 2025  97 Fig.1: the monarch “Wedge” circuit with suggested test points and expected voltages. 98 Silicon Chip Australia's electronics magazine 12BE6 converter valve V1’s local oscillator section uses the common Hartley circuit, with cathode-to-gridone feedback and R1/C5 providing bias for the local oscillator (LO) circuit. The LO tuning section (C7) uses cut plates, giving a different capacitanceto-­rotation characteristic from that of the antenna section (C4) and removing the need for a padder capacitor. The moving plates are identical for both sections, so it’s the stationary LO plates that have the cut profile. The low HT voltage allows the converter screen to connect directly to HT, rather than via the dropping resistor used in most radios. The converter runs without cathode bias, but the high value of series resistor R3 allows some contact potential effect. Added to around -0.4V from the AGC circuit, this sends the converter’s signal grid to about -1.1V. With no external antenna/ground connection, this set relies on the effectiveness of its ferrite rod for signal pickup. This proved to be quite short compared to other sets, as shown in the photo of the chassis taken from above. The rod is original, and part has not broken off, as you might think. Unusually, the converter feeds to a simple LC IF circuit (L3), then capacitively couples (via 30pF C8) to the IF amplifier grid. IF amplifier valve V2, a 12BD6, has similar characteristics to the better-known 6BA6/12BA6. Like the converter, the IF amp runs without cathode bias, but the combination of the AGC line’s -0.4V and contact potential bias across 1MW resistor R2 sends the 12BD6 signal grid to around -1.1V (like the converter’s). The IF amp feeds a conventional IF transformer (IFT1) with a tuned, untapped primary and secondary and ferrite core adjustments. The signal from IFT1 feeds to both diodes in V3, the demodulator/first audio valve, a 12AV6. Capacitor C9 does the IF signal filtering, and the audio signal is developed across 500kW volume control potentiometer VR1. The AGC control voltage is picked off and sent back to the IF amp and converter via 2MW resistor R3, with the audio signal filtered out by 50nF capacitor C2. The audio signal from the volume control is fed to the 12AV6 triode’s grid via 5nF capacitor C10. Contact potential bias for the V3 triode develops siliconchip.com.au across 5MW resistor R5. The amplified audio output from its anode is fed to output valve V4 via 5nF capacitor C12, and any remaining IF signal is filtered out by 250pF capacitor C11. The 50C5 output valve’s grid returns to the chassis via R6, and the stage is cathode-biased by R7. Unusually, there is no cathode bypass capacitor. The circuit for a similar Monacor set shows output transformer T1 with a primary impedance of only 2.5kW, further confirmation of the special characteristics of the 50C5 and its low-voltage applications. ‘Full HT’ types, such as the 6V6/6AQ5, commonly require load impedances in the 5~6kW range. The mains supply connects directly to the anode of the 35W4 half-wave rectifier (V5) and to the series heater chain. This chain has the rectifier first in line, then the output valve. In common with battery-powered sets, the 12AV6 audio amplifier is the last in the chain, so that one side of its heater connects to ground, minimising any induced mains hum. The 35W4 rectifier supplies a halfwave rectified output to the first filter capacitor, C15 (30μF). This point directly supplies the output valve anode. Although the supply is not fully filtered, output pentodes are not very sensitive to power supply hum. This connection has the advantage of taking off the largest current The top view of the chassis shows the very short ferrite rod antenna, which gives mediocre performance consumption before the series filter resistor. To place the anode after the filter would increase the filter’s voltage drop by three or four times. Although the filter resistor R8 has a high resistance of 1kW, the current drain from the converter, IF amp and first audio is modest, so the filter only results in an HT drop of about 30V. The circuit diagram’s voltage callouts show the effect of AGC: on strong signals, the reducing current draw from the signal part of the converter, plus the IF amplifier, allow the RF/IF/ Audio HT to rise by around 20V. Such a voltage rise on strong signals is common, it’s just not often reported. Point-to-point wiring is used in the underside of the “Monarch Wedge” chassis, which truly is wedgeshaped. siliconchip.com.au Australia's electronics magazine The entire circuit is isolated from the chassis metalwork. I have