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Practical Electronics The UK’s premier electronics and computing maker magazine PIC n’ Mix PIC18F development board Make it with Micromite Audio Out Counting pulses and rotary encoders Circuit Surgery Germanium Interference BJT amplifier and noise Toot toot! Complete Arduino DCC Controller WIN! Microchip SAM IoT WG Development Board WIN! NeoPixel gamma correction 7 6 5 4 3 2 1 0 DIGITAL POWER ANALOG IN A0 A1 A2 A3 A4 A5 UNO 5V RES 3.3V 5V GND GND VIN Fun LED Christmas Tree offer! SCL SDA AREF GND 13 12 11 10 9 8 Amazing Nutube miniature valve stereo preamplifier PLUS! Fascinating XOD/Arduino thermometer Techno Talk – Lights out for dangerous bulbs? KickStart – NEW series! – using the 2N7000 MOSFET Net Work – New electric cars and Android TV www.electronpublishing.com <at>practicalelec Jan 2021 £4.99 01 9 772632 573016 practicalelectronics Compact, Powerful MCUs PIC18-Q41 Family for Improved Sensor Interface Designs The PIC18-Q41 family of microcontrollers (MCUs) combines sophisticated analog peripherals and powerful Core Independent Peripherals (CIPs) for small, high-performance data acquisition and sensor-interfacing applications. Available in small 14- and 20- pin packages, these MCUs are equipped with an operational amplifier, a 12-bit Analog-to-Digital Converter with Computation, and 8-bit Digital-to-Analog Converters providing a high level of analog integration for amplification, filtering and signal conditioning. With our comprehensive development tool suite, you can easily configure peripherals and functions, generate application code and simulate analog circuits prior to hardware prototyping to reduce your development time and speed your time to market. PIC18-Q41 MCUs are well suited for IoT edge nodes, medical, wearables, LED lighting, home automation, automotive and industrial process control. Key Features • Up to 64 KB Flash program memory/up to 4 KB data SRAM/512B data EEPROM • Small-footprint packages for sensor interface applications • Faster time to market with MPLAB® Code Configurator and MPLAB Mindi™ Analog Simulator • Easy-to-use development tools to get your design to market faster www.microchip.com/Q41 The Microchip name and logo, the Microchip logo and MPLAB are registered trademarks and Mindi is a trademark of Microchip Technology Incorporated in the U.S.A. and other countries. All other trademarks are the property of their registered owners. © 2020 Microchip Technology Inc. All rights reserved. DS40002255A. MEC2345-ENG-11-20 Practical Electronics Volume 50. No. 1 January 2021 ISSN 2632 573X Contents Projects and Circuits utu e iniature alve tereo Pream lifier by John Clarke 14 Korg and Noritake Itron of Japan recently released their Nutube 6P1 twin triode. t’s party piece is that it can operate from ust a few Complete Arduino DCC Controller by Tim Blythman 26 Build this fantastic Arduino-compatible controller shield that forms the basis of a CC system t can also be used as a CC booster or high-current motor driver Intelligent 8x8 RGB LED matrix by Jim Rowe 34 Master a module with an matri of ‘intelligent’ B E s he E s are controlled via a single wire and multiple modules can be cascaded 7 6 5 4 3 2 1 0 SCL SDA AREF GND 13 12 11 10 9 8 Series, Features and Columns DIGITAL POWER ANALOG IN 5V RES 3.3V 5V GND GND VIN A0 A1 A2 A3 A4 A5 UNO Made in the UK. Written in Britain, Australia, the US and Ireland. Read everywhere. © Electron Publishing Limited 2020 Copyright in all drawings, photographs, articles, technical designs, software and intellectual property published in Practical Electronics is fully protected, and reproduction or imitation in whole or in part are expressly forbidden. The February 2021 issue of Practical Electronics will be published on Thursday, 7 January 2021 – see page 72. Practical Electronics | January | 2021 The Fox Report by Barry Fox HDMI capture dongle update 8 Techno Talk by Mark Nelson Bad boy bulbs 10 Net Work by Alan Winstanley he latest news on electric cars tips and tric s for running an Android-based V system and advice on setting up a two-screen indows PC 12 KickStart by Mike Tooley Part M S E switching devices in linear applications – introducing the 2 38 000 PIC n i by Mike Hibbett Part P C development board 43 Audio Out by Jake Rothman heremin Audio Amplifier – Part 46 Make it with Micromite by Phil Boyce Part 2 Counting pulses rotary encoders and a digital safe 50 Circuit Surgery by Ian Bell Interference and noise 54 a s Cool eans y ax The agnificent lashing E s and drooling engineers – Part 58 Visual programming with XOD by Julian Edgar ight Column hermometer 63 Regulars and Services Wireless for the Warrior Subscribe to Practical Electronics and save money NEW! Practical Electronics back issues DOWNLOADS – great 20-year deal! Reader services – Editorial and Advertising Departments Editorial oo ing forward to 202 Exclusive Microchip reader offer in a Microchip SAM o evelopment Board LED Christmas Tree offer Practical Electronics – get your back issues here! PE Teach-In 9 PE Teach-In 8 Classified ads and Advertiser inde Practical Electronics PCB Service PCBs for Practical Electronics pro ects Direct Book Service Build your library of carefully chosen technical books Next month! – highlights of our next issue of Practical Electronics 2 4 6 7 7 11 25 25 42 66 68 70 72 1 WIRELESS FOR THE WARRIOR by LOUIS MEULSTEE THE DEFINITIVE TECHNICAL HISTORY OF RADIO COMMUNICATION EQUIPMENT IN THE BRITISH ARMY The Wireless for the Warrior books are a source of reference for the history and development of radio communication equipment used by the British Army from the very early days of wireless up to the 1960s. timeframe saw the introduction of VHF FM and hermetically sealed equipment. The books are very detailed and include circuit diagrams, technical specifications and alignment data, technical development history, complete station lists and vehicle fitting instructions. Volume 3 covers army receivers from 1932 to the late 1960s. The book not only describes receivers specifically designed for the British Army, but also the Royal Navy and RAF. Also covered: special receivers, direction finding receivers, Canadian and Australian Army receivers, commercial receivers adopted by the Army, and Army Welfare broadcast receivers. Volume 1 and Volume 2 cover transmitters and transceivers used between 1932-1948. An era that starts with positive steps taken to formulate and develop a new series of wireless sets that offered great improvements over obsolete World War I pattern equipment. The other end of this Volume 4 covers clandestine, agent or ‘spy’ radio equipment, sets which were used by special forces, partisans, resistance, ‘stay behind’ organisations, Australian Coast Watchers and the diplomatic service. Plus, selected associated power sources, RDF and intercept receivers, bugs and radar beacons. ORDER YOURS TODAY! JUST CALL 01202 880299 OR VISIT www.electronpublishing.com Quasar Electronics Limited PO Box 6935, Bishops Stortford CM23 4WP, United Kingdom Tel: 01279 467799 E-mail: sales<at>quasarelectronics.co.uk Web: quasarelectronics.co.uk All prices include 20% VAT. Free UK mainland delivery on orders over £60. Postage & Packing Options (Up to 0.5Kg gross weight): UK Standard 2-5 Day Delivery - £4.95 : UK Mainland Next Day Delivery - £9.95 : Please order online if you reside outside the UK (our website will calculate postage for you). Payment: We accept all major credit/debit cards. Make UK cheques/PO’s payable to Quasar Electronics Limited and include P&P detailed above. !! Order online for reduced price postage and fast despatch !! Please visit our online shop now for full details of over 1000 electronic kits, projects, modules and publications. Discounts for bulk quantities. Solutions for Home, Education & Industry Since 1993 Brightdot Clock Kit - BLACK Edition Official Main Dealer stocking the full range of Kits, Modules, Robots, Instruments, Tools and much, much more... Electronic Kits & Modules We have a massive selection of selfassembly electronic kits and preassembled modules. Please see the full range on our website or call for details. LED Buddy / LED Tester Kit Hold any type of LED to the contact pads to see it's polarity, forward voltage & the recommended series resistor value. Adjustable target current & forward voltage. Great design aid. 1x PP3 battery powered. Order Code: MK198 - £13.92 3-in-1 All Terrain Robot Kit Multifunction tracked mobile robot. Transform into 3 amazing models using different track modules: Forklift, Rover (shown above) and Gripper. The wired controller commands the robot to make it move forward, backward, turn, grip or lift. Solderless assembly. Requires 4x AA batteries. Order Code: KSR11 - £34.40 Snowman Flashing LED Kit Have some educational festive fun with this animated LED snowman gadget. 69 multicoloured LEDs (great for soldering practice). Snowflake effect with PWM controlled LEDs. Random generator for a more realistic effect. 1x 9V PP3 battery or 9-12Vdc wall adapter powered. Makes an ideal gift. Order Code: MK200 - £20.34 Brighten any room or space with this fully Arduino® compatible, ESP32 controlled BrightDot clock kit. This designer black edition features 60 bright RGB LEDs that reflect against the surface on which you mount the clock, hence telling you what time of day it is. ESP32 data cable & power supply included. Order Code: K2400B - £117.43 DIY Electronic Watch Kit Make your own DIY, Arduino compatible electronic wrist watch! 24 amber coloured LEDs are bright enough to be clearly visible in broad daylight! Pre-programmed with an addictive reflex game and of course with a basic time view. You can easily re-program it to your liking by using open-source Arduino® library and the K1201 Custom Cradle Kit or a USB to UART module (neither included). Order Code: K1200 - £23.94 Stereo Ultrasonic Bat Detector Kit Converts high frequency sounds (20 90kHz) normally imperceptible to humans like bat signals into audible noise. Can also help detect failures in machines, engines, etc. Stereo feature adds the possibility to pinpoint the source. Requires 3xAA batteries. 3.5mm jack output for headphones. Order Code: K8118 - £21.59 LED Christmas Tree Kit Ho! Ho! Ho! The classic Christmas kit for the budding electronics enthusiast. 15 blinking blue LEDs. Requires 1xPP3 battery. Get cosy… Order Code: MK100B - £7.19 Card Sales & Enquiries Digitally Controlled FM Radio Kit Build your own modern, high quality FM receiver project with excellent sensitivity powered by a simple 9V PP3 battery (not included. Auto-seeking button. 4 station presets. Volume control. Excellent learning project for schools and colleges. Order Code: MK194N - £20.39 Audio Analyser Display Kit Small, compact LCD display, ideal for panel mounting. Give your homemade audio gear a high-tech look. Upgrade existing equipment. Provides Peak Power, RMS Power, Mean dB, Peak dB, Linear Audio Spectrum And 1/3 Octave Audio Spectrum. Auto / Manual range selection. Peak-hold function. Speaker impedance selection. Order Code: K8098 - £39.54 Electronic Component Tester Kit Build your own versatile component tester. Shows value and pin layout information for resistors (0.1 Ohm resolution, max. 50 MOhm), coils (0.01mH - 20H), capacitors (28pF - 100mF), diodes, BJT, JFET, E-IGBT, D-IGBT, E-MOS & D-MOS. Order Code: K8115 - £44.34 LCD Oscilloscope Educational Kit Build your own LCD oscilloscope with this exciting new kit. Learn how to read signals. See the electronic signals you learn about displayed on your own LCD oscilloscope. Despite the low cost, this oscilloscope kit has a lot of features found on expensive units like signal markers, frequency, dB, true RMS readouts and more. A powerful autosetup function will get you going in a flash! Order Code: EDU08 - £48.54 Practical Electronics Practical Electronics Practical Electronics – N NE E EW W P E D NA – ES M IG E N ! Practical Electronics – N NE E EW W P E D NA – ES M IG E N ! Practical Electronics – N NE E EW W P E D NA – ES M IG E N ! – N NE E EW W P E D NA – ES M IG E N ! UK readers SAVE £1 on every issue Practical Electronics NEW subscriptions hotline! The UK’s premier electronics and computing maker magazine Audio Out Super low-noise power supply for your theremin Practically Speaking Flowerpot speakers! Getting to grips with surface-mount ICs Micromite LCD BackPack V3 The UK’s premier electronics and computing maker magazine A low-cost route to high-quality Hi-Fi PIC n’ Mix WIN! Microchip J-32 Debug Probe WIN! Bargain Class-D Amplifier Steering Wheel Audio Button Adaptor IR control of Robot Buggy Low-noise theremin PSU Net Work – Yubico’s latest Security Key Techno Talk – The benefits of hindsight Electronic Building Blocks – Battery capacity tester www.electronpublishing.com <at>practicalelec Build your own exercise bike generator Make it with Micromite WIN! Microchip Explorer 16/32 Development Kit Aug 2020 £4.99 08 9 772632 573016 practicalelectronics The UK’s premier electronics and computing maker magazine Pedal Power Station! GPS modules with UART communication Arduino-based Digital Audio Millivoltmeter WIN! Ping-pong ball lighting! PLUS! Pedal Power Station! Software tools for the PIC18F Ultrabrite LED bike light The UK’s premier electronics and computing maker magazine Add electronics to the exercise bike generator Practically Speaking Restoring vintage electronic equipment Circuit Surgery Understand analogue multipliers WIN! Microchip Curiosity Development Board WIN! WIN! Constructing the High-power 45V/8A Variable Linear Supply Introducing the K40 laser cutter/engraver Precision ‘Audio’ Signal Amplifier PLUS! PLUS! Cool Beans – Even cooler ping-pong ball lights! Net Work – IP security cameras Techno Talk – The perils of an enquiring mind... www.electronpublishing.com <at>practicalelec Techno Talk – 5G craziness! Net Work – Cybercriminals – honour among thieves? Cool Beans – Subtle fade up/down with NeoPixels practicalelectronics www.electronpublishing.com <at>practicalelec Oct 2020 £4.99 PLUS! 10 9 772632 573016 practicalelectronics For Christmas: a fabulous LED Tree Micromite GPS-based ring clock Theremin amplifier Techno Talk – Every little helps Cool Beans – NeoPixel sophistication Net Work – Internet shopping? – It’s all about trust www.electronpublishing.com <at>practicalelec Circuit Surgery WIN! Microchip SAM IoT WG Development Board WIN! 01202 087631 Robot Buggy Ultrasound sensing! Audio Out Germanium Interference BJT amplifier and noise Five-way LCD Panel Meter / USB Display High-power 45V/8A Variable Linear Supply Meet the Micromite Explore-28 Mastering stepper motor drivers PIC n’ Mix PIC18F development board Toot toot! Complete Arduino DCC Controller WIN! Microchip MPLAB PICkit 4 In-Circuit Debugger The UK’s premier electronics and computing maker magazine Nov 2020 £4.99 11 9 772632 573016 practicalelectronics NeoPixel gamma correction Amazing Nutube miniature valve stereo preamplifier Fun LED Christmas Tree offer! Fascinating XOD/Arduino thermometer PLUS! Techno Talk – Lights out for dangerous bulbs? KickStart – NEW series! – using the 2N7000 MOSFET Net Work – New electric cars and Android TV www.electronpublishing.com <at>practicalelec practicalelectronics Take out a one-year subscription and save £10 over the year. Even better – save £1 per issue if you subscribe for two years – a total saving of £24. 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€6) plus P&P – most recent months still in stock (5) Now you can order SILICON CHIP archives on flash drive: a quality metal flash drive containing any five year block of SILICON CHIP – from the first issue back in 1987 until Dec 2019 (see siliconchip.com.au/shop/digital pdfs for details) Log onto www.siliconchip.com.au for much more information! You might also be interested in: Radio, TV & Hobbies on DVD Take a trip back in time for the entire Radio, TV and Hobbies magazine, from April 1939 through to March 1965 – ready to enjoy at your leisure, again and again and again. Comes in a protective case – and it’s just $AU62 plus p&p (about £31.60/€36) Or you can download the digital edition of Radio, TV & Hobbies: $AU50 (£25.50 /
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Upgrade to the new 20-year bundle for just £9.95 with: PE15to20 Download your collections at: www.electronpublishing.com Practical Electronics Editorial offices Practical Electronics Electron Publishing Limited 1 Buckingham Road Brighton East Sussex BN1 3RA Tel 01273 777619 Mob 07973 518682 Fax 01202 843233 Email pe<at>electronpublishing.com Web www.electronpublishing.com Advertisement offices Practical Electronics Adverts Tel 01273 777619 1 Buckingham Road Mob 07973 518682 Brighton Email pe<at>electronpublishing.com East Sussex BN1 3RA Editor Matt Pulzer General Manager Louisa Pulzer Digital subscriptions Stewart Kearn Tel 01202 880299 Online Editor Alan Winstanley Web Systems Kris Thain Publisher Matt Pulzer Print subscriptions Practical Electronics Subscriptions PO Box 6337 Bournemouth BH1 9EH Tel 01202 087631 United Kingdom Email pesubs<at>selectps.com Technical enquiries We regret technical enquiries cannot be answered over the telephone. We are unable to offer any advice on the use, purchase, repair or modification of commercial equipment or the incorporation or modification of designs published in the maga ine e cannot provide data or answer queries on articles or projects that are more than five years old Questions about articles or projects should be sent to the editor by email: pe<at>electronpublishing.com Projects and circuits All reasonable precautions are taken to ensure that the advice and data given to readers is reliable. We cannot, however, guarantee it and we cannot accept legal responsibility for it. A number of projects and circuits published in Practical Electronics employ voltages that can be lethal. You should not build, test, modify or renovate any item of mains-powered equipment unless you fully understand the safety aspects involved and you use an RCD (GFCI) adaptor. Component supplies We do not supply electronic components or kits for building the projects featured, these can be supplied by advertisers. We advise readers to check that all parts are still available before commencing any project in a back-dated issue. Advertisements Although the proprietors and staff of Practical Electronics take reasonable precautions to protect the interests of readers by ensuring as far as practicable that advertisements are bona fide the magazine and its publishers cannot give any undertakings in respect of statements or claims made by advertisers, whether these advertisements are printed as part of the magazine, or in inserts. The Publishers regret that under no circumstances will the magazine accept liability for non-receipt of goods ordered, or for late delivery, or for faults in manufacture. Volume 50. No. 1 January 2021 ISSN 2632 573X Editorial Looking forward to 2021 Where to begin with the year 2020? It has certainly been a year we’ll never forget: lockdowns, endless statistics, economic uncertainty and sickness. While it’s no antidote to a pandemic, I do hope Practical Electronics has provided a welcome distraction from some very serious problems. With luck, and a huge amount of brilliant work from dedicated researchers, 2021 may well bring a vaccine and the chance to put the virus behind us. In the meantime, I hope you enjoy this month’s Practical Electronics, which marks the very welcome return of PE/EPE stalwart Mike Tooley. He is starting a new series – KickStart – which aims to cover a wide range of topics with useful down-toearth advice, tips and hints. For those of you who enjoy building Hi-Fi with valves we have a special treat in the shape of the Nutube Miniature Valve Stereo Preamplifier. This is based around a fascinating new low-voltage thermionic valve technology that we hope to use again in PE. Thank you, PE writers! Practical Electronics stands or falls with the quality of its writing. I always end a year with a thank you to our contributors. They work very hard every month to bring you high-quality, original content. So, in no particular order – a great big ‘well done and thank you’ to Alan Winstanley, Mike Tooley, Ian Bell, Mark Nelson, Mike Hibbett, Clive ‘Max’ Maxfield, Phil Boyce, Julian Edgar, Barry Fox and Jake Rothman. Just as important are the ‘back office boys’, Stewart Kearn, (again) Alan Winstanley and Kris Thain, who keep the shop and website ticking over. Extra special Christmas present? Even – or perhaps especially – a socially distanced holiday season still comes with the unavoidable ‘What would you like for Christmas?’. For you though, lucky reader, the answer is easy – ‘I just want a subscription to my favourite magazine.’ You can choose paper or online, and as a subscriber you can be sure that you will never miss a copy. From all of us at Practical Electronics, thank you for your support over 2020, have a very happy Christmas and a healthy 2021! Matt Pulzer Publisher Transmitters/bugs/telephone equipment We advise readers that certain items of radio transmitting and telephone equipment which may be advertised in our pages cannot be legally used in the UK. Readers should check the law before buying any transmitting or telephone equipment as a fine confiscation of equipment and or imprisonment can result from illegal use or ownership. The laws vary from country to country; readers should check local laws. Practical Electronics | January | 2021 7 The Fox Report Barry Fox’s technology column HDMI capture dongle update I promised an update report on the absurdly low- priced (under a tenner) HDMI capture dongle which I test-purchased (see The Fox Report, PE, October 2020.) These little devices are now widely available online, direct from Chinese suppliers and via more local importers – at a wide range of prices starting at around £7 (US$10). The generic name is ‘HDMI to USB Video Capture Card 1080P 2.0 For Game / Live Streaming’. Some middlemen are labelling these dongles with their own model names, but the basic specification, and very likely the chipset, remains the same: ‘Input (HDMI) resolution 3840×2160<at>30Hz, output (USB) resolution 1920×1080<at>30Hz. Support 8/10/12-bit deep colour. Support most acquisition software, such as VLC, OBS, Amcap, etc. Support Windows, Android and macOS. No external power supply is required. Video Output Mode: YUV / JPEG. Support Audio Format: L-PCM’. Easy to use No instructions are needed. Just plug one end of the dongle (which is around the same size as a USB memory device) into the HDMI output of pretty much any media player, set-top box, game console or online entertainment system; connect the other end of the dongle by USB lead to a USB socket on a computer which is running capture software, such as free Open Source OBS (Open Broadcaster Software). The capture software ‘sees’ the dongle as a simple USB media device and captures whatever it delivers. It’s that simple. The only complication is the way OBS offers a myriad of setup options and likes to default to settings which mute the sound. Time spent on the OBS help pages and googling user experience is time well spent. It’s a small price to pay for a wonderfully flexible free program. Higher-priced alternatives Super-cheap HDMI capture dongle – it does exactly what it says on the case... ...but unlike its (much) more expensive rivals it does require a reasonably powerful PC. 1552 hand-held plastic enclosures One cautionary note; most of the capture devices sold by big name companies such as Hauppauge and Elgato, and costing ten or twenty times the dongle price, have onboard processing power which takes processing load off the computer running the capture software (such as OBS). This enables glitch-free capture on low-spec computers. Some of these ‘name’ devices also cater for component video and analogue and SP/DIF audio inputs; the new dongles do not. If you use the budget dongle with a budget computer, the captured video and audio will stutter and judder. But use it with a reasonably powerful computer and the dongle delivers smooth and glitch-free results. I tried it with a low-end Acer Windows 10 laptop and ageing HP desktop and the results were unacceptable; but with a Mac laptop it worked like a dream. ! w ne Learn more: hammfg.com/1552 Contact us to request a free evaluation sample. uksales<at>hammfg.com • 01256 812812 8 Practical Electronics | January | 2021 Copyright / copy wrong Tests suggest that the dongle simply ignores the HDCP copy protection which is tied to the HDMI standards; so, it enables copyright infringement. Perhaps sales can and will be banned; perhaps not. We are simply reporting the facts on devices which are now widely on sale, with China apparently the prime source. What you use these dongles for is your responsibility. I have found an HDMI-to-USB dongle a very good way of digitising my own old home videos, by first converting analogue AV from a VCR to HDMI (see The Fox Report, PE, September 2020) with one of the equally cheap converter devices that are now also widely available. The analogue-to-HDMI conversion will often stabilise ageing video. AVAILABLE NOW! PE VOL 49 January to December 2020 P files ready for immediate download All 2 2020 issues for ust £25.95 See page 6 for further details and other great back-issue offers. VCR rescue! Combine an HDMI capture dongle with one of the many cheap and simple SCART-to-HDMI converters to get your tape-based analogue video into the digital domain. Purchase and download at: www.electronpublishing.com STEWART OF READING 17A King Street, Mortimer, near Reading, RG7 3RS Telephone: 0118 933 1111 Fax: 0118 933 2375 USED ELECTRONIC TEST EQUIPMENT Check website www.stewart-of-reading.co.uk Fluke/Philips PM3092 Oscilloscope 2+2 Channel 200MHz Delay TB, Autoset etc – £250 LAMBDA GENESYS LAMBDA GENESYS IFR 2025 IFR 2948B IFR 6843 R&S APN62 Agilent 8712ET HP8903A/B HP8757D HP3325A HP3561A HP6032A HP6622A HP6624A HP6632B HP6644A HP6654A HP8341A HP83630A HP83624A HP8484A HP8560E HP8563A HP8566B HP8662A Marconi 2022E Marconi 2024 Marconi 2030 Marconi 2023A PSU GEN100-15 100V 15A Boxed As New £400 PSU GEN50-30 50V 30A £400 Signal Generator 9kHz – 2.51GHz Opt 04/11 £900 Communication Service Monitor Opts 03/25 Avionics POA Microwave Systems Analyser 10MHz – 20GHz POA Syn Function Generator 1Hz – 260kHz £295 RF Network Analyser 300kHz – 1300MHz POA Audio Analyser £750 – £950 Scaler Network Analyser POA Synthesised Function Generator £195 Dynamic Signal Analyser £650 PSU 0-60V 0-50A 1000W £750 PSU 0-20V 4A Twice or 0-50V 2A Twice £350 PSU 4 Outputs £400 PSU 0-20V 0-5A £195 PSU 0-60V 3.5A £400 PSU 0-60V 0-9A £500 Synthesised Sweep Generator 10MHz – 20GHz £2,000 Synthesised Sweeper 10MHz – 26.5 GHz POA Synthesised Sweeper 2 – 20GHz POA Power Sensor 0.01-18GHz 3nW-10µW £75 Spectrum Analyser Synthesised 30Hz – 2.9GHz £1,750 Spectrum Analyser Synthesised 9kHz – 22GHz £2,250 Spectrum Analsyer 100Hz – 22GHz £1,200 RF Generator 10kHz – 1280MHz £750 Synthesised AM/FM Signal Generator 10kHz – 1.01GHz £325 Synthesised Signal Generator 9kHz – 2.4GHz £800 Synthesised Signal Generator 10kHz – 1.35GHz £750 Signal Generator 9kHz – 1.2GHz £700 HP/Agilent HP 34401A Digital Multimeter 6½ Digit £325 – £375 HP 54600B Oscilloscope Analogue/Digital Dual Trace 100MHz Only £75, with accessories £125 (ALL PRICES PLUS CARRIAGE & VAT) Please check availability before ordering or calling in HP33120A HP53131A HP53131A Audio Precision Datron 4708 Druck DPI 515 Datron 1081 ENI 325LA Keithley 228 Time 9818 Practical Electronics | January | 2021 Marconi 2305 Modulation Meter £250 Marconi 2440 Counter 20GHz £295 Marconi 2945/A/B Communications Test Set Various Options POA Marconi 2955 Radio Communications Test Set £595 Marconi 2955A Radio Communications Test Set £725 Marconi 2955B Radio Communications Test Set £800 Marconi 6200 Microwave Test Set £1,500 Marconi 6200A Microwave Test Set 10MHz – 20GHz £1,950 Marconi 6200B Microwave Test Set £2,300 Marconi 6960B Power Meter with 6910 sensor £295 Tektronix TDS3052B Oscilloscope 500MHz 2.5GS/s £1,250 Tektronix TDS3032 Oscilloscope 300MHz 2.5GS/s £995 Tektronix TDS3012 Oscilloscope 2 Channel 100MHz 1.25GS/s £450 Tektronix 2430A Oscilloscope Dual Trace 150MHz 100MS/s £350 Tektronix 2465B Oscilloscope 4 Channel 400MHz £600 Farnell AP60/50 PSU 0-60V 0-50A 1kW Switch Mode £300 Farnell XA35/2T PSU 0-35V 0-2A Twice Digital £75 Farnell AP100-90 Power Supply 100V 90A £900 Farnell LF1 Sine/Sq Oscillator 10Hz – 1MHz £45 Racal 1991 Counter/Timer 160MHz 9 Digit £150 Racal 2101 Counter 20GHz LED £295 Racal 9300 True RMS Millivoltmeter 5Hz – 20MHz etc £45 Racal 9300B As 9300 £75 Solartron 7150/PLUS 6½ Digit DMM True RMS IEEE £65/£75 Solatron 1253 Gain Phase Analyser 1mHz – 20kHz £600 Solartron SI 1255 HF Frequency Response Analyser POA Tasakago TM035-2 PSU 0-35V 0-2A 2 Meters £30 Thurlby PL320QMD PSU 0-30V 0-2A Twice £160 – £200 Thurlby TG210 Function Generator 0.002-2MHz TTL etc Kenwood Badged £ 6 5 Function Generator 100 microHz – 15MHz Universal Counter 3GHz Boxed unused Universal Counter 225MHz SYS2712 Audio Analyser – in original box Autocal Multifunction Standard Pressure Calibrator/Controller Autocal Standards Multimeter RF Power Amplifier 250kHz – 150MHz 25W 50dB Voltage/Current Source DC Current & Voltage Calibrator £350 £600 £350 POA POA £400 POA POA POA POA Marconi 2955B Radio Communications Test Set – £800 9 Bad boy bulbs Techno Talk Mark Nelson Unwelcome interference is again our topic this month, but on a different kick. This time, the investigation team performed splendidly, tracking down a hapless householder who was jeopardising the safety of the 8.85 million passengers who use Glasgow airport each year. T his month’s EMR nightmare starts with the oldest, and you’d think humblest of electrical items – the lightbulb. Someone unwittingly bought some light bulbs that should never have been put on sale in the first place, and which had defects that were recognised 70 years ago. Mission impossible? Pilots flying aircraft in and out of Glasgow airport complained they were suffering radio blackouts when they were between 6,000 and 10,000 feet in the air. Voice communication between aircrew and the controllers on the ground was wiped out by mystery interference, meaning crews were unable to hear vital air traffic control messages. Locating and identifying the source of the jamming signal aloft was well-nigh impossible on account of the height of the aircraft and their flying speed. Finding the source was a veritable needle-in-a-haystack puzzle, but OFCOM’s investigators were not easily deterred. An ‘area of probability’ was narrowed down by using flight-tracking software to correlate the location of planes reporting the issue with the corresponding location on the ground. With this done, the quest turned into a ground-level investigation centred on a single town. This task involved using receivers aboard vehicles and driving through the suspected area until the interference was heard. Old-fashioned footwork Having pinned down the area where the signal was strongest, the investigators then used handheld equipment to cover the remainder of the search area on foot. The team visited a number of likely properties and they eventually located the source. But what was jamming the sensitive aircraft radios so effectively? If you’re a regular reader of this column (and if not, why not?), you may well be screaming: ‘Oh no, it’s those blessed LED light bulbs that hoot from DC to daylight’, but you’d be wrong. In fact, the culprits were squirrels – or rather their cages. Let me explain, albeit in a roundabout way. 10 The problem with squirrels First, let’s recall a very apposite statement made in 1905 by the philosopher George Santayana in his book, The Life of Reason. It read, ‘Those who cannot remember the past are condemned to repeat it.’ The radio jamming at Glasgow airport proves his point perfectly. Back in 1905, the incandescent lamp bulb was not particularly efficient and to counter this, manufacturers were doing their damnedest to squeeze the maximum amount of light from a glowing filament. One technique involved threading the wire filament up and down the length of the bulb multiple times in a circular fashion using a system of hooks and frames until it resembled a squirrel cage. In those days, keeping pet squirrels in cages was not uncommon, although nowadays it’s considered pretty cruel by most people. After the sale of most incandescent bulbs was phased out by European legislation, lamp manufacturers discovered a loophole that allowed them to sell ‘decorative’ incandescent lamps, and the squirrel cage bulb has returned. These are usually made of tinted glass and have the word ‘rustic’ in their description; for example: https://amzn.to/32C1BTL Get to the point! OK, so what’s the connection between radio jamming and Santayana? Precisely this – OFCOM’s photo of the errant lamp bulb that was wiping out air traffic control frequencies (118 to 121MHz, remember this!) showed it was a squirrel cage ‘deco’ bulb. Electromagnetic radiation does not occur with modern incandescent bulbs that use short, coiled filaments, but the multiple straight wires used in squirrel cage lamps radiate like mad and can affect FM radio transmissions (88 to 108MHz) and the nearby air band (118 to 121MHz). The mechanism is so deeply technical that I cannot paraphrase it in a sentence, but you can read about it here: https://bit.ly/pe-jan21-fm Back in the early 1950s this potential for harmful interference was already recognised in America, where power companies were changing out straight-filament bulbs for consumers (read Popular Science magazine, April 1953, https://bit.ly/pejan21-ps). Nobody cares today, though, because those who do not remember the past are condemned to repeat it. Happy ending? OFCOM confirms that the interference at Glasgow was caused by four ‘vintage’ lightbulbs that the homeowner had recently bought online (not necessarily the ones mentioned above). The house was directly underneath the flightpath of the aircraft and so every time a plane passed and the lamps were in use, the air crew suffered the interference. The bulbs were replaced with non-interfering types and checks with National Air Traffic Services and aircraft operators confirm that the area is now free of interference. OFCOM’s spectrum enforcement team is following up the case with the lightbulb suppliers, to make sure no more dangerous bulbs are sold to unwitting customers. Tracking down the disturbance that was not merely annoying but positively life-threatening was a highly impressive effort. This work, by telecoms regulator OFCOM, confers great credit on their spectrum assurance team, who found the unknowing culprit. So that’s all right then? Hardly. Squirrel cage lamp bulbs are still sold widely; as I write this article, eBay has 1,284 offers of them for sale, while Amazon has more than 2,000. All manner of other online sellers and out-of-town DIY sheds sell them as well. How many of those bulbs have been tested for EMC compatibility? Manufacturers – and UK distributors – are responsible for exercising due diligence regarding the CE marking they place on their products, but how many of them are aware of their legal responsibilities for public safety? And what if they are made on the other side of the world, where CE merely signifies ‘Chinese export’? Local trading standards authorities are responsible for enforcing regulations, but they may not be aware of non-conformant merchandise. So, as always – caveat emptor – let the buyer beware! Practical Electronics | January | 2021 Exclusive offer Win a Microchip SAM IoT WG Development Board Practical Electronics is offering its readers the chance to Boards. The SAM-IoT WG development board is a small win a SAM IoT WG Development Board (EV75S95A) – and and easily expandable demonstration and development even if you don’t win, receive a 20%-off voucher, platform for IoT solutions. plus free shipping for one of these products. Out of the box, the MCU comes preloaded The SAM-IoT WG Development Board with firmware that enables you to quickly features the SAMD21G18 Arm Cortex-M0+ connect and send data to the Google based 32-bit microcontroller (MCU), Cloud Platform. Once you are ready an ATECC608A CryptoAuthentication to build your own custom design, you secure element IC and the fully certified can easily generate code using the free (approx £22.50) ATWINC1510 Wi-Fi network controller, so software libraries in MPLAB Harmony v3. you can quickly and easily connect your The SAM-IoT WG Development Board is embedded application to Google’s Cloud IoT supported by MPLAB X IDE. core platform. The on-board debugger allows you to The SAM-IoT WG development board features two sensors: program and debug the MCU without any additional hardware. Use the mikroBUS sockets to expand • A light sensor – TEMT6000 your design with your choice of MikroElekronika click • A high-accuracy temperature sensor – MCP9808 Worth $29.00 each Free-to-enter competition Microchip SAM IoT WG Development Board How to enter For your chance to win a Microchip SAM IoT WG Development Board or receive a 20%-off voucher, including free shipping, enter your details in the online entry form at: https://page.microchip.com/PE-SAM-IoT.html September 2020 winner Gary Meakin He won a Microchip Explorer 16/32 Development Kit Closing date The closing date for this offer is 31 December 2020. Practical Electronics | January | 2021 11 Net Work Alan Winstanley This month, Net Work has some of the latest news on electric cars, tips and tricks for running an Android-based TV system and advice on setting up a two-screen Windows PC. I n last month’s Net Work, I discussed some of the trends appearing in the electric vehicle (EV) market, starting with the now Chinese-owned ‘MG’ marque, a re-emerging former British car brand that has ambitions for growth in the UK and Europe. After introducing a small range of petrol cars, MG Motors is busily positioning itself as makers of all-electric battery (BEV) and plug-in hybrid (PHEV) cars, as demonstrated in a recent TV advertising campaign for the MG ZS EV that I mentioned last month (see https://youtu.be/mAqeTrA8I3c). We Brits also like our estate cars (station wagons), and MG has launched the MG5 SW, a new 52.5kWh EV station wagon with a range of (up to) 214 miles. The brand is also launching in Ireland and aims to expand further across Europe and Australia, backed by the resources of China’s SAIC motor group. For everyday consumers, MG’s emphasis will be on affordability, promising an era of ‘electric for everyone’. The zany Honda ‘e’ (see last month) is an astonishing mini town car bristling with LCDs and electronics aimed purely at urban drivers. Readers can find an engaging review of Honda’s baby battery car (and its interactive LCD aquarium!) on YouTube at https:// youtu.be/x6G-3_Aasao The same channel also reviewed Volkswagen’s forthcoming ID.3 all-electric car, VW’s third iteration of its ‘world car’ following the VW Beetle and Golf. This ‘1st Edition’ BEV costs nearly £39,000 ($50,000) before government grants (cheaper ones are in the pipeline) and VW reportedly expects to sell three million electric vehicles a year within four years, so expect them (and similar SEAT brand cars) to become a common sight before the decade is out. See https://youtu.be/4yntwHNCDBo for an early review, and details of VW electric cars can be found on VW’s web site at: www.volkswagen.co.uk/electric Electric car WLTP... How to measure fuel economy of an electric car? For those who, like the author, still check their miles per gallon, a new set of rules has been adopted that covers both new petrol/diesel cars and EVs. The term ‘WLTP’ (World harmonised Light-vehicle Test Procedure) is cropping up everywhere, including electric vehicle marketing which aims to give buyers real-world fuel consumption of vehicles. We might have to re-think our motoring habits and get used to the idea of journeys being interrupted by a 30 or 40-minute break to ‘refuel’ our plugin cars at a (hopefully) nearby charging point. More advice about WLTP data can be found on the UK government website at: https://bit.ly/pe-jan21-wltp It is still early days for electric vehicles as new petrol, diesel and hybrid sales are not scheduled to be banned in the UK until the year 2030. Until then, many drivers might prefer to hang on and let the BEV market mature. An emerging trend is that of owners VW’s forthcoming ID.3 BEV aims to be easy to live with and brings affordable electric motoring to the masses. 12 facing potentially ruinous repair bills once their car warranties have expired. Worrying reports circulate of £40,000 hybrid cars suffering major electric failures after just three years, leaving owners with nearly £3,000 in repair bills from workshops unfamiliar with this new technology and car manufacturers reluctant to pitch in to help. Android on TV Set-top box maker Humax has launched the first 4K Freeview Play recorder that also runs Android TV. The feature-filled Humax Aura has a 1TB or 2TB hard drive and can pause or play live TV as well as recording 4K HDR programmes, plus the Aura also runs Android TV 9, which gives access to some 5,000 apps on Google Play Store. Freeview Play offers built-in catch-up TV for mainstream TV channels and it can also record up to four programmes at a time while watching a fifth. Parents might welcome the Kids Zone feature and the remote carries Google Assistant and Prime Video buttons. Its built-in Google Assistant interacts in the usual way and allows smart control of compatible IoT devices, while the Chromecast-like operation allows content on other compatible devices to be cast to the Humax Aura. A not-yet-available Aura Android app promises to allow live or recorded content to be streamed around the house and reminders to be set. Assuming the app will be fully supported in The VW’s dashboard has a central display and smaller screens in front of the driver. Note the ‘Play’ and ‘Pause’ pedals. Practical Electronics | January | 2021 the future, the Aura could be a worthwhile way of adding Android PVR functionality to a 4K or HD TV. The recorder costs £249 or £279 depending on disk capacity. More details at: https://bit.ly/pe-jan21-aura If a fully-fledged 4K Android TV recorder is not for you, Google has finally launched its ‘Chromecast with Google TV’, a 4K dongle with voice remote control sporting Google Assistant, Netflix and YouTube buttons (see Net Work, November 2020). It adds Chromecast-style Android TV services to any TV with an HDMI port, which may give an older TV a new lease of life. All the usual streaming services are available, including Amazon Prime, mainstream catch-up TV, Google apps and, of course, Netflix and YouTube. The dongle aims to make it easy to cast content from other Android and iOS devices. Where Wi-Fi is weak, an Ethernet cable can be connected to a special mains-powered USB adaptor, available separately. The ‘Chromecast with Google TV’ debuted on YouTube at: https://youtu.be/EPluWn8RT3Y and can be purchased for £59.99 at store. google.com, but expect special offers to be available. Amazon users might choose from their latest line-up of Fire TV Stick HDMI dongles and several models are now available in time for Christmas: the cheapest ‘Lite’ version (£29.99) has an Alexa voice remote and the mid-range TV Stick (£39.99) adds TV volume and power buttons which (only) work with ‘compatible’ TVs and soundbars, Chromecast with Google TV offers 4K connectivity with Android TV features and a voice remote control. Amazon’s Fire TV Cube has a built-in speaker and Alexa commands interface with your TV screen. Practical Electronics | January | 2021 The Humax Aura is a 4K personal video recorder complete with Freeview Play and Android TV. 1TB and 2TB disk capacities are available. Amazon says. A more powerful 4K UHD version (£49.99) is also sold, and if your Wi-Fi coverage is patchy then wired Ethernet adaptors are sold separately. For hands-free control with a single TV, Amazon’s Fire TV Cube might be the answer. It has a built-in speaker offering the usual Alexa interface, displaying on the TV screen as well. The Fire TV Cube costs £109.99 and claims to control certain smart devices and selected ‘compatible’ Sky satellite receivers too, but do check the specs carefully. Full details are on Amazon.co.uk and do keep a close eye on prices and reviews: these can drop wildly over a holiday weekend so set up the Camelizer plug-in to alert you of price drops, see: https:// camelcamelcamel.com/camelizer Twin-dows 10! After upgrading a work PC to Windows 10, a 22-inch monitor was left over that still had plenty of life in it. What to do with a surplus screen that was too good to throw away? Provided your PC motherboard or video card has a suitable spare video port, Windows 10 can make short work of adding a second monitor to allow dual-screen operation and increase your desktop’s ‘real estate’. After hooking up a second screen, simply hit [Windows key] + P to ‘project’ your desktop on to the second monitor, choosing the ‘Extend’ mode from the menu. If your main monitor goes blank, keep holding the Windows key and hit ‘P’ to cycle through the options. Dual-screen operation has proved very handy for a busy desktop worker, with the extra screen space great for dragging active windows or website shortcuts sideways out of the way. Using a PC TV tuner, such as a Hauppauge USB type, means you can drop a TV window onto the second monitor or you could watch Freeview TV in a web browser instead; go to: https:// bit.ly/pe-jan21-free and choose ‘watch now’ or ‘catch up’, then drop the new TV browser window onto the second monitor. This idea works very well, though the odd freeze or lockup has occurred in practice. The author’s ordinary video card drives both a DVI and VGA monitor in dual-screen mode without any problem. Spare (or lengthier) video cables are readily available online, as are wireless Wi-Fi display dongles (untested by the author) that enable PC or mobile device screens to be mirrored on the monitor from a distance. Thanks to the simple setup in Windows 10 it’s easy to build a dual-screen option, so if you have a spare monitor gathering dust why not test it on a spare video port and see if dual-screen mode will work for you. Some other handy keyboard shortcuts worth knowing include: Win + P dual monitors Win + V clipboard history Win + I PC system settings Win + Tab 30-day timeline Win + D toggle Windows desktop Win + E File Explorer Win + L Lock your PC Win + R Open the Run... dialogue Win + Pause/Break About page Ctrl + Shift + Esc Task Manager A list of Windows keyboard shortcuts is on Microsoft’s website: https://tinyurl. com/y5o7pwp2 (some are now obsolete). Archiving disks One final tip: after upgrading the aforementioned PC, like many users I was left with a handful of hard disks that contained valuable legacy data, files and photos, so I archived the drives safely in plastic HDD storage boxes sourced on eBay (eg, https://tinyurl.com/y53jtngp). For many years I’ve relied on a handy cataloguing program called SuperCat which can index entire hard disks or data DVDs and catalogue them in a searchable database. I can easily search the catalogue on-screen in order to pinpoint the relevant hard drive. A 30-day trial of this neat little program (32/64-bit, Windows 10 compatible, $35) can be downloaded from http:// no-nonsense-software.com/supercat Wishing all our readers a Happy Christmas and a safer 2021. The author can be reached at: alan<at>epemag.net 13 Nutube miniature valve stereo preamplifier by John Clarke Valves are old hat, right? Not any more, they’re not! Korg and Noritake Itron of Japan recently released their Nutube 6P1 twin triode. Its party trick is a very wide range of operating voltages, from just a few volts up to 200V, and meagre power consumption. That makes it ideal for a battery-powered stereo preamplifier. You’ll enjoy the sound as well as the retro blue glow! A re you one of those people who simply ‘loves’ designed and built similarly to a vacuum fluorescent disthe nostalgic sound of valves, both in power amplifiers play (VFD). So the heater glow looks like two blue squares, and preamps? But valves are relatively expensive, and similar to large VFD pixels. Its performance is pretty good, too. Distortion levels below the high-voltage power supplies typically required make 0.1% are possible across a wide range of frequencies with a building a valve preamp a bit of a pain. Now, however, the latter problem is no longer true with little care during calibration. See the spec panel, Fig.2 and Korg’s Nutube 6P1 twin-triode. It works perfectly fine with a Fig.3, plus Fig.12 to get an idea of how well it performs. This Nutube preamp can run from a DC supply between plate voltage of just 6-12V, and the heater power and voltage 7V and 18V, with only a modest current draw. It can also requirements are also modest. So, building a preamp around it is a cinch, and it’s a suit- be powered using a 9V battery that is housed within the able project for beginners and school students becuase there preamp’s enclosure. If you want to be able to switch between signal sources, are no dangerous voltages involved. (In fact, for this reason alone we anticipate that this will be a very popular student you can match up this Nutube Preamplifier with the Six-way Stereo Audio Input Selector with Remote Control that we project, right up to and including their ‘major work’). Even if you have built valve gear with high voltage supplies described in the June 2020 issue. before, we think you will find the unusual construction of Nutube 6P1 dual triode the Nutube 6P1 dual triode quite fascinating. We’ve taken some care with this design, so that it fits into a Korg developed the Nutube 6P1 in collaboration with Norivery cool (and professional) looking extruded aluminium case, take Itron of Japan. While it is a directly heated triode with a filament, grid and with the inputs and plate connections, outputs at the rear and its construction more a power switch and resembles a vacuum volume knob at the • Power supply: 7-18 VDC; draws 29mA <at> 9V DC fluorescent display front. And of course, • Gain: up to 15dB at maximum volume setting (VFD) than a tradiwe’ve left a window in the clear front panel so • Distortion: around 0.07% at 200mV RMS output from 20Hz to 5kHz (see Fig.1 and Fig.2) tional valve (or tube). Two Nutube triodes that you can see that • Frequency response: 20Hz-20kHz, +0,−0.6dB; −3dB at about 7Hz and 80kHz (see Fig.3) are encapsulated in ‘warm’ blue tube glow. • Channel separation: typically >45dB (see Fig.4) One of the fasci- • Signal-to-noise ratio: 83dB with respect to 270mV in, 2V out, 20Hz-22kHz bandwidth a rectangular glass envelope. Each triode nating aspects of the • Maximum output level: 2V RMS with 9V supply, 2.8V RMS with 12V supply is effectively a singleNutube is that it’s Specifications 14 Practical Electronics | January | 2021 Features • Stereo valve preamplifier • Based on the recently released Korg ‘Nutube’ dual triode • Visible plate glow • 30,000-hour Nutube life • Safe low-voltage supply (7-18V DC) • Low power consumption • Battery or plugpack powered • Onboard volume control • Internal balance and distortion adjustments • Switch-on and switch-off noise eliminated • Power supply reverse-polarity protection • No transformers needed • Inputs and outputs are in phase. pixel VFD. The internal construction has the heater filament as a fine-gauge wire running across the front, with the metal mesh grid located below that. Behind the grid is the plate (also called the anode), which is phosphor-coated and glows when the filament is heated. The filament wire is held taut, and because of this, it can vibrate similarly to a stringed musical instrument. (The Nutube is, after all, sold by a musical instrument manufacturer). This vibration is not necessarily a wanted feature, as it can be the source of microphonics – where external sound can couple to the filament and this alters (or modulates) the audio signal being amplified in the triode. The result is that this vibration is heard in the sound output. The microphonics can be minimised using careful construction methods. This includes protecting the Nutube from surrounding air vibrations, by using flexible wiring and including a vibration-damped mounting. In operation, the Nutube draws very little current, with each filament requiring just 17mA. Total heater power for the two triodes is around 25mW. The grid and plate current total at around 38µA. The Nutube is best operated with a plate voltage between 5V and 30V, and the load-line curves (Fig.1) reveal that within this voltage range, the grid voltage needs to be above the cathode filament. This is different from the traditional triode, where plate voltages are much higher, and the grid voltage is usually negative with respect to the cathode. Yes, it really is a thermionic valve (or ‘tube’ as our Americans friends like to say). But this Nutube 6P1, shown here from the underside, is quite unlike any valve you’ve come across before. For a start, those blue windows (see opposite) really do glow blue! The Nutube operating point would typically be set so that the distortion from each triode is at a minimum and so that maximum dissipation is not exceeded. To achieve this, our design includes two trimpots to set the grid bias of each triode. There are three ways to make these adjustments. One is to adjust the trimpots so the Nutube plate glows brightest for each channel, which will generally give good performance. Another method is to use a signal source and multimeter to adjust the grid bias for maximum output signal level, or better still, by observing the distortion products and setting each trimpot for the desired result. Freely available computer software can be used to measure the distortion and view the waveform. This allows for easy set-up of the desired distortion characteristic. We describe what software you need and how to use it in a panel later in this article. Preamplifier performance Fig.2 shows the total harmonic distortion plus noise (THD+N) figure as a percentage, plotted against frequency and output level. As you can see from Fig.3, the performance is best with an output level in the 100-400mV RMS range. This is a typical level that you might feed into a 100W (or thereabouts) stereo amplifier to get a reasonable listening volume. Such an amplifier would generally have a full -power sensitivity between 1-2V RMS. Below 100mV RMS output, noise starts to dominate the THD+N figure. In other words, preamp performance at lower volume levels is limited by its 83dB ultimate signal-to-noise ratio (SNR). Above 400mV RMS, triode non-linearities dominate. Tiny! At just 115 × 50 × 125mm, and built into this snazzy extruded case from Jaycar, it really looks the part. Performance is no slouch, either! Practical Electronics | January | 2021 15 frequency range of 10Hz-100kHz so you can get an idea of the actual −3dB points. Fig.5 shows the channel separation. This is produced by feeding a signal into the right channel, monitoring the left channel output level and sweeping the test signal across the audible frequency range. The channels are then swapped, and the test is repeated. As you can see, there is more coupling from the right channel to the left, and the separation figures are not amazing, at around 45-68dB. However, this is more than good enough for a stereo system, and sounds panned entirely to the left or the right will still appear to be coming from just one speaker. Fig.6 is a scope grab showing the preamp output (at the top, yellow) at around 200mV, 1kHz, with the ~0.07% Fig.1: load lines for the Nutube triode showing the relationship between anode residual distortion signal below, in blue. (plate) voltage (horizontal axis), anode/cathode current (vertical axis) and gridYou can see that this is primarily third cathode voltage (labels on curves). The area below the black dotted line is the harmonic, with some second harmonic. continuous safe operation envelope. Fig.7 shows the much higher-level The rise in distortion with frequency is mild, with THD+N distortion present in the output if the triode is adjusted furonly increasing by about 50% between 1kHz and 10kHz. The ther away from its ideal operating point. This is around 0.3% measurement shown in red on Fig.2 is with an ultrasonic THD+N, the majority of which is second-harmonic distortion. (80kHz) bandwidth in order to measure the harmonics of Fig.8 shows the noise residual when the output level is higher test frequencies. set much lower. This is a fairly typical wideband white The blue trace gives a most realistic measurement up to noise signal. about 10kHz, which then falls off due to the 22kHz filter Circuit description limit cutting out the harmonics. You may wish to compare Fig.2 and Fig.3 with Fig.12, The full circuit is shown in Fig.9. One of the triodes in the which shows a spectral analysis of the distortion at 1kHz Nutube provides amplification for the left channel (V1a), while the other triode is used for the right channel (V1b). and around 200mV output. As you can see from Fig.12, this method of reading the These are connected as common-cathode amplifiers, where distortion gives much the same result as the Audio Precision the cathode filament is referenced to ground. The signals are applied to the grids, and the resulting amplified signals system used to produce Fig.2 and Fig.3. Fig.4 demonstrates that the preamp has a very flat response, appear at the corresponding anode (or plate). The anode loads are 330kΩ resistors from the positive with no peaks or wobbles. The output is down well under 1dB by 20Hz at the bass end, and an even smaller fraction of a supply, with 150Ω/100µF low-pass filters to prevent supply noise from reaching the anodes. decibel by 20kHz at the upper end. This plot has an extended 10 Nutube Preamplifier THD vs Frequency 23/10/19 12:56:49 10 22kHz bandwidth 80kHz bandwidth 2 1 0.5 0.2 0.1 0.05 0.02 0.01 2 22kHz bandwidth 1 0.5 0.2 0.1 0.05 0.02 20 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k Fig.2: a plot of total harmonic distortion, including noise, Fig.2measurements were made against signal frequency. These at about unity gain, with around 200mV RMS in/out, and with two different filter bandwidths. The blue curve (20Hz-22kHz) includes the distortion products and noise which are audible to the human ear, while the red curve (20Hz-80kHz) includes higher harmonics for more realistic readings at higher frequencies (8kHz+). 