used the ‘ground’ symbol for power and signal returns. Capacitor C1 connects the isolated ground to the chassis at radio frequencies. Restoration The original figure-eight mains cord had shed its insulation just as it emerged from the chassis, shorting it out. It’s a stark reminder to never just plug in a set in unknown condition! Fortunately, the mains cord was secured by a two-part cord anchor, so it was easy to replace the original figure-eight with a new section and secure it against movement. Given the set’s age, I was a bit apprehensive about the valves. Happily, all five tested good after a bit of time on the tester. This is a common as oxide-coated cathode formulations include barium, a highly reactive element. Barium is so reactive that barium powder scattered on a benchtop will spontaneously burst into flames! During manufacture, the applied coating contains the metallic oxides as inert carbonates. After the envelope is evacuated, induction heating and heater activation achieve two outcomes: any entrained gases in the structure ‘boil out’ and are drawn out with the evacuation, and cathode carbonates reduce to oxides. Normal operating temperatures January 2025  99 maintain the oxide compounds, but, on cooling, the highly reactive oxides tend to absorb any residual gases not already ‘cleaned up’ and oxidise to more complex compounds. Such absorption compromises the cathode’s emission and degrades performance – it’s known as cathode poisoning. Rather than a random occurrence, it’s common with valves that have been left unused for extended periods. Fortunately, all that’s needed in most cases is a few minutes of normal operation, and the cathode coating will reduce back to simple oxides. ‘Rejuvenation’, a period of over-running the heater, can accelerate the process. The valve sockets all required a good contact clean. I like to leave the radio off, applying my BWD 216 0~400V power supply to the HT line to test for electrolytic capacitor leakage. There was more than acceptable, but I left power applied, and the current fell as the two filter capacitors reformed. It all seemed to be working OK, and only needed an IF and antenna/LO alignment. The IF was a bit off, but I was able to calibrate it without difficulty. Remember that it’s important to do this for a low output, maybe 10mW, and to reduce the input signal to keep the output low as the set comes into full alignment. This is because, with higher level input signals, the AGC action ‘mushes out’ the tuning response, making the optimal peak difficult to adjust to. Like many sets, there’s no effective antenna alignment at the 600kHz end of the band, so it’s a matter of tuning to 600kHz, then tweaking the LO coil’s slug and looking for an improvement in sensitivity. The top end had trimmers on both antenna and LO sections. I simply used the LO trimmer to align to 1600kHz for full dial rotation, then dropped back to 1400kHz for the antenna trimmer. As usual, I did both ends a few times, as there is some interaction between adjustments. cathode resistor. Popping in a 470μF bypass cap brought the output stage gain up by a factor of two, doubling the sensitivity at every point, including RF sensitivity. Unmodified, its sensitivity (for a 50mW output) was 1.6mV/m at 600kHz and 550μV/m at 1400kHz. The signal+noise-to-noise ratio was 20dB or better in both cases. The IF bandwidth was ±2.2kHz at -3dB; at -60dB, it was ±39kHz. The audio response from antenna to speaker was 210-2500Hz, with a 2dB peak around 1kHz. From volume control to speaker, the audio response was 240Hz to 10kHz. Total harmonic distortion (THD) at 50mW output was 8%. The maximum output was 0.9W at 10% THD. The signal sensitivity was a bit underwhelming, and I suspect that the main culprit is the very short ferrite rod antenna, combined with the lack of cathode bypassing on the output valve. My final test demands good reception from Warrnambool’s ABC station, 3WV, at 594kHz. It was present, but only at full volume, and noisy. This compact marvel is, indeed, just what it appears to be: an economy ‘city and suburbs’ radio. Special handling Although the chassis metalwork is isolated from the chassis, this transformerless set presents an electrocution hazard. Any work with power applied must be done using an isolation transformer. Be aware that variacs and other autotransformers do not give electrical isolation. Would I buy another? I already have this example, but I’m interested in the idea of mass-­ produced minimalist radios. To me, it’s a continuation of the VE301 Volksempfänger (February 2023; siliconchip.au/Article/15671), DKE38 Kleinempfänger (July 