16 23/10/19 12:58:58 5 Total Harmonic Distortion (%) Total Harmonic Distortion (%) 5 Nutube Preamplifier THD vs Level, 1kHz 0.01 0.01 0.02 0.05 0.1 0.2 0.5 Output Level (Volts) 1 2 3 Fig.3: distortion plotted against output level. This graph demonstrates that the Fig.3 output level is the largest determining factor in the preamp’s distortion performance. At low levels, noise begins to intrude, while at high levels, the waveform shape gets ‘squashed’ and so distortion increases significantly. The middle section, where distortion is lowest, is the range in which the preamp will generally be used. Practical Electronics | January | 2021 The Nutube triodes have relatively low input impedances at the grids and high output impedances at the anodes, so op amp buffers are used at both ends. IC1a and IC2a ensure that the grids are driven from low impedances. IC1b and IC2b minimise the anode loading, as they have very high input impedances of 600MΩ, which is effectively in parallel with 1MΩ resistors. These op amps have very low noise figures (3.3nV/√Hz) and distortion (0.00006% <at> 1kHz and 3V RMS) when operated at unity gain. Therefore, these op amps do not affect the sound of the signals. The properties of the Nutube triodes dominate any effect that the op amps have on the signals. We’ll now describe the signal path in more detail, but only for the left channel, as both channels are almost identical. The input signal is fed in via RCA socket CON1a and passes through a 100Ω stopper resistor and ferrite bead (FB1). These, in conjunction with the 100pF capacitor, significantly attenuate RF signals entering the circuit, which could result in unwanted radio-frequency detection and reception. The signal is AC-coupled to 50kΩ volume control VR1a via a 470nF DC-blocking capacitor. This capacitor removes any DC voltage that may be present at the input to prevent pot crackle, and also produces a low-frequency roll-off below about 7Hz. The signal is then AC-coupled from VR1a’s wiper to the non-inverting input (pin 3) of op amp buffer IC1a via a 100nF capacitor. Pin 3 of IC1a is biased near to half the supply voltage via a 1MΩ resistor that is tied to a half supply rail (Supply/2). The input bias current at pin 3 of IC1a will cause the DC voltage level to shift from this half supply level due to the current flowing through the 1MΩ resistor. This causes the signal voltage to rise about 0.5V above the half-supply rail, reducing the maximum symmetrical voltage swing. But since the nominal supply voltage is 9V (down to 7.2V if the 9V battery is getting flat), the signal swing is still sufficient to prevent signal clipping of line-level audio signal levels. IC1a’s output drives V1a’s grid (G1) via a 10µF coupling capacitor. This grid is DC-biased via a 33kΩ resistor with a voltage that’s set using trimpot VR2. This is adjusted to set the operating point, and hence the distortion, produced by V1a. V1a’s plate anode load is a 330kΩ resistor which connects to either the Vaa or 6V supply via a 150Ω decoupling resistor. +3 Filament current Just like a traditional valve, the Nutubes have heater filaments. These are connected between F1 and F2 for V1a, and between F2 and F3 for V1b. So the F2 connection is shared between the two. There are two ways to drive the filaments. One is to supply current to F1 and F3 via separate resistors and have the Nutube Preamplifier Frequency Response 23/10/19 13:01:58 +2 -0 Relative Amplitude (dBr) -1 -2 -3 -30 -50 -60 -70 -80 -5 -90 50 100 200 5k 10k 20k 500 1k 2k Frequency (Hz) 50k Fig.4: this plot shows that the Nutube Preamplifier’s Fig.4 frequency response is commendably flat. This plot extends down to 10Hz and up to 100kHz so that you can see the roll-off at either end. The slight difference between the response of the two channels above 10kHz is likely due to slightly different biasing; we had purposefully biased the two channels slightly differently to see the difference in distortion. Practical Electronics | January | 2021 left-to-right coupling right-to-left coupling -40 -4 10 20 23/10/19 13:10:17 -20 0 -6 Nutube Preamplifier Channel Separation -10 left channel right channel +1 Relative Amplitude (dBr) Which supply is used depends on the position of jumper JP1. When a 9V battery is used for power, using the fixed 6V selection prevents anode (plate) voltage variations as the battery discharges. When used with an external regulated supply, the Vaa setting would be selected. The high-impedance amplified anode signal is again an AC-coupled op amp buffer, IC1b via a 100nF capacitor. IC1b is also biased to half supply via another 1MΩ resistor to Supply/2. This 1MΩ resistor loads the anode, reducing the Nutube anode signal to 75% of the unloaded signal. This is unavoidable in a circuit with such high impedances. Note that the signal at the triode’s anode is inverted compared to that applied to the grid. In some cases, it is important to maintain the phase of audio signals between the inputs and outputs. So the output signal from the triode is re-inverted by op amp IC3a, connected as an inverting amplifier. VR4 is included so that the gain of IC3a can be adjusted. The gain of IC3b in the right channel is fixed at −2.3 times (−5.1kΩ ÷ 2.2kΩ), so the gain for IC3a is typically set at a similar level. The gain may need to be slightly different between the two channels to get equal gains for both outputs, due to variations in gain between the two triodes at similar bias levels. Finally, the signal from IC3a is AC-coupled with a 10µF capacitor to remove the DC voltage and DC-biased to 0V with a 100kΩ resistor. The output is fed through a 150Ω isolation resistor to prevent oscillation of IC3a should long leads with a high total capacitance be connected. To prevent noises when power is switched on and off, the output signal passes to the output RCA sockets via a pair of relay contacts that are open when power is off. At power-on, the relay is only switched on to allow signal through to the output terminals after everything has settled down. At power off, the relay is switched off immediately. This isolates the signal while the power supply voltages decay. -100 20 50 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k Fig.5: this graph shows the Nutube Preamplifier’s channel separation. It is quite decent up to about 2kHz, Fig.5 with more than 60dB separation between channels. The main concern with signal coupling from one channel to another is that it introduces distortion; however, as this is not an ultra-low-distortion device, it isn’t that big of a concern. We included this plot mostly for completeness. 17 6V Vaa 100nF SUPPLY/2 6.8k LEFT IN CON1a 10 F FB1 VR2 10k 470nF 100 100nF VR1a 50k LOG 100pF 3 2 ADJUST G1 BIAS 25V 1M TPG1 33k 10 F 8 25V 1 IC1a 4 IC1: OPA1662 VOLUME Fig.6: the output of the unit with the triode biasing adjusted for lowest distortion. The yellow trace is the output signal, while the blue trace is the distortion residual (ie, the yellow trace with its fundamental removed). It contains significant second and third harmonics. 6V Vaa SUPPLY/2 100nF 5.1k RIGHT IN CON1c ADJUST G2 BIAS 1M VR3 10k 470nF 100 FB2 VR1b 3 50k LOG 100pF 100nF 2 TPG2 33k 8 1 IC2a 10 F 4 25V IC2: OPA1662 POWER S1 DC INPUT 7 – 18V CON2 Fig.8: the output of the preamp with no input signal. Some devices produce more high-frequency or more low-frequency noise. In this case, it appears quite close to white noise. common F2 terminal tied to ground. In this case, the resistors are chosen for 17mA flowing in each filament, giving a total filament current of 34mA. 18 A Vaa K REG1 TPS70960 + CON3 Fig.7: this plot is the same in Fig.6, but the triode biasing has been adjusted away from its optimal condition. Total harmonic distortion has risen to around 0.3%, with the second harmonic now the dominant distortion signal. D4 1N5819 9V BATTERY (BAT1) 1 10 F 25V 3 IN EN OUT GND NC 5 4 2 Fig.9: the input signals from CON1a and CON1c pass through RF filters and volume control pot VR1 before being AC-coupled to ultra-low-distortion buffer op amps IC1a and IC2a. These feed the signals to the grids of V1a and V1b, while VR2 and VR3 allow you to adjust the DC grid bias levels. The inverted output signals at the anodes of V1a and V1b are AC-coupled to the inputs of buffer op amps IC1b and IC2b. The signals are then re-inverted by op amps IC3a and IC3b before being fed to the outputs via the contacts of RLY1. VR4 allows the gain of the two channels to be matched. IC4 controls RLY1’s coil so that it switches on around five seconds after power is applied, and switches off immediately upon power removal, eliminating clicks and thumps. But in our circuit, we connect the filaments in series, so the same 17mA flows through each filament for a 17mA total current, but with twice the voltage across the filaments. This is a more efficient way to drive the filaments, and saves power when using batteries. In our circuit, F1 is tied to ground, F2 is left open and current supplied via a 270Ω resistor from 6V to F3 ((6V – 0.7 – 0.7) ÷ 270Ω = 17mA). Note that F2 and F3 are bypassed to ground with 10µF capacitors. This reduces noise in the circuit. There is one extra consideration when the filaments are in series. As the Nutubes are directly heated, V1b’s cathode will be 0.7V higher than V1a, due to the voltage drop across V2’s filament before the current reaches V1. This changes the Practical Electronics | January | 2021 6V 6V Vaa 150 100 F  ST AS 330k TPG1 8 1M 33k A1 G1 V1a F2 2.2k 5 IC1b 6 2 7 3 8 4 1 RLY1a NC 150 8 4 LEFT OUTPUT CON1b NO 4 F1 3 1 2 10 F 25V 1 IC3a 5 1 VR4 10k 100nF C IC4 TPS70960 4 IC1 – IC3 25V E K K 25V B A A 10 F Vaa OR 6V BC547 1N4148 1N5819, 1N4004 Vaa SUPPLY/2 100k 10 F 25V SUPPLY/2 V1: NUTUBE 6P1 6V 6V Vaa SUPPLY/2 150 IC3: OPA1662 Vaa OR 6V 100 F  ST AS 25V 1M 330k TPG2 5.1k 100nF 33k A2 V1b F2 2.2k 5 G2 F3 IC2b 6 6 7 5 IC3b 10 F 25V 7 RLY1b NC 150 RIGHT OUTPUT CON1d NO 270 100k 6V 10 F SUPPLY/2 25V Vaa JP1 10k Vaa Vaa OR 6V 6V SUPPLY/2 TP6V 2.2 F RLY1 5V 6V K D3 1N4004 A 100 F 10k 25V TPGND CERAMIC 33 6V 100k A 180k K D1 1N4148 47 F 100k 100k 2 3 1M 10 F 6V IC4: LM358 D2 1N4148 8 1 IC4a A 6 K 5 IC4b 4 100k 47 F 270 10k 100k 100k C 7 10k B Q1 BC337 E 5.1k 100k 100k SC Nutube Stereo Valve Preamplifier NUTUBE STEREO VALVE PREAMPLIFIER 2020 bias voltage requirement at the grid (G2) for V1b compared to G1 for V1a. The extra voltage required for G2 is provided by having a wider voltage range for VR3 due to a lower-value resistor connecting it to the 6V supply compared to VR2. Note that the grid bias voltage derived from VR2 and VR3 is relative to the output of 6V regulator REG1. This is a fixed voltage, so the grid bias voltage does not vary with the supply voltage. Power supply When no DC plug is inserted into DC socket CON2, the internal 9V battery supplies power to the circuit, via CON2’s normally closed switch connecting the negative of the battery Practical Electronics | January | 2021 to ground. When a power plug is inserted, power is from the DC input and the battery negative is disconnected. Power switch S1 connects power to the rest of the circuit, whether from the battery or an external source, while diode D4 provides reverse-polarity protection. REG1 is a low-dropout, low-quiescent-current 6V regulator. It is included to maintain a constant grid voltage for the Nutube when power is from a battery, as battery voltage naturally varies over time. The 6V rail also powers relay RLY1. The input of REG1 is bypassed with a 10µF capacitor, while a 2.2µF ceramic capacitor filters the output. This output capacitor has the required low ESR (effective series resistance) to ensure stability at the regulator output. 19 SECURE TO CASE + D4 – CON2 DC in 5819 L NO FB1 33 R FB2 TPS70960 2.2 F CON1 100 NUTUBE PREAMPLIFIER 100 The half-supply rail is derived from a pair of 10kΩ resistors connected in series across the anode supply for V1. It is bypassed with a 100µF capacitor to reduce noise and lower the rail impedance. TP6V CON3 – + 7-18V 20 270 10k 9V BATTERY 100k 1M 100nF 180k 1M 4148 10 F* 100k 100k BAT1 100k 100k 33k 100nF 10 F* 270 100k 10k 100k 100 F 330k 150 4148 100k 47 F 5.1k 150 2.2k 10 F* 100nF 2.2k 100nF 330k 6.8k 100k 33k 10 F* 1M 1M IC1,2,3 : OPA1662 * 25V minimum 01112191 REV.B 4004 1M C 2019 10k RLY1 REG1 10 F* Power switching and output isolation JP1 NC Vaa S As mentioned earlier, the relay contacts at C 100pF 100pF the left and right outputs connect the signals Q1 BC337 COIL some time after power-up and disconnect the 47 F 6V N signals quickly when power is switched off. 470nF 470nF 100k 5.1k D3 IC4, Q1, RLY1 and associated components 150 150 10k provide this signal switching. VR2 10k VR3 10k IC4a and IC4b are two halves of an LM358 single-supply, low-power dual op amp. They are used as comparators with hysteresis. The hyster10 F* 10 F* D2 esis is provided by 100kΩ resistors from their TPG2 GND TPG1 19121110 5.1k outputs to their non-inverting inputs, while the nominal comparator threshold at these inputs IC4 IC3 LM358 is set around 2V when the output is low and 4V 10 F VR4 when the output is high. 10k 100 F* 100 F* So in each case, the output goes high when N the voltage at the inverting input drops below S 2V, and then goes low again when the voltage at D1 the inverting input rises above about 3.5V (you might expect 4V, but the LM358’s output can’t IC2 IC1 swing to the positive rail). In other words, there GND 10  F * 100nF 10 F* is about 1.5V of hysteresis. A1 F2 A2 G2 F1 G1 F3 100nF RLY1 is initially off, and when power is applied VR1 50k Log S S FOAM via switch S1, several things happen. First, power S1 is supplied via D1 to the preamplifier circuitry, NUTUBE 6P1 TWIN TRIODE POWER Volume including REG1, V1 and IC1-IC4. The supply and S = M3 x 15mm LONG STANDOFF signal-coupling capacitors begin to charge up to CABLE N = M3 x 25mm LONG NYLON OR SC TIE their operating conditions. 2020 POLYCARBONATE SCREW WITH NUT At the same time, the inverting pin 2 input to Fig.10: all the Nutube Preamplifier components mount IC4a is pulled high, to near the incoming supply on one double-sided PCB, as shown here. They are voltage, via the 100kΩ and 180kΩ resistors conmostly standard parts, but IC1-IC3 and REG1 are only necting to switch S1. Diode D1 prevents more than 6.5V available in SMD packages. The Nutube (V1) is in a from being applied to this pin. SIL-type package with right-angle leads that are surfaceThe 180kΩ and 1MΩ resistors form a voltage divider so mounted to pads on the top of the board. The whole that their junction tends to sit at around 5.5V when there is assembly slides into an extruded aluminium case. more than 6.5V at the anode of D4. This is above the pin 3 voltage, and therefore the output The value of the 270Ω resistor means that the current of IC4a goes low, near 0V. Pin 3 is therefore around 2V. drawn by the relay coil drops from 30mA initially down to Diode D2 is reverse-biased and pin 6, the inverting input about 12.8mA, extending battery life. of IC4b, is initially held high near to 6V, due to the 47µF When power is switched off via S1, the pin 2 voltage at capacitor being initially discharged. The 10kΩ resistor in IC4a’s input immediately drops to 0V. That voltage is below series with the capacitor reduces the pin 6 voltage down the pin 3 voltage, so IC4a’s output goes high. Diode D2 conto about 5.7V initially. ducts and pulls pin 6 of IC4b above the pin 5 threshold, so This is above the 4V at the non-inverting pin 5 input, so IC4b’s output immediately goes low. Q1 switches off and the the output of IC4b will be low. Pin 5 will be at 2V. The low relay contacts open. This all happens well before the supply output of IC4b means NPN transistor Q1 is off, and the relay capacitors in the circuit have time to drop significantly in is off. The relay contacts will be open, so no audio passes voltage. So the output signals are cut before anything in the through to the output. circuit can misbehave. As the 47µF capacitor charges via the 10kΩ and 100kΩ The 10kΩ resistor between the diode D2 and the 47µF resistors, after about five seconds, the voltage at pin 6 will capacitor is there so that the pin 6 input to IC4b can be imdrop below the voltage at the pin 5 input (2V). The output mediately taken high, without having to wait for the 47µF of IC4b then goes high, driving transistor Q1 and switching capacitor to discharge. on RLY1. The audio signals are then connected to the left and right-channel output sockets. Construction Note the 47µF capacitor with a parallel 270Ω resistor and The Nutube Stereo Preamplifier is built using a double-sided series 33Ω resistor between the collector of Q1 and the coil PCB coded 01112191, which measures 98 × 114mm and is of RLY1. The 33Ω resistor is included so that the 5V-rated available from the PE PCB Service. It is housed in an extruded relay coil is initially driven with 5V rather than the full 6V aluminium enclosure with clear end panels, measuring 115 of the supply. × 51 × 119mm. Fig.10 has the PCB assembly details. Then, as the 47µF capacitor charges, the voltage to the Start by fitting the surface-mount parts. Mostly, these are relay coil is reduced until it is instead supplied current via used because the same parts are not available in through-hole the 270Ω resistor. This reduces relay coil voltage and current, packages. They are not difficult to solder using a fine-tipped saving power but still holding the relay’s contacts closed. soldering iron. Practical Electronics | January | 2021 This photo also shows the completed PCB – use it in conjunction with the component overlay opposite. The flying lead visible in this photo (and the photos below) earths the aluminium case to the PCB to minimise hum. Good close-up vision is necessary, so you may need to use a magnifying lens or glasses to see well enough. These parts are IC1, IC2 and IC3, REG1 and its associated 2.2µF ceramic capacitor. Make sure that each component is oriented correctly before soldering it – ie, rotated as shown in Fig.10. The ceramic capacitor is not polarised. For each device, solder one pad first, check alignment and readjust the component positioning by reheating the solder joint if necessary before soldering the remaining pins. If any of the pins become shorted with solder, solder wick can be used to remove the solder bridge. But note that pins 1 and 2, and pins 6 and 7 of both IC1 and IC2 connect together on the PCB, so a solder bridge between these pins is acceptable. Continue construction by installing the resistors (use your DMM to check the values), followed by the two ferrite beads. Each bead is installed by using an offcut length of wire (from the resistors) feeding the wire through it and then bending the leads down through 90° on either side to fit the PCB. Push each bead all the way down so that it sits flush against the PCB before soldering its leads. Install diodes D1-D4 next. Take care to orient each correctly, as shown in the overlay diagram, and make sure each is in its correct position (ie, don’t get the different types mixed up) before soldering. Following this, fit the IC socket for IC4. Make sure that the socket is seated flush against the PCB and that it is oriented correctly. It’s best to solder two diagonally opposite pins of the socket first and then check that it sits flush with the board before soldering the remaining pins. You could skip the socket and solder IC4 straight to the board. This would improve long-term reliability but would make it much more difficult to swap or replace IC4 should that be necessary. The MKT and the two 100pF ceramic capacitors can now go in, followed by the electrolytic capacitors. The polarised electrolytics must be oriented with the correct polarity; ie, with the longer lead into the pad marked with the ‘+’ sign. Now install the two single-turn trim pots, VR2 and VR3. These might be marked as ‘103’ rather than ‘10kΩ . Next, mount multi-turn trimpot VR4. Orient it with the adjusting screw positioned to the left, as shown. It also may be marked as ‘103’ instead of ‘10kΩ . The next step is to fit Q1 by splaying its leads slightly to suit the hole arrangement on the PCB. Also install PC stakes for GND, TPG1, TPG2 and TP6V. The three-way header for JP1 and the two-way header for the battery lead can be mounted now, followed by RLY1, CON1, CON2 and switch S1. Potentiometer VR1 is mounted, soldered in place and secured against the PCB using a cable tie around the pot body. This stops force on the shaft from breaking the solder joints or lifting tracks off the board. Feed the tie through the holes in the PCB on each side of the pot, and tie it underneath. Nutube V1 is mounted so that the front glass is vertical and with its leads soldered to the top pads on the PCB, similar to a surface-mount component. Pins F1 and F3 at each end of the Nutube use two adjacent leads on the Nutube device. In addition to the leads, it is supported by two 15mm-long tapped spacers, one on either side of the device, which hold a piece of foam against the Nutube envelope. More views of the completed PCB from the front (at left) and the rear (above). Neither photo has the 9V battery in place, but its support standoffs and screws are ready for it. Practical Electronics | January | 2021 21 Where can you buy a 6P1 Dual Triode? The 6P1 is available from RS Components (https://uk.rs-online. com/web/). We have to warn you, though, it’s not a cheap device: RS Components list it as £50 (inc VAT, plus postage – the RS stock number is 144-9016). We would expect prices will eventually come down as they become more popular and more suppliers carry them. Secure these spacers to the PCB using short machine screws fed in from the underside of the PCB. We will later sandwich the foam between the spacers and the Nutube, stopping it from flexing its leads too much. Also fit one 15mm standoff at each end of the battery outline on the PCB (see photos). The sides of the battery are held in by two M3 x 25mm nylon or polycarbonate screws passed up from the underside of the PCB and secured with M3 nuts. Wiring Crimp and/or solder the battery wires to the header socket terminals after cutting these wires 60mm long. Then insert these terminals into the header socket shell, making sure you get the red and black wires in the correct positions, as marked on the PCB. An earth wire is also required to prevent hum injection to the circuit if the case is touched. This connects the metal case to the GND terminal on the board. Solder it to the solder lug at one end and the GND terminal on the board at the other. Heatshrink tubing can be used over the lug terminal and PC stake for GND. When the case is assembled, the solder lug is captured in the top corner end-cap screw, adjacent to the RCA terminals. Powering up and testing If you are planning to use a battery to supply power, connect a jumper shunt in the 6V position for JP1. That way, any voltage changes from the battery will not affect the anode plate voltage. If using a DC plugpack, use the Vaa position for JP1. Initially, set VR2 and VR3 to midway. Apply power to the circuit from a 7-18V DC supply. Check that TP6V is between 5.88 and 6.12V. Also check the relay switches on after about five seconds; you should hear it click in. Adjust VR2 so that the left-hand plate of the Nutube lights up at its brightest. Similarly, adjust VR3 so that the right-hand plate of the Nutube glows brightest. If using a supply that’s over 12V, then make sure the grid voltage is less than 2.5V; otherwise, the device’s maximum dissipation rating will be exceeded. The grid voltage for each triode can be measured at TPG1 and TPG2, relative to the GND PC stake. VR4 adjusts the output of the left channel so that it can match the right channel in level. This can be done by The completed PCB simply slides into the extruded case so that the pot shaft and switch emerge from the front panel. No PCB screws are necessary as it is held tight by the front and rear case ends. connecting up the preamplifier to your sound system and rotating VR4 so both channels have the same output level, just by listening. For more accurate adjustments, you need a signal generator. You can use a standard hardware-based signal generator, or computer software. You will also need suitable leads to connect the generator to the RCA inputs. For connection to a computer, you typically need a stereo lead with RCA plugs one end and a stereo 3.5mm jack plug at the other. Leads for a hardware signal generator will require an RCA plug one end and a connector for the generator, such as a BNC plug, at the other. Apply a 1kHz signal of about 1V RMS to the right channel preamplifier input (red input socket). Monitor the right channel output with a multimeter set to measure AC volts. Set the volume control for about 500mV signal at the output. Adjust VR3 for maximum signal, but when doing this, adjust the volume control so the level does not exceed about 500mV. That’s required to ensure the signal is not clipped. When the maximum level is found, take note of the level reading. Now apply the same signal to the left channel (white RCA input) and measure the left channel output. Do not change the volume setting, but you may need to adjust VR4 for a suitable level, not much more than 500mV. Adjust VR2 for maximum signal as before. Now adjust VR4 so that the measured level is the same as that already measured in the right channel. If you wish to set the grid bias more accurately, spectrum analyser software can be used. The spectrum analyser will show the distortion products of the preamplifier, including the fundamental and harmonics. The fundamental is the reproduction of the actual applied signal. Nutube Preamplifier + + L + Power + + IN Volume SILICON CHIP www.siliconchip.com.au + + 7 to18VDC (Centre +) R + + + OUT IN Volume Fig.14: the 1:1 front and rear panel artwork can also be used as a template. V1 requires a 43 × 15mm cutout; the volume control, a 10mm hole; and the power switch, a 5mm hole. On the rear panel, the RCA sockets require 10mm holes where shown, with a 3mm hole in the middle; the DC socket is 5mm. (Download from the January 2021 page of the PE website.) 22 Nu + + Practical Electronics | January | 2021 With a perfect preamplifier, without distortion, you would only see the fundamental at the output. However, with a real preamplifier, there will be noise and distortion. This will show up in the analyser as other spikes rising above the noise floor. Typically, the distortion will have second, third, fourth, fifth harmonics and so on. For a 1kHz signal, the fundamental (first harmonic) would show as a peak at 1kHz, with the second harmonic at 2kHz, the third harmonic at 3kHz, the fourth at 4kHz... These harmonic distortion products hopefully will be at a lower level than the fundamental, and not all harmonics will necessarily be present. Once you can see this, you can adjust the grid bias for minimum distortion. For that matter, you could also adjust it for maximum distortion, if that’s what you’re after! (See panel on the next page). Final assembly The Nutube Preamplifier PCB is housed inside an aluminium enclosure with clear end panels, measuring 115 × 51 × 119mm. If you are not using a battery for power, unplug the battery clip from CON3 to prevent the contacts from shorting onto a part of the circuit. The end panels include 3mm-thick foam plastic that can be used as padding for the Nutube device. The end pieces just require this foam to be placed within the outer surround, where the end panels connect to the aluminium body. The central pieces that cover the window and the buttonshaped pieces for the corner-securing holes are not required for the case. Cut out a piece of foam 38 × 17mm and place this behind the Nutube. This is held between the two 15mm standoffs at the rear of the Nutube. Note that the enclosure has a specific top and bottom orientation for both the aluminium extrusion and end panels. The front and rear panels have a slightly different profile at the top and bottom edges. While the top edge is straight, the lower edge has a slightly lower moulding below the two left and right corner holes. That matches the same profile on the aluminium extrusion. Holes need to be drilled for the volume potentiometer and power switch at the front and the DC socket and RCA sockets at the rear. The required front panel hole locations are shown on the label artwork of Fig.14. This can be downloaded in a PDF from the January 2021 page of the PE website. A small portion along the top edge of the RCA terminal housing plastic needs to be shaved or filed off because it is slightly too high to fit in the case otherwise. Less than 1mm needs to be removed. You can place the labels on the inside of the panels, cutting around the outside perimeter of each label. Or you can cut out the smaller-sized inner perimeter so the labels can be affixed to the outside of the end pieces. If the panel label is to be inside the end panel, a paper label could be used. For the front panel, the central window in the artwork will need to be cut out with a hobby knife, to expose the Nutube. The RCA sockets should be secured to the rear panel with the self-tapping screw, and with the rear edge of the PCB touching the inside of the rear panel. You can then slide the PCB into the case on the second slot up from the bottom. Don’t forget to attach the GND solder lug to the top corner screw at the rear adjacent to the RCA sockets. The wire end of the solder lug will need to be oriented diagonally inward, so it does not foul the end cap border. Additionally, the anodising layer on the aluminium is a good insulator. It will need to be scraped off at the point Practical Electronics | January | 2021 Parts list – Nutube Valve Preamplifier 1 double-sided PCB coded 01112191, 98 x 114mm from the PE PCB Service 1 set of front and rear panel labels (see text) 1 extruded aluminium enclosure with clear end panels, 115 x 51 x 119mm [Jaycar HB6294] 1 Korg Nutube 6P1 double-triode thermionic valve (V1) [RS Components 144-9016] 1 1A DPDT 5V relay (RLY1) [Altronics S4147] 1 SPDT sub-miniature toggle switch (S1) [Altronics S1421] 1 double stereo horizontal PCB-mount RCA socket assembly (CON1) [Altronics P0211] 1 PCB-mount DC power socket (CON2) [Jaycar PS0520, Altronics P0621A] 1 2-pin 2.54mm-pitch vertical polarised header (CON3) [Jaycar HM3412, Altronics P5492] 1 inline plug to suit CON3 [Jaycar HM3402, Altronics P5472 + P5470A x 2] 1 3-way pin header, 2.54mm pitch with shorting block (JP1) 2 5mm-long ferrite RF suppression beads, 4mm outer diameter (FB1,FB2) [Altronics L5250A, Jaycar LF1250] 1 9V battery 1 9V battery clip with flying leads 1 13-16mm diameter knob to suit VR1 1 8-pin DIL IC socket (optional) 1 100mm cable tie 4 15mm-long M3 tapped spacers 2 M3 x 25mm nylon or polycarbonate panhead machine screws 4 M3 x 6mm panhead machine screws 2 M3 hex nuts 1 No.4 x 8mm self-tapping screw 1 90mm length of medium-duty hookup wire 1 solder lug 4 PC stakes Semiconductors 3 OPA1662AID dual op amps, SOIC-8 (IC1-IC3) [RS Components 825-8424] 1 LM358 dual op amp, DIP-8 (IC4) 1 TPS70960DBVT 6V regulator, SOT-23-5 (REG1) [RS Components 900-9876] 1 BC337 NPN transistor (Q1) 2 1N4148 small-signal diodes (D1,D2) 1 1N4004 1A diode (D3) 1 1N5819 1A schottky diode (D4) Capacitors 3 100µF 25V PC electrolytic 2 47µF 16V PC electrolytic 10 10µF 25V PC electrolytic 1 2.2µF X7R SMD ceramic, 2012/0805 package [RS Components 6911170] 2 470nF MKT polyester 6 100nF MKT polyester 2 100pF ceramic Resistors (all 0.25W, 1% metal film) 5 1MΩ 2 330kΩ 1 180kΩ 10 100kΩ 2 33kΩ 4 10kΩ 1 6.8kΩ 3 5.1kΩ 2 2.2kΩ 2 270Ω 4 150Ω 2 100Ω 1 33Ω 1 dual-gang logarithmic 50kΩ 9mm PCB-mount potentiometer (VR1) [Jaycar RP8760] 2 10kΩ horizontal 5mm trimpots (VR2,VR3) 1 10kΩ top-adjust multiturn trim pot 3296W style (VR4) where the solder lug makes contact with the screw entry point to ensure good contact with the metal. Finally, the rubber feet provided with the enclosure can now be fixed to the base using their self-adhesive. 23 Free audio signal generator and analyser software If you want an audio signal generator that runs on a computer, you can use the free Audacity software (www.audacityteam.org). This is available for Windows, macOS, GNU/Linux and other operating systems. Download and install the version that suits the operating system on your computer. Once installed and running, select Generate -> Tone and then set the waveform to sine, frequency to 1kHz and volume to maximum (ie, set the level value to one). You can also set the duration over which the tone is generated. Press the play button for the audio to start. Another good, easy-to-use option is WaveGene (http://bit.ly/ pe-jan21-wg1). For spectrum analysis, you could use WaveGene in combination with WaveSpectra (http://bit.ly/pe-jan21-wg2). See the setup instructions at: http://bit.ly/pe-jan21-wg3 We used Visual Analyser, available from http://bit.ly/pe-jan21-va, mainly because this allows the actual measured waveform to be seen as a ‘scope’ view, along with the output spectrum. Once you have installed the signal generator and spectrum analyser software, it’s a good idea to use it to analyse the performance of your computer sound interface. That can be done with a cable with 3.5mm stereo jack plugs at each end, with one end plugged into the sound input and one into the sound output. To do this with Visual Analyser, on the main screen, select ‘floating windows mode’ and then the Scope, Spectrum and Wave need to be opened from the top row of selections. Select a 1kHz sinewave for the wave generator, select interlock (that causes both A and B channels to change together) for the output levels and bring up the output level on the waveform generator. Then press the on/off button below the output level slider. The on/off selection at the top left of the main screen also needs to be selected so that the analyser measures the signal. Both will show ‘off’ when the signal is generated and measured. You can choose to view the A channel (left) or B channel (right), or both, in the main settings channel selection. We chose to use a 16,384 sample FFT window and a sampling rate of 44.1kHz in the main menu. Output gain (adjustment along the top row at right) was set just below maximum, yielding the lowest distortion figure of 0.0626%. In our case, noise is mostly more than 80dB below the fundamental (see Fig.11). That indicates that this is not a particularly good sound card, but good enough to evaluate the distortion from the Nutube Preamplifier. Now the Nutube Preamplifier can be connected between the computer sound input and output. Adjust signal levels using the volume control and/or the signal generator level so that the waveform is not clipped (ie, so the top of the sinewave is not plateauing) and shows a clean sinewave. In the main menu, you can select the left channel (A) and adjust trimpot VR2 for the lowest distortion reading, with minimal harmonics – see Fig.12. This shows the waveform as a clean sinewave, with the analyser showing the main 1kHz fundamental at 0dB level and the second harmonic (2kHz) at around –70dB. The third, fourth and sixth harmonics are at a similar level. Once you’ve finished tweaking VR2, select the right channel (B) and adjust VR3 for the lowest distortion reading. VR4 can then be adjusted while viewing in the A channel of the analyser, so that fundamental level is the same as that in the B channel. Fig.13 shows the waveform and spectrum when the grid bias (with VR2) is adjusted incorrectly. The top half of the sine waveform is very rounded, and the second harmonic is only 10dB below the fundamental. The distortion reading is around 30%. 24 Fig.11: a screen grab of the free Visual Analyser PC software performing a ‘loopback’ test, with the sound card output fed directly into its input. This lets you analyse the distortion inherent in the system. In this case, the reading is 0.0626% THD+N at 1kHz. You therefore won’t get a reading lower than that when measuring the performance of external devices like the Nutube Preamplifier. Fig.12: now we have connected the Nutube Preamplifier ‘in the loop’ between the sound card output and input, using two stereo jack plug to red/white RCA plug cables. The output levels have been set to 41% full-scale, which corresponds to around 250mV RMS, The distortion reading has only risen slightly, to 0.07%, because the Nutube Preamplifier and sound card distortion figures are similar. Fig.13: here is the same test as Fig.12, but the triode grid bias voltage adjustment is completely wrong. You can see the heavily distorted sinewave in the ‘Oscilloscope’ window, with many harmonics in the spectrum analysis. The THD reading is 30%. This is about as bad as it gets; more realistically, a slightly misadjusted grid bias voltage can lead to distortion levels in the 0.1-1% range. Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au Practical Electronics | January | 2021 LED Christmas Tree PCB special offer! Here’s a little Christmas bargain to help you build your very own stackable LED Christmas Tree decoration.* Buy a single LED Christmas Tree PCB for £6.95 4 PCBs costs just £14.95 12 PCBs costs just £24.95 20 PCBs costs just £34.95 Visit our shop at www.electronpublishing.com and place your order! Want even more? 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Copy this form if you do not wish to cut your issue. Complete Arduino DCC Controller Digital Command Control (DCC) is a great way to control multiple trains on a model railway layout. Unfortunately, commercial DCC systems can be quite expensive. Here we present an Arduino-compatible controller shield that can form the basis of a DCC system. It can also be used as a DCC booster or even as a high-current DC motor driver. by Tim Blythman Y ou can put together this DCC Controller, which incorporates a Base Station and optionally also a programmer, for a fraction of the price of a commercial unit. Combine it with a PC, and you have a potent and flexible model railway control system. It’s based on the Arduino platform, and it’s easy to build. You can also add boosters to the system easily, just by building a few more shield boards. DCC is still the ‘state-of-the-art’ in terms of off-the-shelf model railway systems, so if you have a model railway layout but don’t have a DCC system (or have a DCC system that’s inadequate for your needs), now is the time to upgrade! We published an Arduino-based DCC Programmer for Decoders in our October 2019 issue. Since then, we’ve had numerous requests for a DCC base station or booster. Therefore, we have created this DCC power shield, which is the final piece of the puzzle. Adding this (and an appropriate power supply) to the Programmer,, in conjuction with DCC-capable locos, results in a complete DCC system. As this is an Arduino-based project, the following description assumes that 26 you are familiar with the Arduino IDE (Integrated Development Environment). To download the free IDE software, go to: www.arduino.cc/en/software We are using version 1.8.5 of the IDE for this project, and suggest that if you have an older version installed, that you upgrade it now. What is DCC? We went into a bit of detail on DCC in the DCC Programmer article, so we’ll only cover the basics here. If you want to learn more, read the aforementioned article from October 2019. DCC is designed to allow multiple model trains to be controlled on a single track, with the same set of tracks carrying power for the trains and also digital control commands. Older command controls systems exist; for example, the Protopower 16, which was based on another system called CTC16. This worked similarly to the system used to control multiple servo motors on model aircraft. However, that system was limited to 16 locomotives, while Digital Command Control has around 10,000 addresses available; probably well beyond the scope of most model railroads (and many full-scale railroads too!). A complete DCC control system can be made by adding an Uno board and the DCC Programmer Shield (which we described in the October 2019 issue) to the DCC Power Shield, as shown here. Fit the DCC Programmer Shield with stackable headers, so it can be sandwiched between the other two boards, and take care that nothing shorts out between the adjacent boards. You may need to trim some of the pins on the underside of the DCC Power Shield. Practical Electronics | January | 2021 Two locos, one track – but both are under individual control of the DCC system. As you can just see, the loco in front even has its headlight on – also switched on or off at will via DCC. Want more than two trains? DCC has up to 10,000 addresses available! The most basic method of model train control is for a single throttle to apply a variable DC voltage to the track, which drives the train’s motor directly. Instead, a DCC base station delivers a high-frequency square wave to the track. The base station encodes binary control data into this signal by varying the width of each pulse (see Fig.1). A digital decoder on each vehicle receives commands and also rectifies the AC track voltage to produce DC. The decoder then uses this to drive the motor and can also control lights, sound effects (like a horn or engine) or even a smoke generator. There are also accessory decoders which can be used to control things such as points and signals using the same DCC signals. The DCC standard is produced by the National Model Railroad Association (based in the US; see http://bit.ly/ pe-jan21-dcca). These standards are available for download, which means that anyone can use them. As a result, many different manufacturers are making DCC-compatible equipment. Our Base Station will work with many commercially-available decoders. There is a vast array of manufacturers of DCC equipment, so we can only test a small subset. All of those we have tested have worked well, as should be expected from a proper application of the standard. Terminology A Base Station – in DCC terminology – is the brains of the system. Typically, it receives commands from attached throttles controlled by people, or perhaps a computer. These commands then dictate what DCC data needs to be sent over the tracks to the trains to control them. The Base Station generates a continuous stream of DCC data packets to control and update all trains, signals and points as needed. Practical Electronics | January | 2021 A Booster is a simple device which takes a low-level DCC signal and produces a DCC signal of sufficient power to drive a set of tracks. Many smaller DCC systems consist of a single unit which combines a Base Station with a Booster, while larger systems might have separate units, including multiple Boosters. Our DCC Power Shield works as a Booster. An attached and properly programmed Arduino board can be used as the Base Station smarts, thus creating a basic DCC system in a single unit. Extra DCC Power Shields can be deployed as separate Boosters, with an Arduino attached to monitor each and check for faults. When programmed with the DCC++ software, the Arduino board and DCC Power Shield can be combined with our earlier DCC Programming Shield to create a compact, economical and fully-featured DCC system. Power source A DC power source is needed to run the DCC Power Shield. The DCC standards suggest that Boosters should produce 12V-22V peak, so your chosen power source needs a regulated DC output in this range. For modest current requirements (up to around 5A), a laptop power supply is a good choice. Many of these have a nominal 19V DC output at several amps. Any fully DCC-compatible trains and decoders should handle this fine, but it’s worth checking any that you aren’t sure about. Decoders are supposed to work down to around 7V. Given that the track, wiring and locomotives are bound to drop some voltage, a 12V ‘power brick’ type supply works well enough for driving trains. However, we found that this sometimes wasn’t enough to allow decoder programming to occur. If you need more current than a laptop power supply can provide, you will need to find a dedicated power supply in the 12-22V range. Many suitable high-power ‘open frame’ switchmode supplies are available from various suppliers. One thing to note is that while some Arduino boards (including genuine boards) can tolerate up to 20V on their VIN inputs, some clones use lowerrated voltage regulators which can only handle 15V. We have provided an option for a zener diode to help manage this variation; read on for more information on how the circuit works. DCC Power Shield circuit The circuit of the DCC Power Shield is shown in Fig.2. Its key function is to turn a steady DC voltage into a DCC-modulated square wave. For this, we need a full H-bridge driver. To keep it simple, we have used a pair of BTN8962 half-bridge driver ICs (IC1 and IC2). Features and specifications • Based on the Arduino Uno • Provides a DCC output of 12-22V peak at up to 10A, or more with some changes • Can operate as a Base Station or Booster • Compatible with DCC++ and JMRI (DecoderPro/PanelPro) software • Opto-isolated input for use as DCC slave • Works with our DCC Programmer shield from the October 2019 issue • Can also be used as a brushed-motor driver • All Arduino pin assignments configurable via jumpers 27 “0" BIT “1" BIT +12V to +22V 0V –12V to –22V 58 s 58 s 100 s 100 s SC 2020 Fig.1: the DCC waveform is a square wave with a frequency around 5-8kHz. Binary data to control trains, signals and points is encoded in the pulse TIME widths. The BTN8962TA ICs we’re using are ideally suited to delivering such a signal at up to 10A or more. See the panel How DCC works on pages 30 and 31 of the October 2019 issue for more information. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 A A A A A A A A 0D D D D D D D D0 C C C C C C C C 1 ADDRESS PREAMBLE START BIT The BTN8962 comes in a TO-263-7 package, which is a surface-mounting part, although quite a large one. It is not difficult to solder. There are two of these, one driving each side of the track. They are supplied with out-ofphase input signals to produce the required alternating output drive. Their supply pins (pins 1 and 7) are connected directly across the incoming DC supply from CON1, labelled VIN. A 100µF electrolytic capacitor bypasses this supply. 100µF may seem like a low value, the current drawn by IC1 and IC2 is quite steady as when one output goes high, at the same time, the other goes low. The outputs of IC1 and IC2 connect to screw terminal CON2, and then onto the tracks. The state of the IN pins (pin 2) determines whether the output pins (4 and 8) are driven high or low. The SR input pin controls the output slew rate. We’ve tied this to ground to give the fastest possible slew rate. The ‘INH’ pins (pin 3) need to be brought high to enable the outputs. These are connected together and have a 100kΩ pull-down resistor so that the outputs default to off. The enable signal connects back to an Arduino pin via a 10kΩ resistor and jumper JP1, allowing the Arduino to enable or disable the outputs as DATA START BIT START BIT CHECKSUM END PACKET BIT required. JP1 lets any Arduino digital pin connect to the enable signal, to suit the software used. The IS pins (pin 6) on IC1 and IC2 are outputs that source a current proportional to the current being drawn from the output of each IC (plus a small offset current, which is compensated for in software). These currents are combined in a ‘diode-OR’ circuit formed by diodes D1 and D2 and then fed to a 1kΩ resistor to convert the combined current into a voltage. This then passes to an RC low-pass filter (20kΩ/100nF) for smoothing. The 2ms time constant means that peaks in the current due to the rapidly changing DCC signal are ignored, but faults can still be detected quickly. The resulting smoothed voltage is fed to one of the Arduino analogue input pins via jumper JP2, to allow the Arduino to monitor the track current. JP2 allows any of the Arduino analogue inputs to be used to monitor track current, again allowing us to choose whichever pin suits the Arduino software. The IS pins will also source current if IC1 or IC2 detects an internal fault condition; as far as the software is concerned, this is equivalent to a very high current being drawn from the output and is treated the same way. Bridge driving signals The input signal to pin 2 of IC2 comes from another one of the Arduino digital outputs via a 10kΩ series resistor. Once again, any Arduino digital pin can be used, and this too is selected by a jumper shunt on JP1. A simple inverter circuit produces the out-of-phase signal to drive the IN pin of IC1. The signal that goes to pin 2 of IC2 is also fed to the base of NPN transistor Q1 via a 1kΩ resistor. Q1’s collector is pulled up by a 10kΩ resistor to the ENABLE line. So as long as ENABLE is high, meaning the outputs of IC1 and IC2 are active, input pin 2 of IC1 is inverted compared to input pin 2 of IC2. Opto-isolated input To allow a separate Base Station to be used, an optoisolated input is provided at CON3. This can accept a logic-level DCC signal, or even a ‘track voltage’ (12-22V) signal from another DCC system. The signal at CON3 passes through a 2.2kΩ series resistor and into the LED of OPTO1. 1N4148 diode D3 is connected in reverse across this LED, to protect it from high reverse voltages. If a logic-level DCC signal is applied to CON3, then the polarity markings need to be observed, as current will only flow through OPTO1 when the voltage at pin 2 is high. A bipolar DCC signal can be connected either way around. OPTO1 is a 6N137 high-speed optoisolator which has a nominal forward current of 10mA. Thus the 2.2kΩ resistor is suitable for voltages up to around 22V, ie, the maximum expected from a DCC system. The output of OPTO1 is supplied with 5V from the Arduino board, bypassed by a 100nF capacitor. A 330Ω pull-up resistor sets the logic-high level. The output from OPTO1’s pin 6 is fed via a 1kΩ protection resistor to jumper JP3. This allows the DCC signal to be fed directly to the input of bridge drivers IC1 and IC2. The three PCBs which make up the DCC system: on the left is a ‘standard’ Arduino UNO board (or one of its many clones); centre is the optional DCC Programmer (from our October 2019 issue) while at right is the DCC Power Booster Shield. All three boards are made to conveniently plug together. 28 Practical Electronics | January | 2021 VIN VIN POWER IN 2 100 F 35V 1 +5V CON1 2.2k DCC IN + 1 2 D3 1N4148 1 7  3 A CON3 C 6 B 10k Q1 BC549 1k IC2 ENABLE JP3 DIR OPTO JP1 1k 7 2 BTN8962TA CON2 VS 6 IS 5 SR 10k DIR DCC OUT VIN A 1k ENABLE 1 1 D2 1N4148 K 4,8 GND 10k ENABLE OUT CONTROL LOGIC 2 IN 3 INH E 5 4 BTN8962TA 6 IS 5 SR 330 8 2 K 7 VS A K 100nF OPTO1 6N137 IC1 D1 1N4148 CONTROL LOGIC 2 IN 3 INH OUT 4,8 GND 100k 1 1k VIN 2 4 6 K  E A 1N4148 K K LED1 A  BC549 B K A LED2 ENABLE 6N137 1 A K BTN8962TA 8 20k 8 C ZD1 A 1k A5/SCL A4/SDA 1 3 A3 A2 A0 A1 VIN GND GND +5V +3.3V +5V RESET DC VOLTS INPUT 5 ICSP ARDUINO UNO, DUINOTECH CLASSIC, FREETRONICS ELEVEN OR COMPATIBLE LEDS +5V +5V D1/TXD D0/RXD D3/PWM D2/PWM D4/PWM D5/PWM D7 D6/PWM D8 D10/SS D9/PWM D12/MISO D11/MOSI GND D13/SCK AREF SCL USB TYPE B MICRO SDA DIR 4 1 4 7 JP2 ISENSE A 100nF ZD1 (OPTIONAL) K +5V SC DCC DCC Controller/Booster CONTROLLER/BOOSTER 2020 In this case, a jumper on JP1 can be used to feed the same signal to one of the Arduino’s digital pins, which would then be configured as an input. Due to the open-collector output of OPTO1, this signal is inverted compared to that applied to CON3. But this can be solved simply by reversing the connections from CON2 to the tracks. This reversibility of the DCC signal is a necessary feature, as a locomotive may be placed on the track either way and must be able to work with an inverted signal. The only time this matters is when different boosters feed two adjoining tracks. In that case, you will need to make sure that the signals are in-phase. Other features Status LEDs LED1 and LED2 are connected to the ENABLE signal with 1kΩ current-limiting resistors to GND and 5V respectively. So if ENABLE is high, the green LED1 lights up, and if it’s low, then the Practical Electronics | January | 2021 Fig.2: as with many Arduino shields, the circuit’s smarts are on the Arduino itself. The shield consists primarily of two integrated half-bridge drivers (IC1 and IC2), a transistor inverter (Q1), a high-speed optocoupler for feeding in external DCC signals (OPTO1), two LEDs for status monitoring and some headers to allow the Arduino pin mappings to be changed if necessary. red LED2 lights up instead. If ENABLE is high-impedance, such as when the Arduino is in reset, neither LED lights. A single bi-colour LED could be fitted either for LED1 or LED2 to achieve the same effect. If fitted, ZD1 feeds DC from CON1 to the VIN input of the Arduino board. Its value is chosen to limit the Arduino input voltage to a safe level at the maximum expected voltage from CON1. For example, for 22V into CON1, ZD1 can be an 8.2V type, so 13.8V is fed to the Arduino VIN pin. A 1W, 8.2V zener diode can pass up to 120mA, which should be enough to power the Arduino and any connected shields. We’ve left enough space to fit a 5W zener diode if you need more current than that, although if you’re going to be applying less than 22V to CON1, you could also use a lower-voltage zener, which could then pass more current before reaching its 1W limit. For situations where the voltage on CON1 is suitable for direct connection to VIN (typically under 15V for clones or 20V for genuine Arduino boards), then a wire link can replace ZD1. However, it would still be a good idea to fit a low-voltage zener (eg, 3.3V) as this will reduce the dissipation in the Arduino’s regulator. Just make sure that the voltage fed to the Arduino’s VIN pin will not drop below 7V. If you aren’t sure whether your Arduino can handle more than 15V, check the onboard regulator. It’s usually in an SOT-223 three-pin SMD package with a hefty tab. Genuine Arduino Uno boards usually have an NCP1117 regulator, rated to handle up to 20V. Clones often have an AMS1117 instead, which is only rated to 15V. If ZD1 is left off, the supplies are separate (although their grounds will be connected). This allows the Arduino to be powered via its USB connector, eg, from a controlling computer. DCC Programming Many DCC Base Stations have a separate output for programming decoders. 29 CON2 09207181 Rev F 4148 10kW <OPTO 1 0 2 4 #3 #5 7 8 Q1 10kW 100nF 5V GND VIN ANALOG + – CON3 DCC IN 2.2kW D3 20kW 1kW 330W OPTO1 6N137 4148 10kW 1kW A0 A1 A2 A3 A4 A5 In other words, programming is not done via the main high-current output driver, which is usually kept connected to the layout. For this reason, you may wish to have the DCC Power Shield and October 2019 DCC Programmer shield plugged into the same Arduino. The DCC++ software is designed to handle this. However, this does complicate the power supply arrangements a bit. First, the DCC Programmer Shield has a maximum supply voltage of 15V, so regardless of the type of Arduino board you are using, you will need to ensure that the VIN pin is no higher than 15V. Also, in this system, it would be best to build the DCC Programmer Shield without the MT3608 boost module, and fit the jumper shunt on CON8 between pins 1 and 2, so that the VIN supply is used for programming power. The DCC Programmer Shield can draw up to 200mA from VIN, so the dissipation of ZD1 will increase substantially. You will need to choose its value carefully, or use a 5W zener. Another option, if the system will always be connected to a computer, is to build the DCC Programmer Shield with the MT3608 boost module and fit it below the DCC Power Shield, then leave out ZD1 from the Power Shield. The DCC Programmer Shield will then be powered from the computer’s 5V USB supply, while the DCC Power Shield is still powered via CON1. Construction The DCC Power Shield is built on a double-sided PCB in a typical Arduino shield shape, coded 09207181, measuring 68.5 × 55mm and available from the PE PCB Service. Use the overlay diagram, Fig.3, as a construction guide . Start by fitting IC1 and IC2. Although these are surface-mounting components, they are quite large. Because of this, and the fact that they sit on large copper pours, it will require quite a bit of heat to make good solder joints. 