2017; siliconchip.au/Article/10728) and their English counterpart, the “Wartime Civilian Receiver Utility Set”. Given that I only need the English unit to make a complete set, I might just check eBay once in a while. Further reading The set appears as the Monarch Wedge on Radiomuseum (siliconchip. au/link/ac1y). There’s also a 220V version, which looked identical at first glance. How did they soak up the extra mains voltage? A series resistor? No. An old and rarely used trick – pop in a series capacitor with the required reactance! Just 2.1μF will do. See siliconchip.au/ SC link/ac1z How good is it? For what it’s meant to be, pretty good. I did notice the lack of a bypass capacitor across the output valve’s 100 Silicon Chip Repairing the mains cord and cleaning the valve sockets had the radio back to operating condition. Australia's electronics magazine 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 Question about FlexiDice project I have just assembled the Flexi­ Dice kit I received recently from you (November 2024 issue; siliconchip.au/ Article/17022) and it is working well. I noticed that there are SPK1 tracks on the back panel. Was there a plan to include a speaker? I also built the Surf Sound Simulator from the same issue, which works OK too. Thanks for a couple more fun kits. (D. C., Beachmere, Qld) ● Thanks for the feedback. The pads are for a small surface-­mounting speaker, but there was no intention of the FlexiDice to have sound. We thought that readers could use the same PCB to make a small games console, in which case the speaker could be used to provide sound. We may publish new firmware to do that at some point in the future. Is a Murphy Minstrel radio worth restoring? I found my very old Fisher and Paykel M-401 radio hidden away and was surprised to find it was still operable but it needs some TLC. It is the 1958 Murphy Minstrel model. I would love to know if a Vintage enthusiast think it worth restoring or if it is actually worth something. (J. N., Tauranga, New Zealand) ● Associate Professor Graham Parslow responds: Thanks for enquiring about your Fisher and Paykel M-401 radio. The colourful New Zealand Bell radios are notably collectable. Radios that are not mainstream tend to have low market value unless you can find someone who has a sentimental attachment to that radio. This is only a guess, but I think you would struggle to get $100 unless you found a keen buyer. The odd knob is a value-killer, even though the rest looks in fair condition. This does not mean that you should not take on restoring the radio as a project for yourself if you have the skills. siliconchip.com.au When I declare something to be a project, I discount all time and expense because I am doing it for the satisfaction of the restoration. There are keen restorers in NZ that could help you. However, why let anyone else have the fun? NZ has produced a range of well-­ regarded vintage radios and the favourite in my collection is the Cromwell. You will enjoy having a visit to www. vintageradio.co.nz – your radio is also mentioned on that site. Questions about Surf Sound Simulator design I was greatly taken by this project in the November 2024 issue (siliconchip. au/Article/17018) as it seems a great improvement over another one by Craig Sellen in Circuit Notebook, July 2011 (siliconchip.au/Article/1102). I have a couple of questions about it. Firstly, why use a BC549C in particular? Do high current gain transistors have higher noise levels in reverse breakdown? Due to the lesser availability of non-polarised 33µF electrolytic capacitors, I recalculated the integrator timing resistors for 22µF capacitors. Would they be OK and should you give these values as an option for constructors? I changed the 680kW resistor to 1MW (-1.96% compared to the calculated value of 1.02MW). I changed the 330kW resistors to 510kW (+3.03% compared to the calculated value of 495kW). Furthermore, I changed the 150kW resistor to 220kW (-2.22% compared to the calculated value of 225kW). It seems that the very minor timing errors for the triangle oscillators would be negligible in this role. Would it be better to go up to 1.1MW and 240kW for the resistors with negative errors? Also tried calculating resistor values for 47µF capacitors, but they are much larger physically and the resistor errors are larger (up to 5.03%). Congratulations on such an interesting analog(!) project. ● BC549C transistors seem to be better at generating more noise in reverse breakdown compared to standard types. The circuit is easily experimented with by changing values, so you can test your different values. We suspect it would work