4148 DCC POWER SHIELD 1kW A 1 ZD1 1 D1 LED2 K JP2 D2 LED1 109207181 8170290 100nF 1kW 1kW 100kW ENABLE IC1 BTN8962 30 TX RX JP1 IC2 BTN8962 DCC OUT DIGITAL #6 DIR #9 13 12 #11 #10 100mF RESET 3V3 +DC IN– GND 1 CON1 AREF SCL SDA Fig.3: the seven-pin halfbridge driver ICs are mounted on the left, near the power input (CON1) and track (CON2) terminals. The jumper positions shown here are those required to use both the open-source DCC++ software and our example sketches. The jumpers are mostly handy if you want to use this shield as a DC motor driver, so that you can connect the required SC Ó2020 functions to PWM pins. Flux paste and solder braid (wick) will come in handy, as will tweezers. Apply some flux paste to the pads first, to make soldering easier. Working on one at a time, start by tacking one of the end pins in place to locate the device. Once you are happy that each is centrally located within the footprint, load some solder on the tip of your iron and apply it to each of the smaller pads. Ensure that the resulting solder fillets are solid. Use the solder braid to remove any solder bridges. The two end pins, numbers 1 and 7, are ground and power respectively. It’s a good idea to add a bit of extra solder to these pins to help with current and heat handling. Finally, solder the large tab of each device. Hold the iron tip at the point where the tab meets the pad on the PCB. Heat the pad until it melts solder applied to it. Feed in solder until a rounded, but not bulging fillet is formed and allow it to cool. Next, fit the 12 resistors. The PCB silkscreen is marked with the values, and you should check these match with a multimeter as they are fitted, to ensure they are the correct value. Solder close to the PCB, then trim the leads close to the underside. Then install the three small 1N4148 diodes (D1-D3) where shown (Fig.3) ensuring that they are correctly oriented. If fitting ZD1, do that now. Make sure that its cathode band faces towards the top of the PCB. Then mount the rectangular MKT capacitors, which are not polarised. Now install NPN transistor Q1, with its body oriented as shown. You may need to crank the leads out to fit the PCB pads. Solder it in place, ensuring it is pushed down firmly against the PCB. If you plan to fit another shield above this one, then its top should not be more than 10mm above the PCB. The electrolytic capacitor should be mounted on its side to allow another board to be stacked above this one. Its longer, positive lead must go in the pad towards the top of the board, as shown. Fit OPTO1 next. Check that its notch or pin 1 dot faces in the direction shown. Carefully bend the pins to allow it to fit into the PCB pads and solder it in place. Headers The various headers should be fitted next. Note that if you already know which Arduino pins will be used for the DIR, ENABLE and ISENSE signals and they will not change, you could omit JP1-JP3 and fit wire links in their places. To connect to the Arduino, you can use either regular headers or stackable headers. We recommend using the Arduino board as a jig to ensure that the pins are square and flush to the PCB. Stackable headers can be more tricky to mount as they need to be soldered from below. If possible, use those with 11mm-long pins (some that have 8mm pins don’t leave much room to solder). Thread the headers through the shield and into the Arduino board. Flip the whole assembly over so that the shield is resting flat against the pins, then solder the end pins of each group in place to secure the headers. You can then remove the shield from the Arduino board and solder the remaining pins in place, before retouching the end pins. It’s easiest to use single-row pin headers for JP1-JP3 – snapped to length and soldered side-by-side for JP1 and JP2. If you are snapping 40-way headers to do this, you will need at least two. Rather than fitting JP3 as a separate two-way header, you can make the top two rows of JP1 longer by one pin (ie, 15 pins rather than 14). The last step in the construction is to fit the two screw terminals to CON1 and CON2, with their wire entry holes facing the outside edge of the board. Ensure that they are flat against the PCB; this is particularly important if you need to stack a shield above this one. Practical Electronics | January | 2021 You may need to trim the underside of CON2, as this could foul the DC jack of an attached Uno board. Similarly, the underside of CON1 comes close to the metal shell of the USB connector of an attached Uno. It’s a good idea to add a layer of electrical tape on top of the USB connector on the Arduino board, to make sure they can’t short if the boards flex. Jumper settings We suggest that you connect DIR to D10, ENABLE to D3 and ISENSE to A0, as shown in Fig.2 and Fig.3. This suits our software. Note that there are triangular silkscreen markings on the PCB to indicate the default jumper locations for JP1. To use the board as a DCC Booster with our supplied software, just add a fourth jumper across JP3 at upper-right. Software There are a few different ways this shield can be used, and each has its own software requirement. We’ll describe a few of these possibilities. The following assumes that you have fitted the jumpers to the default locations described above. DCC++ We mentioned the DCC++ software in our October 2019 article. It is designed to work with either an Uno or Mega board; we paired it with the Uno previously, and the discussion in this article assumes the same. The Uno is adequate to work with the JMRI (Java Model Railroad Interface) software and will naturally cost less than a Mega. The DCC++ project also includes a Processing-based GUI application for your PC that can interface with the Base Station, although this has been customised to work with a layout which belongs to the DCC++ software designer. Alternatively, you can use JMRI. We also covered this software in the previous article. JMRI can be downloaded from http://bit.ly/pejan21-dccb There are versions for macOS, Windows and Linux. It can even be run on Raspberry Pi single-board computers. Carefully follow the installation instructions, including installing Java if necessary. As we mentioned, our hardware is compatible with DCC++ in base station mode. There is more information, including the required Arduino sketch, available for download from: http:// bit.ly/pe-jan21-dccc Practical Electronics | January | 2021 Parts list – Arduino DCC Controller 1 Arduino Uno or equivalent 1 12-22V DC high-current supply (see text) 1 double-sided PCB coded 09207181, 68.5mm x 55mm, from the PE PCB Service 1 set of Arduino headers, standard male or stackable (1 x 6-way, 2 x 8-way, 1 x 10-way) 2 2-way 5/5.08mm pitch PCB-mount screw terminals (CON1,CON2) [Jaycar HM3172, Altronics P2032B] 2 15-way pin headers (JP1,JP3) 1 14-way pin header (JP1) 2 6-way pin headers (JP2) 4 jumper shunts/shorting blocks Semiconductors 2 BTN8962TA half-bridge drivers, TO263-7 (IC1,IC2) [Digi-key, Mouser] 1 6N137 high-speed optoisolator, DIP-8 (OPTO1) 1 BC549 100mA NPN transistor (Q1) 1 green 3mm LED (LED1) 1 red 3mm LED (LED2) 1 1W or 5W zener diode to suit your situation (ZD1; see text) 3 1N4148 signal diodes (D1-D3) Capacitors 1 100µF 35V electrolytic 2 100nF MKT Resistors (all 1/4W 1% metal film) 1 100kΩ 1 20kΩ 3 10kΩ This software is designed to work with several commonly available Arduino motor driver shields. But these shields need some modifications to work, whereas our hardware only requires the correct jumpers to be set. The default setting in DCC++ for the MOTOR_SHIELD_TYPE of ‘0’ will work with our hardware. Open the Arduino IDE, select the Uno board and its serial port via the menus and open the DCC++ Base Station sketch that you’ve downloaded. Then upload the sketch to the Uno. If you open the serial monitor at 115,200 baud, you will see a banner message; this indicates that the Base Station software is working as expected. You can also interact with the Base Station through serial commands. The protocol is detailed in the PDF file that is included in the DCC++ Base Station project ZIP file. Once you’ve tested this, close the Serial monitor and open the JMRI DecoderPro program. Go to Edit -> Preferences, and under Connections, choose DCC++ as System Manufacturer, DCC++ Serial Port as System connection. Ensure the serial port setting matches the Uno’s. Save the settings and close DecoderPro, so that it can reload the new settings. Re-open DecoderPro and under Edit -> Preferences, choose Defaults, and ensure that the name of the new connection name is used for all connections (instead of ‘Internal’). Unless you have other hardware you want to use, you should select DCC++ for all options. 1 2.2kΩ 5 1kΩ 1 330Ω Save, close and re-open DecoderPro again. Click the red power button in DecoderPro and ensure that the power button turns green. The LED on the DCC Power Shield should switch from red to green. The simplest way to drive trains is to select Actions -> New Throttle, set the locomotive address and manipulate the controls (see Screen1). JMRI can do a lot of different things, so we suggest you read its manual to find out about its capabilities. The JMRI project also includes PanelPro, which can be used to design track and signal diagrams for controlling a model layout. Adding the DCC Programmer If you have already built the DCC Programmer, then the Arduino board is already programmed to work with the DCC Power Shield, and the DCC Power Shield can be added to the stack, ideally at the top. As we noted earlier, the choice of zener diode and power supply will be more complicated if you want to construct an all-in-one setup. Since this is likely to be a smaller system, we suggest that a modest power supply will be suitable. Using the DCC++ software with JMRI is the same as discussed above. Using it as a booster When a signal is fed in via the optoisolated input (CON3), the DCC Power Shield is effectively working as a Booster. The signal can be from 31 another Base Station or system, with the DCC Power Shield turning that signal into a more powerful DCC signal that can be used to drive trains. While it might not seem that an Arduino is needed in this case, it’s a good idea to have one as we can program it to monitor the DCC signal and intervene if there is a problem. So we’ve written a sketch to allow an Arduino to take on this supervisory role. There are two main conditions to check for. First, we want the Booster to be able to protect the shield if too much current is being drawn from it. This could be due to an overload or even a short circuit, such as a metal object being dropped across the tracks. Thus, our sketch continually monitors the voltage present on its A0 pin via its internal analogue-to-digital converter (ADC). If it gets above a certain threshold, the power to the track is cut by pulling the ENABLE pin low. A timer starts and the sketch attempts to re-apply power after it expires. If the short circuit is still present, then the over-current condition re-occurs, power is cut again and the timer re-starts. The other condition we need to consider is if the incoming DCC signal is lost. This could be for any reason, such as if the connection to CON3 is broken or the upstream DCC Base Station has a fault. In any case, when there is no signal at CON3, the input to IC1 is held high and IC2’s input is low. There is then an unchanging DC voltage across the tracks. This may not sound like a problem, but some DCC locomotives can be programmed to undergo ‘DC conversion’. When a locomotive decoder detects that there is a steady DC voltage present, the locomotive behaves as if it was on a conventional ‘single-throttle’ layout and will typically set off in one direction at full speed (hopefully not towards the end of the track…). This feature was initially added to allow DCC locomotives to be used on conventional layouts, perhaps as an aid to owners transitioning to DCC from DC systems. Fortunately, the DC conversion feature can be turned off in the decoder by setting a configuration variable. You can use a DCC Programmer such as from our October 2019 article to do this. In any case, the sketch detects that the DCC signal is no longer changing and pulls the ENABLE line low, disabling the track output and preventing such runaways. To enable the use of the optoisolated input, add a jumper across JP3. Leave the jumper on ‘DIR’ for pin D10 in place; D10 is set as an input in the software and is used to monitor the incoming DCC signal. Screen1: while JMRI’s DecoderPro program has many features, it also has a set of basic tools for controlling trains. This throttle window allows speed, direction and light functions to be controlled. You can even switch track power directly; the green icon at upper right mimics the status LEDs on the shield. 32 The Booster sketch is called DCC_ Shield_passthrough_supervisor.ino. This uses a library to perform the precision timing needed to generate the DCC waveform, called TimerOne. This can be installed via the Library Manager by searching for ‘timerone’ or from the ZIP file we have included with our software package. Open the sketch, select the Uno and the serial port and upload it. Disconnect the USB cable and connect your power source to CON1. The red LED should light. Connect a valid DCC signal to CON3 and the green LED should light. You should then have a valid DCC signal at CON2. A standalone sketch We’ve also created a simple standalone sketch that produces a DCC signal, suitable for controlling a single locomotive. The decoder identification number has been set to 3 (which is the default for new, unprogrammed decoders), although it can be changed in the code. We suggest you use this option if you want to try out DCC for the first time. We can’t offer advice on fitting decoders; there are so many options for both decoder choices and how they are connected. The companies that manufacture the decoders do offer advice (and many have custom decoders to suit specific locomotives). After all, they want to make it easy for you to buy their products. Our standalone sketch also requires the Timer One library mentioned above, so make sure that is installed. Screen2: while very basic, our standalone sketch named DCC_Single_Loco_Control.ino allows power, speed, direction and lights to be controlled by commands in the serial monitor – download from the January 2021 page of the PE website. The software can be modified to control multiple locos. Advanced Arduino users could use it as the basis of an automated layout control system. Practical Electronics | January | 2021 Set the jumpers on the shield to the default positions and connect the Uno to the computer. Open the DCC_Single_Loco_Control.ino sketch and select the Uno board and its serial port. Press the Upload button to compile and upload the sketch, then open the Serial Monitor at 115,200 baud (see Screen2). You can now enter commands as numbers which correspond to the desired locomotive speed, in 128 steps. Thus, numbers from −127 to 127 are accepted. You should ensure that 28/128 step speed mode is set on your locomotive decoder. Type ‘P’ (upper case) to turn track power on and ‘p’ (lower case) to turn it off. The power will automatically turn off if current over half an amp is detected. You can also use ‘A’ and ‘a’ to turn on and off the loco’s headlights. The program is elementary, but it has several unused functions to send all manner of DCC packets to the track. If you are comfortable with the Arduino, you should have no trouble adapting it to do something more advanced. Current limitations Using the specified components and the DCC++ software, the shield can easily deliver up to 10A. This is mostly limited by the screw terminal connectors. The DCC++ software also has a hard-coded current limit which kicks in at around 10A. Of course, the software limit is easy to change, but any hardware changes should be done with care. The output driver ICs are capable of handling around 30A, with the PCB tracks topping out around 20A. In any case, everything runs cool well below the 10A limit, so maintaining this limit is good for component longevity. DCC has a wide range of operating voltages, so to increase output power, it may be easier to increase the supply voltage. Most locomotives use PWM speed control on their motors, so a higher supply voltage simply means a lower PWM duty cycle (and thus current consumption) for the same speed. We haven’t done any tests above 10A, but if you are set on increasing the current capacity of the DCC Power Shield,, then you should ditch the screw terminal connectors and solder thick copper wires directly to the board (ideally, to the power pins of IC1 and IC2). Practical Electronics | January | 2021 If the wires can handle 20A, then your modified DCC Power Shield should have no trouble doing that. To go higher than this will probably mean that IC1 and IC2 need some heatsinking, as well as even thicker wires. We suggest that you instead consider using more, smaller boosters. For example, you could modify the Booster sketch to monitor and drive multiple DCC Power Shields stacked above it. A larger system If you are planning a system with multiple Boosters, either because you need the power or it otherwise makes sense to do so, then there are a few minor caveats. When running multiple Boosters, avoid daisy-chaining the DCC signal from one Boosters to the next. Instead, fan out the DCC signal from one Base Station to all the Boosters. Many commercial Base Stations have a low-powered DCC signal output (Digitrax names this Railsync), which is ideally suited for this purpose. The first problem with a daisy-chain configuration is that if one Booster goes down, then so do all those downstream, as the DCC signal will be shut off. Second, each Booster also has a small but measurable delay in propagating the signal. In our case, this is around 4µs, due to the switching time of the BTN8962s. This delay is not usually a problem, but it may become one at the boundary where the tracks from two Boosters meet (where there would typically be an insulator, to prevent one Booster feeding another Booster’s section of track). Where the tracks meet, a train may be briefly fed by both the Boosters. If there is Using the DCC Booster Shield as a motor driver The DCC Booster Shield can be used as a high-current motor driver shield. In this case, the signal on the DIR pin determines the motor direction, and a pulse-width modulated signal is applied to ENABLE to control the speed. The BTN8962 has active freewheeling, so no external diodes are needed. If used like this, LED1 and LED2 will both appear to be on at the same time, with green LED1 becoming brighter and red LED2 dimmer as the duty cycle increases. As noted earlier, the 100µF electrolytic capacitor is adequate for a DCC application. A larger value may be needed for motor driving. We suggest leaving ZD1 off, as larger motors will create hefty spikes at the end of each drive pulse. Keeping the two supply rails separate will prevent this from damaging the Arduino board. a delay between the signals from the two Boosters, then it may appear to be a short circuit if the two Boosters are delivering opposite polarity voltages at that instant. This is less likely to occur if the Boosters are well synchronised, which should be the case if all are being fed the same signal. You should also ensure that the Boosters are fed with similar supply voltages, so that one Booster does not try to power another Booster’s track when the train bridges their join. You must also ensure that the Boosters are wired with the correct polarity where the tracks meet. Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au The DCC Power Shield can be combined with an Arduino Uno and DC power supply to create a basic DCC system. Using our standalone sketch or JMRI’s DecoderPro program, this combination can be used to control DCC-equipped trains, points and signals on a model railway layout. 33 Using Cheap Asian Electronic Modules by Jim Rowe Intelligent 8x8 RGB LED matrix This month, we’re looking at a module with an 8x8 matrix of 64 ‘intelligent’ RGB LEDs. Each LED can display over 16 million different colours, or primary colours at 256 brightness levels. The LEDs are controlled serially via a single wire, and multiple modules can be cascaded to build a much larger display, enabling all sorts of useful applications. W e looked at some 8x8 LED display modules in an earlier article in this series, back in the July 2018 issue. We thought it was worth writing this one up too, as it is significantly more flexible and just generally more useful. It uses RGB (red/green/blue) LEDs rather than monochrome (single colour) LEDs. Each LED can display up to 256 brightness levels for each of the three colours, to give a total of 16,777,216 (256 × 256 × 256) different colours. In this module, each RGB LED has its own built-in serial data register, latch register and decoder/driver, so no separate controller is needed. All 64 LEDs of the module are connected in sequential (daisy-chain) fashion, so that serial data can be fed into the first LED of the module and passed through to the other LEDs in turn. If you want to use multiple modules, the data output from the 64th LED on the first module can be fed to the first LED of the next module to program its LEDs as well. And so on. This module is based on an impressive device: the WS2812B intelligent control LED made by WorldSemi, based in Dongguan, Guangdong province, China (between Guangzhou and Shenzhen, and near Hong Kong). I should note that some of the modules currently available use a ‘clone’ of the WS2812B device, the SK6812, made by another Chinese firm: Shenzhen Sikewei Electronics. Although the timing specs for the SK6812 differ a little from those of the WS2812B, they are quite compatible with most of the available software. You can find these WS2812B/ SK6812-based 8×8 RGB LED modules on the internet from various vendors, many of them available via sites like eBay or AliExpress (www.aliexpress. com/item/32671025605.html). The prices vary quite a bit, but you can find them from around US$4 shipped! Now let’s look at the WS2812B IC to see how it works. (This description also applies to the SK6812.) The WS2812B LED chip This small (5 × 5 × 1.6mm) four-lead SMD package, shown in Fig.1, houses a trio of LEDs as well as a serial controller IC. It looks deceptively simple, but you can see from the block diagram (Fig.2), there’s quite a lot inside. It includes a 24-bit shift register, a 24bit latch, three eight-bit DACs (digitalto-analogue converters) coupled to a driver for each LED and a buffer amplifier to boost and reshape the serial data output, ready for the next WS2812B. Fig.3 shows how a string of 64 WS2812B devices are connected to Fig.1: the SMD package size and pinout of the WS2812B (and equivalent) chips. Internally, it’s made from multiple semiconductor dies, tied together with bond wires and encapsulated with a plastic lens on top. Note that the package orientation marking is located on pin 3, rather than pin 1. ► Fig.2: as well as the red, green ► and blue LED dies, the WS2812B incorporates a controller/driver IC, which includes a serial latch plus three linear LED drivers with 8-bit DACs. 34 Practical Electronics | January | 2021 Fig.3: cascading multiple WS2812B devices is simple. The DOUT (data out) pin of one device is simply connected to the DIN (data in) pin of the next device. The 5V and GND pins are all connected in parallel, with a 100nF bypass capacitor close to each device. make up the module. This is simplified by showing just three of the 64 devices. The data stream from the MCU is fed into pin 4 (DIN) of the first device, while the output from pin 2 (DOUT) is connected to pin 4 of the next device, and so on. One of the slightly unusual features of this chip is that unlike other daisychained shift registers, it doesn’t feed the top-most ‘overflow’ bit of the shift register to the output, for feeding into the next device. Rather, the output is held in a static state until all 24 bits have been shifted into the register (presumably, tracked via a counter register), at which point it no longer shifts in any new bits. The input is then connected to the output buffer via an internal switch. This means that the first 24 bits of data shifted into the daisy chain determine the state of the first device. With the more typical (and simpler) shift-through design, the first bits of data end up in the last device – ie, you have to shift in the data in reverse order. So, presumably the reason for this unusual scheme is to avoid the need to reverse the order of data being sent to an array of these devices. The only other components are the 100nF bypass capacitors on the +5V supply line, with one next to each device. The 1000µF reservoir capacitor is external to the module. The physical layout of the 64-LED array, which measures 65 × 65mm, is shown in Fig.4. The input connections for the module are at lower left, while the output connections are at upper right. Each WS2812B device can draw up to 18mA from the +5V supply during operation, so a single 64-LED module can draw as much as 1.152A. That’s why it’s recommended that even using a single module, the +5V supply for the module should not come from your MCU (eg, Arduino or Micromite), but from a separate DC supply. It’s even more important to do this when you’re using several modules in cascade. This is also why that 1000µF capacitor is needed on the +5V supply line. Driving the module The LEDs in these modules are programmed serially via a single wire, as mentioned earlier. But they use a special pulse-width modulation (PWM) coding system for the data, shown in Fig.5. The timing for a zero bit, a one bit and the RESET/LATCH pulse for a basic WS2812B device are shown at the top of Fig.5; this is used in most of the currently-available 8×8 modules. The corresponding timings for the latest WS2812B-V4 version of the device are shown adjacent. There are subtle differences in data bit timing between the two versions. The main difference is that the WS2812B needs a RESET/LATCH pulse lasting more than 50µs, while the WS2812B-V4 needs a longer pulse of more than 280µs. Timing for the SK6812 device is similar to that for the WS2812B, with a zero bit composed of a 300ns high followed by a 900ns low, a one bit composed of a 600ns high followed by a 600ns low, and the RESET/LATCH pulse needing to be 80µs or more. The centre section of Fig.5 shows the 24-bit data packet used to program a single WS2812B LED. There are eight bits for each of the three colours, with each colour’s data byte sent MSB (mostsignificant-bit) first. So the total time needed to refresh one LED is either 30µs or 26.4µs, depending on the version of the WS2812B chip. Fig.5 also shows the colour data sent in GRB (green-red-blue) order, but some of the WS2812B or equivalent devices used in these modules require the data to be sent in RGB order. As a result, much of the software written for these modules allow the colour byte order to be changed to suit the specific devices being used. The 64-LED data stream used to program all of the WS2812B LEDs in a single 8×8 module is shown at the bottom of Fig.5. As you can see, the 24 bits of data for each of the 64 LEDs are sent in turn, followed by a RESET/LATCH pulse. This pulse instructs all of the WS2812Bs to transfer the data in their shift register into the latch register, changing the colour and brightness of its LEDs to the new values. So one complete refresh cycle for an 8×8 module takes very close to 1970µs Fig.4: this shows the layout of the 8×8 RGB LED matrix. As you would expect, the LEDs are laid out in a grid. The data input is at lower left and data output at upper right (along with the supply pins), so that multiple modules can be daisy-chained. It’s a pity that the output isn’t at lower right, as that would make chaining modules considerably easier. Practical Electronics | January | 2021 35 While the underside of this module uses headers for external connections, some modules provide SMD pads rather than holes. It can be worthwhile to shop around, but there is a risk that you may come across clones which are not fully compatible. rainbow pattern, sending