much the same as the original design with 22µF capacitors and those new resistor values, but you should also scale the 100kW & 120kW resistors connecting to IC1a & IC1d. The slight drift between oscillators gives a more realistic effect compared to when there is a larger difference. The Fisher & Paykel M-401 “Murphy Minstrel” radio. Australia's electronics magazine January 2025  101 Regulator substitute for Vintage Radio Power Supply I am trying to order the parts to build the Power Supply for Battery-­Powered Vintage Radios (December 2020; siliconchip.au/Article/14670). I have everything sorted except the S-812C33AY-B2-U voltage regulator. It is only about $2 but the postage from DigiKey or Mouser is prohibitive. It looks like it is just a 3.3V low-dropout regulator. Can I just use a generic lowdropout regulator? Would a switch-mode regulator work instead? I am also worried about having two Lithium-ion cells in parallel. If someone were to put one cell in backwards, there would be a short-circuit and fireworks. Would it be OK to put a PTC similar to the ones used for reverse polarity protection into one of the links on the battery holder to prevent that? (P. C., Balgal Beach, Qld) ● We think a switch-mode regulator is overkill for this application. There are two things to watch out for if substituting the 3.3V regulator: 1. The input voltage rating. It could have around 7.5V applied (two fully charged Li-ion cells in series minus one diode drop), so a regulator with an input voltage rating of, say, 6V would be inadequate. 2. The input and output capacitors are both 1μF ceramic types. Not all 3.3V regulators will be stable under those conditions. You could consider using an LP2950ACZ-3.3G from element14 (Cat 2845118). It has a high maximum input voltage and is stable with 1μF of output capacitance. The only problem is that its pinout is different, but it isn’t too difficult to bend the leads of a TO-92 device to fix that. The original regulator has a pinout (looking at its flat face with the leads at the bottom) of GND, In, Out from left-to-right. The LP2950 is Out, GND, In. So you could bend the left-most lead around the other two until it is on the right. As for the cells, you are right that it would be bad if one were inserted backwards compared to the other. You could just use one cell rather than two. Alternatively, a 500mA fast-blow fuse between the two positive cell connections could prevent any high current flow for any significant length of time should one have reversed polarity. Continued: using Vintage Radio Power Supply with one battery Thanks for your help in getting my Vintage Radio Power Supply working with an alternative regulator. I have now run into another problem. I decided to just use one battery, so I removed the components not needed as per the text. However, with no “A” battery, there is never any drive to Q8, so the circuit will not work. Is it OK to just short out Q8? (P. C., Balgal Beach, Qld) ● Yes, you can short the collector-­emitter of either Q7 or Q8 if you are only using one battery. If only connecting the A battery to CON2, you should short the C-E of Q8. If only connecting the B battery to CON1, short the C-E of Q7. That way only one transistor has to be switched on to enable the outputs. However, you may prefer the result with your changes. Troubleshooting the 30V 2A Bench Supply I have built the October & November 2022 version of the 30V 2A Bench Supply (siliconchip.au/Series/389). Upon powering it up for testing and calibration, some magic smoke may have escaped, as the 100W resistor appears slightly discoloured. I got a reading of -2.9V at TP4, which rises to almost zero, 27V at TP25, 2.5V at TP1 and -0.1V at TP2. Apart from TP25, these are way out. The limit LED comes on instantly and, after a few seconds, the relay can be heard and the load LED comes on. The screen does not power up until 102 Silicon Chip the unit is powered down. I checked all component orientations and they seem correct. Is there anything glaringly obvious to you? Any more tests to reveal the fault? (J. S., Lidster, NSW) ● We suspect the wiring to voltage taps on the transformer are incorrect, since there should be -8V at TP4 if the transformer connections are correct. Presumably you managed to obtain the correct transformer specified in the parts list. We published a revised version of the supply after the stocks for the original transformer became exhausted (September & October 2023; siliconchip.au/Series/403). Both 100W resistors shouldn’t draw any significant current as they each have a 100kW resistor in series across the 25V supply. We can’t see how they would