a ‘3’ produces a display of all LEDs glowing mid-green, sending a ‘6’ produces a pattern of white dots ‘chasing’ each other, and so on. While this may not sound terribly exciting, it should give you a good idea of what’s involved in driving these modules from an Arduino. (1.970ms) or 1969.6µs (1.969ms), depending on which version of the WS2812B is being used. As a result, the display can be refreshed up to 500 times each second (or a fraction of this with multiple modules, eg, 100 times per second for five modules daisy-chained). Driving it from an Arduino Thanks to the single-wire data programming system used by the WS2812B device, it’s physically quite easy to drive this module from an Arduino. As shown in Fig.6, all that’s needed is a wire connecting the module’s GND pin to one of the Arduino GND pins, together with a wire with a 390Ω series resistor connecting the module’s DIN pin to one of the Arduino’s digital I/O pins. Wires from the module’s +5V and GND pins are then used to supply it with 5V power, with a 1000µF capacitor used as a reservoir to ensure that the 5V power remains constant. Writing the required Arduino ‘sketch’ (program) is a little complicated due to the unusual PWM coding system used. Luckily, several Arduino software libraries have been written to drive a string of WS2812B/SK6812 devices. You’ll find suitable programs in various places on the Web, most of them fairly simple and straightforward. Many of them make use of a library of routines for the Arduino written by the Adafruit people and called Adafruit_NeoPixel. To get you started, I’ve written a sketch called RGBLED_Matrix_sketch.ino, available for download from the January 2021 page of the PE website. It uses the Adafruit_NeoPixel library, which can be downloaded from https://github.com/ adafruit/Adafruit_NeoPixel (or via the Arduino IDE’s Library Manager). This sketch allows you to produce one of nine different patterns on the module, simply by sending a digit (from 1 to 9) to the Arduino from your PC’s serial port (eg, via the IDE’s Serial Monitor). For example, sending a ‘1’ produces a changing 36 Driving it from a Micromite Driving one of the modules from a Micromite again isn’t easy, mainly because of the PWM bit encoding scheme. After trying to make unorthodox use of MMBasic’s built-in SPI communications protocol (with no luck), I realised that I would need an embedded C function similar to Geoff Graham’s SerialTX module. CFUNCTIONs allow native ‘machine language’ code to be added to an MMBasic program. This would let me send the serial data streams to the LED module with the right encoding and at the right speed. This 8×8 RGB LED module uses WS2812B ICs. The data and power connections are made via two 3-pin male headers on the PCB’s underside. I was rather daunted at the prospect of writing this CFUNCTION. But Geoff Graham advised me that a suitable function had already been created by Peter Mather, one of the Micromite ‘gurus’ on The BackShed Forum (http://bit.ly/ pe-jan21-shed). I eagerly downloaded Mr Mather’s CFUNCTION, and tried using it with a small MMBasic program to drive a module with 64 WS2812B LEDs. The results were a bit disappointing, with a variety of unexpected errors. This prompted me to try using my DSO to check the pulse timing of the bitstream being sent to the WS2812B LEDs, to compare it to the required timing shown in Fig.5. I subsequently found a few differences, which seemed likely to explain the problems I was having. Fig.5: the WS2812B uses a custom 1-wire serial protocol, with the duration of the positive pulse distinguishing between a zero and one bit. Unfortunately, different versions of the chip require different timings, although it is possible to choose timings which will suit all versions. Note the much longer latch pulse required for the V4 chips. Also, while many chips expect colour data in the green, red, blue order shown here, some use the more standard red, green, blue order. Practical Electronics | January | 2021 Fig.6: it’s effortless to hook up an Arduino module to one of these LED arrays. You just need to connect the grounds together, plus connect a 390Ω resistor from any of the Arduino I/O pins to the DIN pin of the module. As mentioned in the text, due to the LED current demands, a separate >1A 5V DC supply is needed to power the module(s). After an exchange of emails with Mr Mather, I learned that his CFUNCTION had been written about four years ago to suit the original WS2812 LEDs. He suggested a couple of changes to it to make the pulse timing more compatible with the WS2812B, SK6812 and WS2812B-V4 devices, and also guided me regarding how to make the changes easily without having to recompile his code. I made the suggested changes and tried it all again. Now the timing of the pulse stream was much closer to that needed by the WS2812B/SK6812 devices, and, lo and behold, the modules gave the correct displays from my test program. I then proceeded to write an expanded version of my original MMBasic test program to provide readers with a suitable demo program to run on a Micromite. This program is called RGB LED matrix test program.bas, and again, you can download it from: the January 2021 page of the PE website. This program displays a ‘rainbow’ of coloured stripes on the 64-LED SW2812B/SK6812 module. It then clears the module’s display for another five seconds before repeating the cycle. While simple, again I hope it will give you a good idea as to how a Micromite can be used to drive these LED modules. To achieve different kinds of display (including dynamic displays), all you need to do is use the MMBasic part of the program to change the ‘pixel’ data stored in the colours() array. You can find some useful information on this module in the following links: http://bit.ly/pe-jan21-8x8a http://bit.ly/pe-jan21-8x8b http://bit.ly/pe-jan21-8x8c Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au Fig.7: driving a ‘neopixel’ LED array from a Micromite is nearly identical to an Arduino: the two grounds connected together, and a 390Ω resistor (or just a direct connection) from one of the Micromite’s I/O pins to the LED array DIN pin. The software is a bit more complicated, but if you start with our sample code, it should work straight away. Practical Electronics | January | 2021 37 KickStart b y M i k e To o l e y Part 1: MOSFET switching devices in linear applications – introducing the 2N7000 ‘Swiss Army Knife’ Our occasional KickStart series aims to show readers how to use readily available low-cost components and devices to solve a wide range of common problems in the shortest possible time. Each of the examples and projects can be completed in no more than a couple of hours using M OSFET devices are available in various forms, including N- t y p e an d P -typ e, an d enhancement or depletion mode types. The 2N7000 is an N-type MOSFET designed for enhancement mode operation (see Fig.1.1). This simply means that to turn the device ‘on’ (ie, to make it conduct) it is necessary to apply a positive voltage between the gate and source of the device. For the 2N7000, the required gate-source voltage is in the range of 2V to 3V. The device will conduct very heavily when the gate-source voltage exceeds about 3V, in which case the resistance between drain and source (RDS) will fall to the very low value required for switching applications. However, with gate-source voltages (VGS) of less than 2.5V the device can be used in a wide variety of linear applications. This is where this incredibly versatile ‘Swiss Army Knife’ finds a whole new variety of applications! Fig.1.2. 2N7000 pin connections. Fig.1.1. Construction of an N-channel MOSFET device. 38 off-the-shelf parts. As well as briefly explaining the underlying principles and technology used, the series will provide you with a variety of representative solutions and examples, along with just enough information to be able to adapt and extend them for their own use. This first part shows you how to use a lowcost MOSFET switching device in a variety of linear applications. In keeping with the KickStart philosophy, we’ve provided sufficient information for you to be able to design and build your own circuits using this handy semiconductor device. MOSFET basics The construction of a typical N-channel MOSFET device is shown in Fig.1.1. The device consists of a series of semiconductor layers onto which an insulating metal-oxide layer is deposited. Conduction takes place between source and drain in a narrow N-type channel. The degree of conduction in this region is dependent on the positive potential Fig.1.3. N-channel MOSFET test circuit. present on the gate terminal. As the gate-source voltage (VGS) is raised beyond the threshold for conduction (usually above about 2V for the 2N7000) conduction increases and the drain and source currents (which are identical) increase as a result. Thus, the voltage present between the gate and source controls the current flowing in the drain. In manufacturers’ data sheets, device properties are usually summarised both in the form of tabulated data and as a series of characteristic Fig.1.4. Output (drain) characteristics for a 2N7000 operating under small-signal conditions. curves. Key data for the 2N7000 is shown in Table 1.1, and series of plots showing the variation the two most important characteristic of drain current (ID) with drain-source curves are shown Fig.1.4 and Fig.1.5. voltage (VDS) for different values of The test circuit that we used to obtain gate-source voltage (VGS). There are a these plots is shown in Fig.1.3. Note few things to note about these curves. that the drain current (ID) needs to be First, we have plotted these curves using relatively small values of drain current kept to a safe value in order to limit that would be used in linear (rather than the total power dissipation of 350mW switching) applications. Second, note quoted in Table 1.1. the bend that occurs for values of VDS The output characteristics (shown in Fig.1.4) consist of a below about 2V. . Practical Electronics | January | 2021 Table 1.1: Key data for the 2N7000 Specification Abbrev. Value Maximum drain-source voltage VDS(max) 60V Maximum drain-gate voltage VDG(max) 60V Maximum gate-source voltage VGS(max) ±20V Maximum drain current ID(max) 200mA Maximum total power dissipation PD(max) 350mW Maximum gate-source threshold voltage VGS(th.max) 0.8V Minimum gate-source threshold voltage VGS(th.min) 3V Minimum forward transconductance gfs 0.1mS Maximum input capacitance Ciss 60pF The transfer characteristics (shown in Fig.1.5) provide us with another family of plots. They show how the drain current (ID) varies with gate-source voltage (VGS) for different values of drain-source voltage (VDS). The three curves shown correspond to VDS values of 3V, 6V and 11V. As can be seen, they are all very similar. It is worth noting that we have plotted these curves using relatively small values of drain current (ID) and, as with the output characteristic curves shown in Fig.1.4, the values of drain current are very much less than those shown in the data sheets published by semiconductor manufacturers. The slope of the transfer characteristic is of particular significance. This is known as the forward transfer conductance (gfs) and is defined as a small change in drain current ( ID) divided by the corresponding small change in gatesource voltage, ( VGS) at a given value of drain-source voltage (VDS), thus: The value of gfs is quoted using the unit of conductance (siemen (S), or even ‘mho’ (which is simply ‘ohm’ backwards!) by some manufacturers). However, since the values are usually fairly small, we use mS (milli-siemen) or mmho instead. Putting this into the context of Fig.1.4, a change in drain current from 6mA to 10.9mA will be produced by a change in gate-source voltage from ΔID 2.3V gfs =to 2.4V.SHence, at this point on the GS transferΔV characteristic when VDS = 6V, we can determine the forward transfer conductance from: gfs = ΔID S ΔVGS gfs = (11 - 6.0 ) mA = 5mA = 50 mS ( 2.4 - 2.3) V 0.1V gfs = (11 - 6.0 ) mA = 5mA = 50 mS ( 2.4 - 2.3) V 0.1V Using MOSFET devices in linear applications As previously mentioned, Fig.1.6 shows the basic components that are used in a simple common-source MOSFET amplifier stage. In order to use a 2N7000 in linear mode it is necessary to provide a gatesource bias voltage of around 2V. The gate bias (Vbias) can be applied to the Fig.1.5 Transfer characteristics for a 2N7000 operating under small-signal conditions. Practical Electronics | January | 2021 Fig.1.6. Bias and load arrangements for a simple common-source MOSFET amplifier stage. gate via a high-value resistor, RB. The value of RB is not critical but is usually in the range 100kΩ to 1MΩ. The output voltage is developed across a load resistor (RL) of suitable value connected in the drain circuit. Capacitors CIN and COUT (respectively) are used to couple the AC signal into and out of the stage. Load lines To understand how the load resistor works it is worth taking a look at Fig.1.7 which shows how a load line can be superimposed on the output characteristics that we met earlier. This particular load line corresponds to a value of 500Ω for RL. The two ends of the load line correspond to the extreme values that would occur when TR1 is either fully conducting (on) or non-conducting (off). The slope of the load line is thus the inverse of RL. The operating point is the point at which corresponding values of VDS and ID will occur when no signal is applied (this is sometimes referred to as the ‘quiescent condition’). When a signal is applied to the stage, VGS will change and the Fig.1.7. Load line superimposed on the 2N7000 output characteristics. 39 Fig.1.8. A basic amplifier using the 2N7000. Fig.1.10. An improved ‘gain block’. half the supply voltage (ie, around 4.5V neglecting the small voltage dropped across R4). Typical operating voltages are shown on Fig.1.8, but in practice, and due to variations in the characteristics of individual components and MOSFET devicFig.1.9. An amplifier stage with a gain of about 40. es, variations of around ±10% can be expected. If significant differences are operating point will move up and down encountered, then the values of R1 and/ the load line. It is then possible to read or R2 can be changed accordingly. off corresponding values of VDS and use The voltage gain of the stage is them to infer the shape of the output approximately 14. This means that a voltage waveform, as shown in Fig.1.7. signal input of 100mVpk-pk will result With the conditions indicated on Fig.1.7, if a 300mV pk-pk signal is in an output of 1.4Vpk-pk. If more gain superimposed on a standing bias voltage is required then a capacitor can be of 2.35V, the value of drain-source voltage introduced in parallel with R4, as shown (VDS) will swing down to a minimum of in Fig.1.9. This capacitor will bypass the signal voltage component that appears 3.2V and up to a maximum of 8.2V. Hence across R4, reducing the amount of negative an input of 350mVpk-pk will produce an feedback and increasing the voltage gain output of 5Vpk-pk corresponding to a as a result. With R4 bypassed to signals, modest voltage gain (AV) of a little over 14. The required bias voltage (2.35V in the case of Fig.1.7) can be obtained in various ways, but one of the most basic arrangements is nothing more than a resistive potential divider derived either from the DC supply rail or from the drain connection. the circuit shown in Fig.1.9 provides a voltage gain of around 40 so that an input of 100mVpk-pk will produce an output of 4Vpk-pk, and so on. The input resistance of both circuits is approximately 500kΩ (determined largely by R1 and R2) and the measured frequency response extends from less than 10Hz to well over 200kHz at the –3dB points. Negative feedback and bias stabilisation The circuit of Fig.1.8 employs two forms of negative feedback: shunt voltage feedback from drain to gate via R3 and series current feedback using R4 in the connection from source to ground. These two feedback loops help to stabilise the DC operating conditions, ensuring that the operating conditions remain within the desired range despite variations in MOSFET parameters and temperature. They also make a significant improvement in the overall linearity of amplifier. Operation of the shunt feedback loop is as follows. If the gate-source voltage increases the drain current will increase as a direct consequence. The increase in drain current will result in an increased potential drop across the load resistance A basic 2N7000 amplifier A basic common-source amplifier is shown in Fig.1.8. The load for the stage is provided by R3, while the gate-source bias voltage (which needs to be approximately 2.2V for optimum operating conditions in this circuit) is defined by the potential divider formed by R1 and R2. Optimum operating conditions are those that will give the maximum undistorted output voltage swing. This condition occurs when the drain voltage is about 40 Fig.1.11. Using Virtins Multi-Instrument PC-based software to analyse and measure the total harmonic distortion (THD) produced by the circuit of Fig.1.10. Practical Electronics | January | 2021 on the drain load of the first stage (TR1) and produces a nominal voltage gain of 10. The measured frequency response extends from around 4.5Hz to 450kHz. Notice how negative feedback (via R1) is applied over both stages (ie, from the source of the second stage to the gate of the first stage). Distortion Fig.1.12. Using Virtins Multi-Instrument PC-based software to analyse and measure the total harmonic distortion (THD) produced by the circuit of Fig.1.10. No amplifier is perfectly linear, and no amplifier can provide a perfect representation of its input. The result of non-linearity is distortion and, while a small level of distortion may be undetectable by ear, designers usually go to great lengths to reduce the level of distortion introduced by an amplifier. It is important to note that, as an amplifier is driven harder, the distortion that it produces will increase. The circuit shown in Fig.1.10 Fig.1.13. A twin-tee oscillator based on the two-stage gain block in Fig.1.10. Fig.1.15. 8MHz crystal oscillator using a 2N7000. (RL). As a result, the drain voltage will fall, which, in turn, reduces the gate bias voltage. Capacitors, C1 and C2 respectively, are used to couple signals into and out of the stage. Put simply, these capacitors pass AC signals but keep the DC gate bias and drain voltages safely within the amplifier stage. An improved ‘gain block’ An improved two-stage amplifier is shown in Fig.1.10. The first stage acts as a common-source amplifier with a high input impedance; the second as a sourcefollower with a low output impedance. The two-stage ‘gain block’ arrangement reduces the effect of output loading typically produces around 0.25% total harmonic distortion (THD) when a signal of 100mVpk-pk is applied, as shown in Fig.1.11. (Note that the THD increases to around 6% for the circuit of Fig.1.9 with the same signal.) It is therefore important to avoid overdriving an amplifier, particularly where the internal gain is appreciable. A high-gain amplifier A high-gain amplifier is shown in Fig.1.12. This arrangement uses two cascaded common-source amplifier stages. This circuit produces a typical voltage gain of 250 with C3 not fitted, increasing to around 370 with C3 fitted (as shown in Fig.1.12). Note that signal levels must be kept low to avoid overdriving the second stage. Sinusoidal oscillators Fig.1.14. Using Tina Pro to check and optimise component values for the twin-tee oscillator shown in Fig.1.13. Practical Electronics | January | 2021 As well as its use in small-signal amplifiers, the 2N7000 can be used in a wide variety of sinusoidal oscillator applications. Fig.1.13 shows a simple twin-tee oscillator based on the twostage gain block in Fig.1.10. The circuit provides an output of around 8Vpk-pk at approximately 100Hz. The frequency of 41 Table 1.2: Going Further with the 2N7000 MOSFET Topic operation can be altered by varying the values used in the twin-tee network (see Going further below). Note that popular Spice-based simulation software can often be used to model and optimise the operation of MOSFET circuits, as shown in Fig.1.14. Finally, Fig.1.15 shows a crystalcontrolled oscillator based on a single 2N7000 device. This provides a typical output of 2Vpk-pk and has been tested with quartz crystals between 2MHz and 20MHz. This arrangement forms the basis of the author’s own simple Crystal Checker circuit, which he uses in his workshop. Going further Our Going further table (opposite) will help you locate the component parts and further information that will allow you to quickly progress with your own designs and modifications. It also provides you with background reading that will help you get up to speed with the necessary underpinning knowledge for key topics discussed. GET T LATES HE T COP Y OF TEACH OUR -IN SE RIES AVAIL AB NOW! LE Source Notes 2N7000 MOSFET The 2N7000 is available from many suppliers, including CPC/Farnell, Mouser, RS Components and numerous online suppliers. Data sheets can be downloaded from manufacturers’ websites including ON Semiconductor, Fairchild and Vishay Prices range from a few pence to about 50p depending on the quantity purchased Audio amplifiers For all your audio amplifier requirements, the PE column Audio Out provides a treasure chest of tips, hints, designs and ideas, drawing on Jake Rothman’s decades of experience in teaching, designing and building a huge range of audio circuits. All Audio Out columns are available via back issues at: www.electronpublishing. com Circuit simulation The author’s own book, Electronic Circuits: Fundamentals and Applications (Fifth Edition 2020 published by Routledge 9780367421984) provides an introduction to circuit simulation based on the popular SPICE-based Tina Pro software Tina software can be obtained from www.tina. com. A cut-down version can be freely downloaded from Texas Instruments at: www.ti.com/tool/TINA-TI Distortion A detailed explanation of different types of distortion can be found in Part 8 of Electronics Teach-In 7 www.electronpublishing. com/product/electronicsteach-in-7 Negative feedback For a useful introduction and relevant theory see Part 8 of Electronics Teach-In 7 www.electronpublishing. com/product/electronicsteach-in-7 Transistor characteristics and load lines The author’s own book, Electronic Circuits: Fundamentals and Applications (see above) provides an introduction to transistor characteristics, load lines and amplifiers. Order direct from Electron Publishing PRICE £8.99 (includes P&P to UK if ordered direct from us) EE FR -ROM CD ELECTRONICS TEACH-IN 9 £8.99 FROM THE PUBLISHERS OF GET TESTING! Electronic test equipment and measuring techniques, plus eight projects to build FREE CD-ROM TWO TEACH -INs FOR THE PRICE OF ONE • Multimeters and a multimeter checker • Oscilloscopes plus a scope calibrator • AC Millivoltmeters with a range extender • Digital measurements plus a logic probe • Frequency measurements and a signal generator • Component measurements plus a semiconductor junction tester PIC n’ Mix Including Practical Digital Signal Processing PLUS... YOUR GUIDE TO THE BBC MICROBIT Teach-In 9 Teach-In 9 – Get Testing! A LOW-COST ARM-BASED SINGLE-BOARD COMPUTER Get Testing Three Microchip PICkit 4 Debugger Guides Files for: PIC n’ Mix PLUS Teach-In 2 -Using PIC Microcontrollers. In PDF format This series of articles provides a broad-based introduction to choosing and using a wide range of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints and tips on using, and – just as importantly – interpreting the results that you get. The series deals with familiar test gear as well as equipment designed for more specialised applications. The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics. The series crosses the boundaries of analogue and digital electronics with applications that span the full range of electronics – from a single-stage transistor amplifier to the most sophisticated microcontroller system. There really is something for everyone! Each part includes a simple but useful practical test gear project that will build into a handy gadget that will either extend the features, ranges and usability of an existing item of test equipment or that will serve as a stand-alone instrument. We’ve kept the cost of these projects as low as possible, and most of them can be built for less than £10 (including components, enclosure and circuit board). © 2018 Wimborne Publishing Ltd. www.epemag.com Teach In 9 Cover.indd 1 01/08/2018 19:56 FREE COVER-MOUNTED CD-ROM On the free cover-mounted CD-ROM you will find the software for the PIC n’ Mix series of articles. Plus the full Teach-In 2 book – Using PIC Microcontrollers – A practical introduction – in PDF format. Also included are Microchip’s MPLAB ICD 4 In-Circuit Debugger User’s Guide; MPLAB PICkit 4 In-Circuit Debugger Quick Start Guide; and MPLAB PICkit4 Debugger User’s Guide. ORDER YOUR COPY TODAY JUST CALL 01202 880299 OR VISIT www.electronpublishing.com 42 Practical Electronics | January | 2021 PIC n’Mix Mike Hibbett’s column for PIC project enlightenment and related topics Part 3: PIC18F development board W e continue this month with  Flexible – the ability to select which Simplifying soldering the process of creating a development board for the PIC18F processor. In this and the following article we are going to cover the design of the board, ensuring it supports a number of different externally connected technologies: GPS location, digital compass, various display types, speech output, connectivity to a PC connection via USB, Wi-Fi communication, interfacing with Amazon Alexa, servomotor control and analogue input. The focus for this article series will be on making use of the processor, rather than diving into the finer details of internal processor peripherals – a hands-on, practical approach. In the previous article we selected the processor we will be using – the PIC18F47K42 in a 40-pin DIL package. We chose that package because it provides lots of I/O signals yet is very easy to solder (via a 40-pin socket) making it easy to replace if damaged. There is a lot of work to do before we jump in and start designing a PCB. Even before drawing the schematic, we need to be clear in our objectives for this board, writing down some requirements, and check if any of our items might clash with each other and (hopefully) find solutions to those issues. Let’s start by writing down our key requirements:  Easy to assemble – no tiny surfacemount soldered parts. We want this to be easily assembled, by any hobbyist features are fitted at any one time, without major re-work  Re-usable – easy to re-purpose to different projects  PC serial communications interface using an on-board UART-to-USB converter chip, the MCP2221A-I/P  Header connector for an ESP-01 Wi-Fi interface  Header for a Micro SD Media card module  Several three-pin headers for servomotors  Header for a colour touchscreen LCD  FET power switches for external device control  PICkit 4 header for programming/ debugging  Header pins for external devices with power and I2C or SPI buses  Two configurable op amps, for analogue input signal conditioning  32kHz crystal for very low-current operation  Plenty of analogue input and digital I/O headers  Single status LED, and a power LED  Power input from a standard DC power brick. The first requirement is driven by a desire to enable this board to be built by as many hobbyists as possible, avoiding the use of difficult-to-solder surface-mount parts. This is easy to achieve with most components, but the USB and SD Media connectors are only available in hard-to-solder formats. Thankfully, this has been recognised by a number of electronics suppliers who provide very simple and low-cost preassembled carrier boards, as shown in Fig.1 and Fig.2. We will also use a Wi-Fi module with a similar simple header interface (shown in Fig.3). These little boards enables us to avoid the difficulty of soldering ICs that are only available in surface-mount format. Another benefit of providing these as plug-in features is that they are optional – buy them only if you want to use them. On our PCB, USB connectivity to a PC is provided by a special IC, the Microchip MCP2221A-I/P. Our processor does not have USB built in, and the choice to use an external IC rather than choose a processor with a USB peripheral is deliberate – in our experience the USB software libraries provided by Microchip (and other vendors) are very complicated to use. The MCP2221A provides a simple USB-to-UART conversion, so our processor we will connect the IC to one of our serial ports, and the USB cable will look to the PC like a standard serial port. This will make software development Fig.1. USB connector board. Fig.2. SD-Media connector board. Practical Electronics | January | 2021 That’s quite a list, but many of these will be easy to implement, and simply remind us to include certain components in the schematic. Many requirements have a consequence – adding a 32kHz crystal, for example, means that two GPIO pins become unavailable. Fortunately, however, there are many GPIO pins, so that won’t be an issue for us. Fig.3. Wi-Fi interface board. 