overheat unless the associated Australia's electronics magazine 100kW resistor is the incorrect value. If the display only lights when the supply is switched off, that suggests the wiring to the display is incorrect. Check the MV+ and VS+ wiring at CON5. Also check the transformer AC voltages. We found on the transformer we purchased for the prototype, the order of the terminals was different to the specification. Check that the voltages at each secondary transformer tab increment correctly; they should be 0, 18, 24 and 30V. If it is otherwise, the supply will not work properly. Alternative for Currawong transformers I had this wild idea of making a valve amplifier and decided to have a go at the Currawong (November 2014 to January 2015; siliconchip.au/ Series/277). I believe there was a kit, but it seems to no longer be available, so I decided to build it from scratch. Then I discovered that the original toroidal transformers and the recommended substitute are unobtainable. Do you have any ideas for replacements, or is it a lost cause? (M. C. P., Armidale, NSW) ● We don’t think it will be too difficult to find a transformer to power the Currawong. You just need secondary windings totalling close to 116V AC for the HT, at around 100VA, and 12V AC at around 24VA for the rest of the circuitry. The only other requirements are that they have 230V primaries and will physically fit in the box. We found one possible option from element14 (1785735). It’s rated at 100VA with both 115V and 230V primaries. It has two 115V secondaries that you could connect in parallel and then between pins 1 and 3 of CON7. Then you just need to add a small 12V transformer like Altronics M4912C or element14 1785738 to wire to CON8. Capacitor Leakage Meter troubleshooting I have assembled your Capacitor Leakage Meter from December 2009 (siliconchip.au/Article/1657). When the test leads are shorted together, all ranges from 10V to 50V work fine, giving 1mA <at> 10V, 1.6mA <at> 16V, 2.5mA <at> 25V, 3.5mA <at> 35V and 5mA <at> 50V. However, for the 63V and 100V continued on page 104 siliconchip.com.au MARKET CENTRE Advertise your product or services here in Silicon Chip FOR SALE FOR SALE USED ITEMS FOR SALE Retired Silicon Chip staff member Jim Rowe is trying to find good homes for the following items: 1. A Sony VPL-CSI LCD data and SVGA video projector ($100). 2. A Teac PC-10 portable stereo cassette recorder with 'Dolby System' and AC power pack ($75). 3. A Chinon 506-SM-XL Super-8 sound camera ($50). 4. A Pioneer VSX-D506 5-channel amplifier with a Dolby Digital decoder, and 100W output from each channel ($100). 5. An AKG D19C dynamic wideband cardioid microphone ($50). 6. An LG BP125 Blu-Ray player ($75). 7. A Toshiba SD-2500 DVD player ($40). 8. A Hantek DSO-2250 USB PC oscilloscope, with two 100MHz channels, plus an operating manual and a small software CD ($50). All of the above are available to be picked up from my home in Arncliffe, Sydney. Also available are quite a few mini file drawers with electronic components such as capacitors, resistors, transistors, ICs, LEDs and diodes, etc. These I'd be happy to give away if someone would be prepared to call and take them away. Please contact me by email to jimrowe<at>optusnet.com.au if any of the above is of interest. LEDsales KIT ASSEMBLY & REPAIR 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 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 PMD WAY offers (almost) everything for the electronics enthusiast – with full warranty, technical support and free delivery worldwide. Visit pmdway.com to get started. KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith: 0409 662 794 keith.rippon<at>gmail.com Lazer Security PCB PRODUCTION WE OFFER KITS, LEDs, LED assemblies and all sorts of quality electronic components, through-hole and SMD, at very competitive prices. Check out the latest deals at www.lazer.com.au 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. 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 January 2025  103 ranges, I only get 5mA or so test current for both. It should be 6.3mA for 63V and 10mA for 100V. What should I do? (A. J., via email) ● The test voltages at those settings could be incorrect, or the step-up circuit isn’t able to supply enough power. Check the open-circuit voltage with a multimeter to see if 63V and 100V are being generated. If not, check the divider resistors at pin 5 of IC1 for correct values. If the voltages are correct, then with the multimeter connected to the battery ground and the positive test terminal, short the test terminals. The voltage shouldn’t drop. If it does, perhaps the windings on transformer T1 are incorrect or the insulation on the Advertising Index Altronics.................................23-26 Beware! The Loop......................... 8 Blackmagic Design....................... 