43 comes pre-loaded with software that is easy to use and communicates with our processor over one of the UART interfaces (we have two UARTs on our processor, so simultaneous Wi-Fi and USB connectivity will be possible.) Power supply design Fig.4. Plug-in option boards. on both the PC and the processor simple. Plus, the MCP2221A can run at 3.3V or 5V, so it’s very flexible. An SD Media card communicates over an SPI interface, so we will route one of the SPI buses to it. SD Media cards operate at a voltage of 3.3V maximum, so bear that in mind if you are thinking of running your board at 5V. An external adaptor to level-shift the signals will be required if you want to use an SD Media card for data storage and run the board at 5V simultaneously. This will be true for the Wi-Fi interface too. The downside of using standard components in a design like this is that the PCB will larger than the more usual Microchip development boards, which use tiny surface-mount components. This is not really a big issue – the board is for development purposes, so easy soldering and easy access to components is more important than size. Plug-in options Although this will be a general-purpose development board, we do have in mind some external devices that we would like to be able to connect to it. Fig.4 shows the current selection from our lab – An LED matrix, touch-input colour LCD screen, GPS module and a bag of over 30 different random sensors that were purchased cheaply on eBay. We will provide links for the parts used in upcoming articles, as we make use of them. Needless to say, these will just be examples, you are free to attach whatever you want to your board. Wi-Fi interface Choosing a Wi-Fi interface board presented some interesting challenges. The board has to be cheap, easy to purchase, easy to solder and easy to use. We settled on a relatively old device, the ESP-01, which is based on the ESP8266 processor. This is such a popular design that it has become available from dozens of different suppliers and is incredibly cheap – as low as £3 including delivery, if you do not mind the long delivery times from Asia. The module 44 With those thoughts behind us, it’s time to make our first key design decisions – how will we power the board, and how will we direct the processor’s 36 I/O pins to onboard and external connections? We’ll start with the power supply design. So, first, we must decide at what voltage to run the board: 3.3V or 5V? These two common operating voltages have for years been a bone of contention for us, too often we have tried to pair a 5V external device with a 3.3V processor development board (servomotors with a Raspberry Pi for example) or a 3.3V module with a 5V processor board (a GPS module with an Arduino, for example.) It’s been such an issue over the years that we have decided to design a board that can operate at either voltage, selected by a simple jumper. The key ICs in our design have been selected as ones that can operate at either voltage, so we only need a power supply design that can be switched between the two voltages – and we can do that with an LM317 regulator and a few resistors. We will use a beefy TO220-style regulator, as powering some external devices, particularly servo motors, may bring our current-consumption requirements up to around 500mA or so. Plus, we make a mental note: leave space around the regulator for heatsinking, the LM317 is a linear regulator, and will get warm at these higher currents. We could have gone with a much more efficient switching-regulator design, but that would add complexity. We will typically be powering this board from a wall socket power supply, so power supply efficiency is not a design requirement. Studying the datasheet of the LM317 we spot that is has a requirement for a minimum 3V input to output voltage differential, so to generate a 3.3V or 5V output will require an input power supply running at a minimum of 8V DC. 9V and 12V DC power supplies are common and cheap, so the regulator choice works well. We will include a power supply isolation header pin, so 3.3V or 5V can be supplied to the board directly, if desired. This will enable the testing of low-power designs, or allow the addition of a more efficient power supply should you want to use a battery. A barrel jack will be accommodated to support plugging in a standard DC power brick. There are several different standards for barrel jack connectors (different pin diameters) so we have to make a decision on which to go with – we chose the 2.5mm P I C 18 F 4 7 K 4 2 MC LR / R E3 1 4 0 R B 7 R A 0 2 39 R B 6 R A 1 3 38 R B 5 R A 2 4 37 R B 4 R A 3 5 36 R B 3 R A 4 6 35 R B 2 R A 5 7 34 R B 1 R E0 8 33 R B 0 R E1 9 32 VDD R E2 10 31 VSS VDD 11 30 R D7 VSS 12 29 R D6 R A 7 13 28 R D5 R A 6 14 27 R D4 R C 0 25 26 R C 7 R C 1 16 25 R C 6 R C 2 17 24 R C 5 R C 3 18 23 R C 4 R D0 19 22 R D3 R D1 20 21 R D2 Fig.5. PIC18F47K42 processor pin-out. version, as that seemed to be the most common size in our lab. Header pins and solder pads will also be provided for power input, as these cost nothing but add flexibility to the choice of power connection. External header pins We have left the most complex design decision until last. The processor, shown in Fig.5, has 36 I/O pins – how will we connect these? Thankfully, the pins are highly configurable; internal peripherals such as SPI, I2C and UART interfaces are all able to be programmed to appear on any pin from a selection of the 36 pins available. While this simplifies circuit design and PCB layout, it does add complexity during software design. With a general-purpose development board design, where we make all pins available, this task is very complicated – but that task can be simplified by writing up a spreadsheet for the pins, indicating where each one will go. The approach taken here is that multiple headers are provided for generic SPI and I2C devices, several servomotor headers (using the standard header layout) and specific headers for the Wi-Fi, USB and Micro SD Media boards. Two FET-controlled power output headers are provided, and then all I/O pins will go to 0.1-inch pitch header strips. Even the I/O signals that are optionally connected to other headers on the board will be routed to the generic I/O headers. Three I/O pins will not be used. PORTE.3 is shared with the master reset pin MCLR, which we will keep as a reset pin. PORTC.0 and PORTC.1 are multiplexed with the external crystal inputs, which will be taken up with our 32kHz crystal, for ultra-low power operation. To finish off, two op amps will be implemented on the board, but left unconnected, with headers to allow user choice of where they connect. Practical Electronics | January | 2021 GP I O 6 ( UA R T 2) GP I O 7 ( UA R T 2) W i- F i GP I O 4 ( UA R T 2) US B GP I O 18 ( P W M) O p am p S ervo GP I O 5 ( UA R T 2) GP I O 8 GP I O 9 O p am p GP I O 10 GP I O 11 GP I O 19 ( P W M) I 2C S ervo GP I O 29 ( I 2C 1) P ower input GP I O 30 ( I 2C 1) GP I O 12 ( S P I 1) GP I O 20 ( P W M) S ervo GP I O 13 ( S P I 1) Micro- S D Media card GP I O 12 ( S P I 1) GP I O 14 GP I O 15 I 2C A dditional power headers GP I O 29 ( I 2C 1) GP I O 30 ( I 2C 1) GP I O 1 GP I O 16 GP I O 2 GP I O 12 ( S P I 1) F ET F ET GP I O 3 S tatus LED GP I O 21 P O R T C 0 GP I O 22 P O R T C 1 32kH z crystal GP I O 12 ( S P I 1) S P I GP I O 13 ( S P I 1) GP I O 14 ( S P I 1) GP I O 13 ( S P I 1) GP I O 14 ( S P I 1) LC D MC LR GP I O 23 GP I O 24 GP I O 25 ( A DC ) GP I O 26 ( A DC ) S P I GP I O 12 ( S P I 1) P O R T B 7 GP I O 13 ( S P I 1) P O R T B 6 Deb ug GP I O 14 ( S P I 1) GP I O 27 ( A DC ) GP I O 28 ( A DC ) Fig.6. High-level schematic, focusing on the I/O signals. A standard PICkit 4 8-pin header will be included for programming and debugging. Working with the above specification, a high-level circuit design focusing on the I/O signals can be drawn, shown in Fig.6, which shows all the required signals, without specifically naming them. From this diagram we can now work through mapping processor pins to the diagram. This is a bit tedious and is best done by writing down a simple spreadsheet listing the pin numbers and their use, confirming for each pin that a chosen peripheral can be mapped to the desired pin. The resulting pin mapping is shown in Fig.7. High-level design – done! Having worked through those requirements, and followed the thought process discussed above, we have arrived at a clear high-level design. At this stage, we have no further questions to answer – the schematic can be drawn up, and a board layout created, with the risk of unexpected issues cropping up significantly reduced. Finishing touches Before getting to the PCB layout we note a few addition requirements that we want to capture. Four holes in the PCB corners will be provided for the mounting of simple feet. 3.4mm-diameter holes will be used to allow the use of cheap M3 bolts to be used, but there will be space for off-theshelf rubber feet. Space will be left around the LM317 voltage regulator to allow for a small heatsink, should your attachments require large currents. The silk screen of the PCB will provide clear pin labels for each header pin. This is an important requirement and will increase the size of the PCB slightly as space beside the header pins will be needed to allow the text to be visible. Although it will not impact the PCB design, it’s worth noting that all ICs (the processor, the USB interface IC and the op amp) will be fitting using sockets. Although this adds a few pounds to the cost, it will make the board easier to repair should accidents happen. You are of course free to ignore this advice! That’s all the hard work completed – now to the PCB design itself, which we will pick up in the next article. References Micro SD-card holder www.pololu.com/product/2597 Wi-Fi interface www.addicore.com/ESP8266-ESP01-p/130.htm USB Micro-B connector www.pololu.com/product/2586 Fig.7. I/O pin mapping. Practical Electronics | January | 2021 LED Display www.ebay.co.uk/itm/191736670249 45 AUDIO OUT AUDIO OUT L R By Jake Rothman Theremin Audio Amplifier – Part 3 L ast month, we completed a silicon-transistor-based amplifier fo the PE Theremin, but I concluded the article with a promise to look at a germanium-transistor version. This actually turned into quite a fascinating project, as you will see. Lost values When I get a break from eyeball-stressing SMT (surface-mount technology) industrial jobs, it’s a relief to get back to some old-fashioned electronics. I recently got really fed up with unmarked SMT components. It started with capacitors, but now even resistors are being supplied unmarked, as shown in Fig.1. This makes fault finding on pre-production prototypes very difficult. When the unit fails a few years later it can’t be fixed without full service data, and who supplies that today? delivery of components rendered obsolete by RoHS directives, my childhood electronics excitement came back. Behold! – there were unopened packs of 1974 Siemens germanium transistors. There was also a bundle of Siemens books, including one called Design Examples of Semiconductor Circuits, from 1969. The section titled, ‘Audio-frequency Amplifiers’ on p.7 compared the use of germanium output transistors with the then relatively new silicon planar transistors. It said the germanium types were superior for low voltage, which is to be expected, since their turn-on voltage is around 0.1V as opposed to silicon’s 0.6V. What intrigued me, however, was the statement that they had better linearity, which should mean lower distortion. The amplifier designer and Bradford University academic Arthur Bailey also said the same in his Wireless World article, ‘High Performance Transistor Amplifier’ (see p.543, November 1966). I’m always a bit wary about ‘linearity’ and other spurious audio claims, but there is only one way to be sure – build and compare. On pages 14 and 15, the Siemens book had two almost identical four-transistor amplifier circuits for a 9V supply and 8Ω speakers, see Fig.3. The main difference was one used the germanium output transistors (AC153K and AC176K) in the box and the other used silicon (BC140 and BC160). Biasing requirements accounted for the other circuit differences. The claimed output power was 1.1W for the germanium and 0.7W for the silicon. This sort of germanium amplifier was used in Robert’s Radios, such as the R505, until the early 1970s. They still sound good today – and they don’t eat batteries. Maybe their designers were onto something. Nice old junk Sifting through some musty boxes (Fig.2) courtesy of yet another lock-down Fig.1. It started with ceramic chip capacitors. Now, even some resistors have no markings, such as these Vishay CRCW080522K0FKEA resistors from RS. Just black and blue rectangles (R84 and R15). This is what I have to put up with in my professional life – I am a human, not a pick-and-place machine! 46 Fig.2. A good dollop of old ‘proper’ components with wires and markings. Siemens germanium output transistors and Philips ‘Minipoco’ 1% tolerance lead-foil polystyrene capacitors. Still 1% accurate after 35 years. Practical Electronics | January | 2021 A udio input – 9 V Note: A C 153K and A C 17 6 K transistors are germ anium , the rest are silicon. Ω 0Ω O utput 1. 1W 15m A total I q Max supply current 19 0 m A A C 153K 0Ω Ω + B A 10 3 or 1N4 14 8 . kΩ 0kΩ 1µ F 50 0 µ F 0. Ω NT C 1 0Ω 0. Ω 10Ω B C 16 8 330 pF + n Comparison circuit Fig.3. The Siemens recommended circuit for their transistors. It’s very efficient but needs matched output devices. Also, the signal reference 0V and speaker reference 0V are different, which gives poor power supply rejection ratio. We won’t build these. A C 17 6 K 10 µ F + 100kΩ B C 258 1. kΩ + 250 µ F 0 V kΩ 10 µ F + 9 V + Ω 0Ω Note: all transistors are silicon. 0Ω B C 14 0 3x + O utput 0 . 7 W 22m A total I q Max supply current 150 m A 50 0 µ F Power supply Since germanium NPN transistors are rare, the circuit was inverted to use a negative power rail and positive earth. This way only one NPN transistor was needed, the rest are PNP devices, normal Ico = grounded em itter leaka ge current 0 . 15m A <at> 25° C , typically doub les ev ery 5° C 1Ω B A 10 3 To perform a practical germanium vs silicon comparison, I decided to use the PE Theremin Amplifier circuit and PCB from November/December 2020’s Audio Out. I installed transistor sockets on the board – always a good idea when trying unusual parts. I used cut strips of SIL turned-pin IC sockets for this (see Fig.4), since real transistor sockets are expensive. I decided to go for a standard 9V, 8Ω set-up to see if I could replicate the Siemens amplifiers’ results. There is no point in trying to go for a micro-power design with germanium, since their high leakage currents prohibit this. When experimenting with germanium transistors it is important to check the leakage current (I cbo) of old stock devices. This is the current that flows between collector and emitter with the base open-circuit. I use a Peak DCA75 Analyser for this (Fig.5), but a 5V power source and multimeter, as shown in Fig.6, will also do the job. . kΩ 0kΩ A udio input 1µ F 2. 2nF + 250 µ F 100kΩ 4 . 5V O C 7 1 ( typical type) + B C 10 8 B C 16 0 B ase lef t open hence the ‘ o’ in Ico 3. 3nF 1. kΩ – mA 1Ω B C 17 8 + . kΩ 0 V 10Ω 0 V Fig.6. Circuit for checking leakage current. Fig.4. When trying out unusual transistors, use SIL sockets for easy substitution during R&D. Practical Electronics | January | 2021 Fig.5. It is important to check the leakage currents of germanium transistors. Anything above 0.25mA is suspect, except for power types, where the limit is 2mA. 47 1. 7 m A R 5 1. kΩ R 6 1. kΩ C 4 4 7 0 µ F 16 V + C 3 22µ F 10 V + – 5. 4 V C lip R 14 Not used T R 4 * B C 14 3 * T R 4 / 5 with T O 5 clip- on heatsink s 7 20 m W output R 7 1kΩ T R 3 B C 327 + D1 B A T 8 6 VR 2 kΩ R 3 100kΩ C 11 10 µ F 6 V R 11 0. Ω R 12 0. Ω C 7 4 7 0 µ F 10 V – 4 . 2V + R 2 47kΩ R 13 10Ω I q set C 9 Not used R 4 47Ω VR 1 00Ω VR 1: DC m id- point adj ust R 8 1 0Ω C 2 4 7 µ F 6 V R 9 47Ω + – 9 V – 4 .8 V T R 1 B C 557 B D2 R ed A udio input I q = 29 m A Ω C 6 10 0 nF T R 5* B C 138 + + R 1 4.7kΩ C 8 10 0 0 µ F 16 V + 10 m A T R 2 B C 557 B C 10 Not used C 1 10 µ F 10 V T ant R 10 10Ω C 5 10 0 µ F 10 V 0 V P ositive earth Fig.7. To start the development of the germanium amplifier, a silicon amplifier was developed using a negative power rail with positive earth. This way, only one NPN device was needed, with the rest being PNP types, paving the way for step-by-step substitution with germanium devices. creating your own ‘Robert’s Radio sound’ then I’ve even got some old alnico magnet Celestion 6×4-inch speakers for sale). for germanium. Of course, when doing this ‘mirroring’, all the electrolytic capacitor and diodes have to be reversed too. This is no problem with the silicon circuit, since the transistors cost peanuts, whatever the polarity. Later I’ll use this positive-earth amplifier design with some germanium fuzz circuits and Mullard LP1171/69 radio modules, as used in the Robert’s R600. (If you fancy e b underside c T O 5 B C 138 / 4 0 underside R 6 pin vi ew 1. kΩ e b c ( case) + C 1 10 µ F 10 V T ant R 1 4.7kΩ C 4 + 4 7 0 µ F 16 V 1. 7 m A R 5 1. kΩ + R 14 Not used C 9 Not used VR 2 kΩ + C 11 10 µ F 6 V R 12 0. Ω + C 8 10 0 0 µ F 16 V + + 9 V C lips at 19 0 m A supply current * T R 4 / 5 with T O 5 clip- on heatsink s R 11 0. Ω + 4 . 2V 7 20 m W output C 7 4 7 0 µ F 10 R 13 10Ω C 6 10 0 nF Ω + VR 1 00Ω R 8 1 0Ω C 2 4 7 µ F 6 V R 9 47Ω T R 5* B C 14 3 I q = 29 m A S tandard silicon am plif ier with negative earth I q set R 4 47Ω VR 1: DC m id- point adj ust R 7 1kΩ T R 3 B C 337 D1 B A T 8 6 R 3 100kΩ T R 4 * B C 138 + 4 .8 V T R 1 B C 54 9 C C lip R 2 47kΩ 10 m A T R 2 B C 54 9 C D2 R ed A udio input R 10 10Ω C 3 22µ F ? ? V C 10 Not used + 5. 4 V The first task is to get the silicon version working. That way any transistor blowups will cost 5p rather than possibly £1. To do this, we’ll take the original circuit (Fig.9, p.65, PE November 2020) and + T O 9 2 B C 54 9 C pin vi ew Germanium gestation C 5 10 0 µ F 10 V 0 V Fig.8. The silicon circuit with standard negative earth, a useful medium-power amplifier. 48 scale up the currents by a factor of three for the lower speaker impedance of 8Ω, rather than the original 25Ω. This entails dividing most resistor values by three. Capacitors will have to be increased by three to take into account the lower impedances. Finally, the power rails and all other polarised components are flipped. There are then resistor tweaks to bring the biasing into line. This new circuit is shown in Fig.7. An interesting high-frequency instability occurred where the frequency of oscillation was so high my 40MHz ‘scope couldn’t see it, but it was enough to completely mess-up the DC conditions and drive one output transistor to partly cut off. The clue was when I put my finger near R6 and the problem just cured itself, a sign of VHF oscillations. Ironically, removing stabilising capacitor C10 fixed it. When the new silicon circuit was rebuilt using a standard positive rail (Fig.8) the instability vanished and there was no problem with C10. I have heard that there is more likely to be problems with silicon PNPs; they have more junction capacitance modulation (Early effect) than NPNs. The germanium transistors were then gradually plugged in, checking along the way with a multimeter, ‘scope and signal generator. The only significant changes were to the DC biasing, with R2 being changed from 47kΩ to 15kΩ. Of course, the output current bias (Vbe multiplier) transistor has to be germanium if the output devices are. This can be an AC153 or a special low-voltage transistor designed/selected especially for the job, such as an AC169. Using an NKT214 didn’t work because the Iq could not be turned down to zero. Do note that some AC169s only have two wires, they are essentially just a 0.13V diode. The metal-cased three-wire ones have to be checked on a transistor tester to determine the connections. Germanium-alloy junction transistors are generally ten-times slower than silicon planar types, giving inferior square-wave and high-frequency response. With TR1 and TR2 (in Fig.9) being 2MHz-Ft (frequency at which gain falls to unity) NKT214 audio types, the –3dB point was about 15kHz, fine for guitar and AM radio. The square-wave response at 1kHz 2V pk-pk output had overshoot on it from TR2. Connecting a 33pF capacitor (C10) removed this. The high-frequency response was brought up to Hi-Fi (40kHz bandwidth) standards by putting in germanium 2SA12 RF transistors, as used in AM radio oscillators and leaving C10 out. These have an Ft of around 10MHz. Finally, we had a germanium amplifier with good performance. The circuit is shown in Fig.9. Practical Electronics | January | 2021 A C 17 6 K / A C 153K , X O 4 packa ge R ed dot ( NK T 214 only) b Mounting hole e c underside e T O 1 packa ge NK T 214 / 2S A 12/ A C 153 c b 1. 7 m A R 5 1. kΩ 17 m A R 6 1. kΩ * Use C 10 with NK T 214 f or T R 2 R 1 4.7kΩ VR 1 00Ω + – 9 V C lips at 19 0 m A supply current www.poscope.com/epe Germ anium am plif ier with positive earth 8 8 0 m W output R 7 C lip 0Ω T R 3 A C 16 9 + VR 2 kΩ C 11 10 µ F 6 V R 11 0. Ω R 12 0. Ω – 4 . 3V R 8 1 0Ω C 2 4 7 µ F 6 V R 9 47Ω T R 5 A C 17 6 K on heatsink C 7 4 7 0 µ F 10 V R 13 10Ω I q set R 4 47Ω VR 1: DC m id- point adj ust 3m A D1 C G9 2 or O A 9 1 R 3 100kΩ C 9 Not used T R 4 A C 153K on heatsink I q = 22m A + R 2 1 kΩ T R 2 NK T 214 or 2S A 12 C 8 10 0 0 µ F 16 V + – 4 . 5V D2 5. 1V A udio input R 14 Not used T R 1 NK T 214 or 2S A 12 R 10 10Ω C 3 22µ F 10 V C 10 * 33pF – 4 .7 V C 4 4 7 0 µ F 16 V + C 6 10 0 nF Ω + + C 1 10 µ F 10 V T ant + c I ndent C 5 10 0 µ F 10 V 0 V P ositive earth Fig.9. The final circuit using germanium transistors. Output transistor selection For the silicon amplifier, standard TO5 metal-can devices were used; in this case BC138 and BC143 – just because they were in the drawer. They can comfortably deliver output powers up to 2W with standard clip-on heatsinks. The germanium equivalents are the AC153K (PNP) and AC176K (NPN) shown in Fig10. The type with the K suffix was used because they have a convenient hole to mount on a metal bracket for heatsinking. The XO4 and TO1 germanium cases are electrically isolated, the heat passing from the junction to the case via a white paste filling of aluminium oxide and silicone grease. There are few germanium complementary pairs available. Alternatives are AC128/176 and AC188/187. For higher powers up to 6W the AD161/162 are often used (Fig.11). These TO66 devices are the germanium equivalent of the silicon TO126 BD135 and BD136 types. Next month That’s all for this month. In Part 4, we’ll finish with the component options and examine the frequency responses. Fig.10. The Siemens AC176K and AC153K complementary output transistors. These are still used today in the very expensive EMS VCS3 synthesiser for their ‘musical’ sound. Practical Electronics | January | 2021 Fig.11. The Mullard AD161/2 output transistors (right). Very popular in the 1970s. Robert’s used them in their RM50 table radio along with the Celestion speaker in Fig.18. Note their small TO66 case compared to the standard TO3 (left). - USB - Ethernet - Web server - Modbus - CNC (Mach3/4) - IO - PWM - Encoders - LCD - Analog inputs - Compact PLC - up to 256 - up to 32 microsteps microsteps - 50 V / 6 A - 30 V / 2.5 A - USB configuration - Isolated PoScope Mega1+ PoScope Mega50 - up to 50MS/s - resolution up to 12bit - Lowest power consumption - Smallest and lightest - 7 in 1: Oscilloscope, FFT, X/Y, Recorder, Logic Analyzer, Protocol decoder, Signal generator 49 Make it with Micromite Phil Boyce – hands on with the mighty PIC-powered, BASIC microcontroller Part 24: Counting pulses, rotary encoders and a digital safe T his month, we are going to explore three useful MMBASIC commands that relate to counting digital pulses. Using these built-in commands greatly simplifies tasks such as measuring the frequency of a signal, measuring the time period of a pulse cycle, or simply counting the number of pulses present on an input pin. Linked to these topics is something that follows on nicely from last month, where we used a rotary potentiometer to generate a voltage between 0V and 3.3V, which in turn was used to control the position of a servo actuator arm. Several readers contacted us and asked if there was an alternative to a potentiometer; something that can instead be continually turned in either direction (unlike a potentiometer which is mechanically limited – typically to around 270°‚ not even a complete 360° revolution). This is where rotaryencoders come into play, and if you have never used one before, they can be an extremely useful input device. To make this a fun, we will show you how to use a rotary encoder to simulate entering a digital safe combination number. What exactly are digital pulses? Digital pulses comprise transitions between low and high logic levels on a signal line. The result is a digital signal. These can vary considerably, yet are often simply drawn as the examples shown in Fig.1. The pulses in a digital signal can be repetitive and symmetrical; an example is a square-wave – see Fig.1a. Here they are shown with a fixed frequency (ie, the length of a pulse cycle is constant), and with a duty cycle of 50% (meaning that for 50% of the pulse cycle, the logic level Questions? Please email Phil at: contactus<at>micromite.org 50 is high, and for the remaining P ulse cycle time it is at a low logic level). We have seen, and used, square-waves earlier in the 1b series when we used a PWM signal to drive a piezo sounder to generate musical notes. 1c Pulses can also be repetitive but non-symmetrical ie, where the frequency is constant, but Fig.1. Examples of various digital signals. 1a represents the duty cycle is not 50% – a square wave, meaning a duty cycle of 50%. 1b see Fig.1b. A great example represents a digital signal with the same frequency (ie, of this is a servomotor signal fixed time-cycle) as 1a, but with a lower duty-cycle. 