5 Dave Thompson........................ 103 Emona Instruments.................. IBC Jaycar............................. IFC, 51-54 Keith Rippon Kit Assembly....... 103 Lazer Security........................... 103 LD Electronics........................... 103 LEDsales................................... 103 Microchip Technology.............OBC Mouser Electronics....................... 3 OurPCB Australia........................ 11 PCBWay......................................... 7 PMD Way................................... 103 SC Bridge Rectifiers.................... 83 Silicon Chip PDFs on USB......... 50 Silicon Chip Shop................ 67, 77 Silicon Chip Subscriptions........ 59 The Loudspeaker Kit.com............ 9 Used Gear.................................. 103 wires has not been properly scraped off where it is soldered to the PCB. How to add a volume control to any amplifier My question is around fitting a potentiometer for volume control on the Hummingbird Amplifier (December 2021; siliconchip.au/Article/ 15126). I bought a single kit of the Hummingbird Amplifier from Altronics (Cat K5158). I have been through the instructions and read the last two pages about setting it up. I just want to set it up and run it with one channel of audio from ±15V DC. I bought a 30V centre-tapped transformer (Jaycar MM2005). I have built the Universal Power Supply board and I’m going to run it with the ±15V DC configuration. Where do I fit a potentiometer to control volume? I couldn’t see any mention of this in the instructions. Do I need to buy a separate volume control module? (E. M., Hawthorn, Vic) ● Referring to the accompanying circuit: 1. Connect the input signal ground to the ground of CON2. 2. Connect the input signal conductor to the clockwise end of the potentiometer track (‘B’) via a series capacitor. With the pot shaft facing you and the pins down, this will be the righthand pin. 3. Connect the pot wiper (middle pin, ‘W’) to the signal input terminal on CON2. 4. Connect the remaining pot pin (‘A’) to the signal ground (either at CON2 or the input connector, whichever is convenient). The value required for the series capacitor depends on the potentiometer value. For example, if using a 10kW potentiometer, use a minimum of 2.2µF (we’ve shown it as 10µF in the circuit, which allows a non-polarised electrolytic to be used). You could use a 2.2µF greencap or similar. You can get away with lower values Next Issue: the February 2025 issue is due on sale in newsagents by Thursday, January 30th. Expect postal delivery of subscription copies in Australia between January 24th and February 7th. Silicon Chip SMD markings are inconsistent I bought the SC6988 SMD kit for the Discrete Ideal Bridge Rectifier (September 2024 issue; siliconchip.au/ Article/16580). I got a bit stuck trying to identify the numerous SOT-23 parts. The confusing aspect is that if you search for a BC856, for example, you might find a package code of “3D”, depending on the variant, not “9AC”. By searching on the package codes, I think I have correctly identified the parts – it would be useful for others to have these codes – providing you continue to provide the same variants in the kit. Qty Code Device 2 1D MMBTA42 2 2D MMBTA92 4 9AC BC856CMTF 4 1C BC847C 4 Y2 BZT52C12/BZX84C12 Is that correct? (I. T., Duncraig, WA) ● The codes you list look correct. It is unfortunate that the same part from different manufacturers can have different markings. Each part should ideally have a standard marking for a given package. The BC84x we supply is actually a BC846C and it is indeed marked 1C. We have BC856Cs marked both 15S and 9AC; obviously, the latter is what we used for your kit. Y2 is the code listed in the Fairchild data sheet for the BZX84C12. Our supplier lists the MMBTA42 as 1D (the ST Micro data sheet says it can also be A42) and MMBTA92 can be 2D or A92. SC You can add a volume control to most amplifier modules like this. Wagner Electronics..................... 10 104 if the pot resistance is higher, but don’t make it too high, or it will introduce noise. 5-20kW is ideal for a low-noise amplifier, although value at the lower end of that range will load the signal source more. Most modern signal sources shouldn’t be bothered by that. If any of this requires significant cable runs, use shielded cable. You can use the shield to make the ground connections. 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