1c (ie, PWM signal) where, as is a random signal with a varying frequency (and cycle we saw last month, the duty time period) and with varying duty-cycles. cycle controls the position of a servo motor actuator arm. Digital pulses that the boundaries of each pulse cycle are may also be random (ie, non-repetitive indicated by the dotted lines in Fig.1a and and non-symmetrical) – see Fig.1c where Fig.1b. In terms of logic-level transitions, neither the frequency nor the duty cycle a pulse cycle in a digital signal is shown is constant. in Fig.1 as starting with a low-to-high Consider each of these digital signals transition followed by a high-to-low, and being drawn as a graph where the x-axis finishes on the next low-to-high transition represents time, and the y-axis represents (which is effectively the start of the next the digital logic level; ie, either a logic pulse cycle, and so on…). Understanding high (3.3V) or logic low (0V). The point this concept of a pulse cycle allows us here is that at any moment in time, the to measure two important parameters – state of the digital signal can be considered time period and frequency – by simply as either a low or a high logic state. detecting logic-level transitions. If we were to use a Micromite input pin to read the logic level of a digital Pulse time-period signal, then by detecting the transitions So, having just explained what a pulse between the low/high states within the cycle is in terms of logic-level transitions, signal, we can start to measure certain it now makes it easy to explain the pulse parameters about the digital signal. We’ll time period (sometimes referred to as ‘cycle discuss the theory first and then look at time’). The time period is simply the time the MMBASIC commands that simplify taken to complete one pulse cycle. In other the whole process. words, relating to the square-wave pulse cycle highlighted in bold in Fig.1, it is the time taken between two consecutive lowPulse cycle to-high logic-level transitions. More on A pulse cycle within a digital signal can this later when we discuss the MMBASIC be considered as a ‘complete wave’ that command to measure this timing. simply comprises a high-level logic pulse and also a low-level logic pulse (it doesn’t matter which comes first). Fig.1a has one Signal frequency square-wave pulse cycle highlighted in Frequency is often regarded as the number bold to make this a little clearer. Note too of pulses per second, and it is measured Practical Electronics | January | 2021 The SETPIN command As we have seen earlier in this series, the SETPIN command is used to configure a Micromite pin to behave in a certain way. The simplest syntax of this command is: SETPIN pin-number, configuration, option where: pin-number is the value of the physical pin number on the Micromite chip. It must be a valid value otherwise an error is generated (see the Micromite User Manual for specified values). configuration is a parameter that determines the function of the specified pin. When we first introduced the SETPIN command early on in the series we used it to define a pin as a digital input with SETPIN x,DIN (where x is a valid pin number) and used SETPIN x,DOUT to define a digital output. Last month, we also used SETPIN x,AIN to define an analogue input pin to which we connected a potentiometer, which supplied an analogue voltage between 0V and 3.3V. Fig.2. This rotary encoder comes with a built-in RGB LED and a push-button (activated by pressing on the shaft). A breakout board is available, making it a breadboard-friendly device. in hertz (abbreviated to ‘Hz’); but more technically correct, it is the number of complete pulse cycles per second. To determine the frequency of a digital signal, we could start a ‘one-second timer’ on a low-to-high transition, and then count the number of subsequent low-to-high transitions while the timer is ‘active’. When the one-second interval is up, the count reached would represent the frequency (in hertz) of the digital signal. An alternative is to use the wellknown equation: Freq = 1/(time period) If we measured a time of 0.1 seconds for a single pulse cycle, then the measured frequency would be 1/0.1 = 10Hz. Note that by using the equation method, we do not have to wait for the ‘one-second’ timer duration. The theory described above shows that for MMBASIC to measure a time period or the frequency of a digital signal, all it needs to do is detect logic-level transitions and accurately measure the time between such transitions. We could write a BASIC program to do this, but what if we want to measure a high frequency signal – there would come a point where our program code would simply be too slow to keep up. To overcome such a limitation, MMBASIC has three commands built into the firmware that automatically detect these logic-level transitions at an extremely fast rate. Practical Electronics | January | 2021 option is an optional parameter that may define further details; for example, previously we have used PULLUP or PULLDOWN in conjunction with DIN when using a push-button as an input. This avoided having to use a physical resistor to tie the input signal to a default logic level (with a push of the button then setting the opposite logic level). So, why have we explained all this? The answer is that the pulse-counting commands that we are exploring this month are also implemented with the SETPIN command, but with different configuration parameters. The FIN, PIN, and CIN parameters The SETPIN command pin-number parameter must be a valid value, and referring to the Micromite User Manual, it explains that four pins (pins 15-18) are valid for use as ‘COUNTing’ inputs on the 28-pin Micromite. Note that ‘COUNTing’ pins means that FIN, PIN and CIN can be used on any of these pins (and not just the counting (CIN) functionality). Now let’s demonstrate each in turn, starting with how the MKC can measure the frequency of a digital signal. Note that the voltage levels of any digital signal that we wish to measure must not exceed 5V, and ideally be at 3.3V for the MKC. If you wish to measure a digital signal that has a higher voltage, then simply add a potential divider between the high-voltage signal and 0V. For our demonstration purposes, we will simply use a Micromite output pin as the source of the digital signal, and so it will not exceed 3.3V for a high logic level. Using FIN To demonstrate how to measure the frequency of a digital signal, we will use pin 16 as the input pin. We could also use pin 15, 17 or 18, but pin 15 may already be in use if you have a touch-screen connected and enabled. For a digital test signal, we will use the PWM command to output a square wave on pin 4. There is no diagram for this circuit, since all we need to do is connect a jumper wire between pin 4 and pin 16. Once you have done this, connect your MKC to your computer, start the Terminal app, and enter the following: Freq=2000 SETPIN 16, FIN PWM 1, Freq, 50 DO PRINT PIN(16) LOOP Before you RUN the code, let’s explain what we’re doing. The first line sets a variable named Freq with a value of 2000. This will be the frequency (in hertz) of the generated square wave from the PWM pin (ie, the frequency of the digital test signal). The second line configures the appropriate input pin (pin 16) to which we are connecting the data signal that is having its frequency measured. The third line starts to output a square wave on pin 4 (PWM channel 1A). It is a ‘perfect’ square wave with a 50% duty-cycle; and the frequency is set to the value stored in the Freq variable. Next, a DO…LOOP, in which we continually PRINT the value of PIN(16) on the terminal screen. Pin 16 is configured with the FIN parameter, so when you PRINT PIN(16) you don’t get the logic value (as you would if we configured pin 16 with DIN), but instead the firmware measures the frequency by counting the number of low-to-high transitions that occur over one second. RUN the program and you should see is a stream of ‘0’s (zeros) initially displayed on the screen, followed possibly by a stream of values close to 2000, and finally after one second it should settle on a stream of 2000. If you do not see this, check your code matches that shown above, and also check you have pin 4 and pin 16 connected to each other. Note that the 2000 shown is a value in hertz; ie is equal to 2kHz. Gate time The reason for the initial zeros (and potentially a few non-2000 values) is that there is a one-second period of time during which the firmware is busy counting the low-to-high transitions in the background. This period of ‘counting time’ is referred to as the ‘gate time’; it must elapse before 51 A C lockw ise B Low- to- high transition A better to measure the pulse time period and then use the formula Freq = 1/(Time Period), This is what we will examine next by using the PIN configuration parameter instead. Using PIN To measure the cycle time period, the Micromite B firmware just needs to accurately measure the time between two Fig.3. The rotary encoder outputs two digital signals: ‘A’ successive low-to-high and ‘B’. By detecting a low-to-high transition on ‘A’, we transitions. By using can read the logic level of ‘B’ to determine the direction SETPIN 16, PIN in the of rotation. above program, we can see a frequency count can be provided. Gatethis time period measurement in action. time, therefore, determines how often the Change the second line (with no output result is updated (important if the option parameter), and also set the frequency of the data signal varies). value of Freq to 50 (ie, 50Hz). On running Now refer back to the syntax of the the program, you will see the value 20 SETPIN command and you’ll see there displayed, which means the period of is an option parameter. An important the 50Hz signal is 20ms. The result is point here is that when you use the always in ms – a value of 20 means 20ms FIN configuration parameter, the (0.02s). As a sense check, note that Freq option parameter defines the gate time = 1/(Time-Period) = 1/0.02 = 50Hz. in milliseconds (ms). So why would we When the configuration parameter want to adjust the gate time? Well, if the PIN is used with the SETPIN command, frequency of the data signal is very high, the option parameter can be used to then you could count many pulses in determine over how many pulse cycles a shorter space of time and achieve an the measurement is taken to average the accurate result quicker. In this scenario, period measurement. Valid values are 1 the output would be updated more often (default) to 10000. You would tend to set (shorter gate times mean the count result this to a higher value when measuring the is displayed sooner). Similarly, if the data time period of higher frequency signals. signal has a very low frequency, then a To see this in action, set the Freq value much longer gate time is required in order to 5000 (ie, 5kHz), and run the program. to count a number of pulse cycles in order The resultant time period displayed is not to measure the frequency. You couldn’t really accurate, so now add the option use a gate time of one second to measure a parameter with a value of 1000 so that signal that only changed every 10 seconds! 1000 pulse cycles are timed and averaged. To see the above in practice, stop the On running the program, you will now see program (Ctrl-C), and then EDIT the the value 0.2 displayed. This represents program to set the Freq variable to 200000 a time period of 0.2ms = 200µS, which (ie, 200kHz) – then RUN the program is the period of a 5kHz signal. again. We are still using a one second In summary, when measuring highgate time, so there is still an observable frequencies, it is better to use FIN, and ‘delay’ before the result is shown. when measuring low-frequencies, it is Next, shorten the gate time to 10ms by better to use PIN. changing the second line to SETPIN 16, FIN, 10 and on running the program Using CIN you should see the resultant frequency The configuration parameter CIN is displayed much quicker (every 10ms). used if you simply just want to count In summary, when using the F I N pulses (or to be precise, count low-toconfiguration parameter, ensure you high transitions). On configuring a pin use an appropriate gate time to allow with CIN, an internal counter is reset to MMBASIC to count a decent quantity a value of zero. Then, on every low-toof pulse cycles (which is ultimately high transition, the counter increments determined by the frequency of the input by one. Then, whenever you use PRINT data signal). The gate time parameter PIN(x) (where x is one of the four valid value must be between 10 and 100,000; COUNT pins), the value of the counter will and the outputted value is always in be displayed. To reset the counter back to hertz, regardless of the gate-time value. zero, simply use SETPIN x,CIN again. When measuring signals with a Note that the Micromite can detect input frequency less than 10Hz, it is often pulses as brief as 10ns, and hence if CIN A nticlockw ise 52 is used, for example, to try and count button presses on a push-button, then any mechanical bounce as the button is pushed will also be counted (which is why it’s important to debounce mechanical switches). Therefore, CIN is much more suited to counting pulses in a true digital signal as we will now explore. Note, there is no option parameter when using CIN. Using our existing code, set the Freq value to 1000, ie 1000 pulses will be generated every second. Then change the second line to SETPIN 16, CIN and run the program. You will see the pulses being counted, with effectively the number of ‘thousands’ being the number of seconds the program has been running for. Have a play by changing the value of Freq, and see how this affects the displayed value after each second. The above shows how to use the Micromite to perform various counting tasks on a digital signal. We will now look at a slight variation to this in the form of detecting pulses from a rotary encoder. Rotary encoders A rotary encoder is a rotary position sensor, but it can also be used as a ‘digital potentiometer’ which has no limitation on its rotation – it can be continually rotated in either direction. Instead of containing a variable resistance, a rotary encoder simply outputs a sequence of digital pulses, often as two separate digital outputs. By knowing how to interpret these pulses, we are able to determine the direction (and speed) of rotation – more on this shortly. Rotary encoders come in many different styles; however, we really like the one shown in Fig.2. It has a built-in RGB LED which can be made to illuminate through the clear plastic shaft; and by pushing down on the shaft, you can activate the built in push-button. In addition, while rotating the shaft, it has a soft ‘click action’ which provides nice tactile feedback. These features are not all normally found on a rotary encoder; and when you consider that it is readily available online at low cost, you can see why it is our favourite rotary encoder. To make it breadboard friendly, an optional breakout-board is also available (see Fig.2). Decoding the pulses The pulses generated by this rotary encoder are output on two digital signal lines – labelled ‘A’ and ‘B’ on the breakout board. These pulses are actually generating a 2-bit Grey-code signal. We won’t explain the details of Grey code here, but Fig.3 shows the kind of signals it creates. The point to observe here is that there are two possible rotations – you can turn it clockwise, or anti-clockwise. So how do we decode these pulses? The answer is actually quite simple once you apply the Practical Electronics | January | 2021 PULLUP option is used to tie the default logiclevel high (with the A C B Eq uiva lent circuit rotary encoder simply 3.3V pulling it low whenever it needs to output a low R ed Green B lue pulse). The second line NC R 1 R 2 configures pin 18 as an 470Ω 470Ω input so that the logic level on signal ‘B’ can 22 24 21 be read when required. + B SW G R The third line sets a R 1 470Ω NC variable that we have 22 3.3V named Rot_Value with a value of 50 – it also 24 R 2 prints this value on the 21 470Ω terminal screen. A simple DO…LOOP is Fig.4. The rotary encoder connects to the MKC with three included, and acts as the connections to read the output pulses. The Digital Safe project main program. It does adds connections for the RGB LED and the push-button. not do anything here, but is required so that the program can continue to run while waiting for a lowfollowing technique. Referring to Fig.3, to-high transition on pin 17 (ie, from the consider the moment in time when there rotary encoder’s signal A output. is a low-to-high transition in signal A. If The interrupt subroutine is the part that signal B is at a low logic level (highlighted then determines the direction of travel by the blue circle), then the shaft is rotating by reading the state of pin 18. Depending clockwise. However, if signal B is at a on the state of pin 18, Rot_Value is high logic level (highlighted by the red either incremented by 1 (clockwise), circle), then the shaft is rotating in an or decremented by 1 (anticlockwise). anti-clockwise direction. It’s that simple! And before the subroutine is exited, the Note that you could detect the transition new value of Rot_Value is displayed of signal B instead, and then read the on the screen. Note that the PAUSE logic level of signal A to determine the command adds a small delay to remove direction. And you can also use a highany mechanical contact bounce generated to-low transition – whatever option you by the rotary encoder. select, just stick with it. RUN the program to ensure it works Let’s put this into practice with a few correctly by spinning the rotary encoder. lines of code. First, refer to Fig.4 for the Upon each ‘click’, you should see the circuit diagram. Note at this stage we only value displayed on the screen increase need to connect the three contacts labelled or decrease according to the direction ‘A’, ‘B’, and ‘C’ to the MKC (respectively of rotation. to pins 17,18 and 19). A and B are the two If you see the above work, but ‘in signal lines, and C is effectively a common reverse’ (ie, increase with an antithat is connected to 0V. Connect this part clockwise rotation), then you can either of the circuit, and then enter the following make a hardware change (by swapping short program: the connections to pins 17 and 18), or you can change the code inside the interrupt SETPIN 17, INTH, MyInt, PULLUP subroutine – I will leave it to you to work SETPIN 18, DIN, PULLUP out what to change! Rot_Value=50 : PRINT Rot_Value 17 0V 18 DO : PAUSE 1 : LOOP SUB MyInt PAUSE 0.1 IF PIN(18)=0 THEN Rot_Value = Rot_Value + 1 ELSE Rot_Value = Rot_Value - 1 END IF PRINT Rot_VALUE END SUB The first line of code simply configures pin 17 to trigger an interrupt subroutine (called MyInt) whenever a low-to-high transition is detected on signal ‘A’. The Practical Electronics | January | 2021 Digital safe Having seen how to use a rotary encoder to increase and/or decrease the value of a variable depending on which direction it is ‘spun’, it is now time to have some fun by demonstrating how a rotary encoder can be put to good use in a practical project – we will simulate entering a safe’s combination-lock number in a short demo program. This will require the use of the red and green LEDs built into the rotary encoder, as well as the built-in push-button. Now connect the rest of the circuit shown in Fig.4 (ie, connect the MKC to the push-button SW contact, and LEDs R and G via 470Ω resistors). Next, download and RUN the DigitalSafe.txt program (available from the January 2021 page of the PE website). On running the program, you will see the current ‘combination dial’ value displayed on the terminal screen. Simply turn the rotary encoder in the appropriate direction to enter the three required values of the combination: 3, 46, 22. When you reach the first required value, push the encoder shaft to activate the push-button, which in turn ‘submits’ the number displayed on the terminal screen. Continue with the second and third required values above in a similar manner. If you entered the three-number combination correctly, then the green LED will illuminate, otherwise the red LED will illuminate, meaning you failed to open the safe! The code is commented throughout so we won’t go into the details here, but do take the time to look through the code to see how easy it is to create this ‘digital safe’. Challenges Once again, there are many things you can change in the program code to affect the way it works. In this example, why not add hardware too – here are some challenges: 1. Increase the range of each value from between 1-50 to between 1-99 (or greater) 2. Make the safe more secure by increasing the length of the combination number from three values to five values (or more) 3. Add a time-out feature; ie, the combination has to be entered within a certain amount of time 4. Add a piezo sounder to give audible feedback to signify a correct combination entry, and a different sound if incorrect 5. Add a relay and a solenoid to complete the lock! Next month We have covered a wide range of topics throughout the series, and along the way encouraged you to experiment and build your own Micromite-based projects. Several of you have written in to ask if there are more powerful versions of the Micromite. For example, devices that can run faster, or have more memory, or have additional features and commands for controlling different hardware, such as SD cards for storing data, or for driving larger displays with higher resolutions. The answer to all of these is very much yes, there are indeed different Micromites available, as well as different versions of MMBASIC. Next month, we will provide a summary overview of what else is available for those of you wanting to advance beyond the power of the Micromite Keyring Computer. Until then, have fun coding! 53 Circuit Surgery Regular clinic by Ian Bell Interference and noise R ecently, Michael Lamontagne posted a question on the EEWeb forum concerning a possible ground loop problem when measuring an op amp circuit with an oscilloscope. Rather than looking at this specific case in detail (partly because we are not certain enough about exactly how everything was wired up) we will take a quick look at unwanted signals in general, specifically noise and interference. Noise and interference Unwanted signals present in electronic systems get often referred to as ‘noise’, although we can be more precise with our terminology. In audio system it may make its present felt as hiss, hum, buzzes and crackles. In sensor systems, it limits measurement of low-level signals and degrades accuracy of measurement. Noise may already be present as part of an input signal (eg, it may come from a sensor along with the wanted sensor signal), or it may be introduced by the circuitry (eg, amplifier) used to process the signal. For example, all resistors generate random electrical noise – you cannot prevent this, it is part of their basic physics. Circuits also produce nonrandom unwanted changes to signals – distortion, which we will not be looking at in this article. Unwanted signals may also come from outside or elsewhere in the system, coupled or picked up inadvertently and added to the signal being processed – this is often called ‘interference’ to distinguish it from random noise. Crosstalk is interference between multiple channels or signal paths. Random noise Radom noise causes the instantaneous value of a signal to deviate from its ‘true’ value, with decreasing probability for larger deviations. The specific mathematical probability function depends on the type of noise, but may be the well-known Gaussian or normal distribution, familiar to all statisticians. There are a variety of types of random noise generated within electronic circuitry; these include thermal noise, shot noise, flicker noise, and avalanche noise. This noise generated is fundamentally due to the discrete nature of electricity at the atomic level – electric charge in circuits is carried in packets of fixed size (eg, electrons). The waveform in Fig.1 shows a random voltage variation with time. This gives us some simple insight into what noise ‘looks’ Fig.1. Random signal (noise). 54 like, but in general, plotting random noisy signals against time is not particularly useful. When dealing with noise we often need to look at the spectrum of the signal – the variation of signal level against frequency. Unwanted signals may look like random noise (eg, on an oscilloscope), but they can actually have significantly different characteristics. For example, the noise on the power supply of a digital circuit may look random, but a look at the spectrum will show that certain frequencies, related to the system clocks will be dominant. Pure random noise has a smooth continuous spectrum – for example, that shown in Fig.2. Random noise may be classed according to the shape of its spectrum. White noise has the same power throughout the frequency (f) spectrum, whereas 1/f noise (or pink noise) decreases in proportion to frequency. For 1/f noise there is the same amount of noise power in the bandwidth of say 100Hz to 1kHz, as there is in 1kHz to 10kHz, whereas for white noise there would be 10 times as much power in the bandwidth 1kHz to 10kHz as 100 to 1kHz because it is 10-times larger. Amplifiers (and other circuits) typically exhibit a mixture of pink and white noise, with pink noise dominating at low frequencies. The frequency at which the dominant noise component changes between pink and white noise is called the corner frequency or noise corner (see Fig.2). The fact that the components in any electronic circuit or system generate random noise means that there is always a certain level of noise, even with no signal present. This is known as the noise floor, which is important because the circuit cannot meaningfully process input signals that are smaller than the noise floor. As noise floor relates to noise within the circuit, this is different from noise within the input signal. If the properties of the required signal are known then there are techniques which can extract signals that are smaller than noise present within the signal. The difference between the signal and the noise is often important; it is expressed as the ‘signal to noise ratio’ (SNR), usually in decibels (dB) and based on the ratio of noise power (hence the v2 terms in the equation). Larger values indicate better performance. 𝑣𝑣%& 𝑣𝑣% 𝑆𝑆𝑆𝑆𝑅𝑅!" = 10 𝑙𝑙𝑙𝑙𝑙𝑙#$ * & , = 20 𝑙𝑙𝑙𝑙𝑙𝑙#$ . / 𝑣𝑣' 𝑣𝑣' Fig.2. Typical spectrum of amplifier noise. Practical Electronics | January | 2021 points. Careful circuit design and construction can greatly reduce these problems. S ource of interf erence; eg, digital clock line Capacitively coupled interference (see Fig.3) can be reduced using screening, which C ircuit 1 C ircuit 2 effectively grounds the interference coupling capacitance. Screening is implemented using a) coaxial (screened) cable to link (for example) a Ground sensor to a circuit, and by enclosing the sensitive circuits in a grounded screening box. The source of interference can also be screened to reduce A lternative connection C ircuit 1 ground vi a screen; its effect on other circuits. Choice of where the usef ul f or ‘ f loating’ sensors screened cable is grounded may have an effect C ircuit 1 C ircuit 2 C ircuit 1 on circuit performance due to the possibility S creened cab le of creating grounding loops if the screen/signal return path is grounded at both ends (more on b) Ground this a little later). For differential signals, we can also use screened wires – the two signal wires form a twisted pair and are enclosed by the screen Fig.3. Capacitively coupled interference. (see Fig.4). Here, grounding at both ends is less of a potential problem as the ground does not carry the signal. Magnetic interference is worse when physically large loops Here, vs is the rms signal voltage and vn is the rms noise voltage. occur in the circuit (see Fig.5) but can be reduced by avoiding When using or quoting SNR values, the bandwidth (range of such loops – for cables, use of twisted pairs of wires is an effective signal and noise frequencies considered) should be quoted approach. For PCBs, use a ‘ground plane’ on one side of the because noise power is frequency dependent and noise may be board and for ribbon cables each signal can be given an adjacent present well outside the range of signal frequencies of interest. ground wire. Circuits can be shielded against magnetic fields, but this is not used as commonly as shielding for capacitive Interference coupling as it requires special high-permeability materials such External signals may get into your circuit through electrostatic, as Mu-Metal. These materials are expensive and since they may electromagnetic and magnetic coupling. In electrostatic coupling, need to be quite thick the screening may be bulky. a high-impedance part of your circuit acts like one plate of a Power supplies can be a significant source of unwanted capacitor; in magnetic coupling, a loop in your circuit acts like signals. The circuit in Fig.6 illustrates how supply resistance the secondary of a transformer; and in electromagnetic coupling, leads to errors or interference. The supply current taken by parts of your circuit act like antennas. Mains hum signals (at a circuit causes a voltage drop across the supply wiring; so, 50/60Hz) and radio frequency interference from other electronic for example, the ‘ground’ voltage at each subcircuit will not systems such as phones and computers are obvious examples actually be the same (as measured from the same reference of external interference. The amount of external noise a circuit point). Fig.6 is a simplification – every branch of the supply is subject to will vary greatly depending on its location. The wiring will have resistance and hence cause voltage drops. problem will tend to be worse close to things like power lines, Supply and group voltage drops may cause problems if we are electrical machines and transmitters such as mobile phones. trying to accurately process voltage signals. If the supply currents Signals in one part of your circuit can find their way into other are constant, then the error will be an offset (DC error) but the parts of the circuit where they cause problems. A common example ground voltage is not necessarily constant; as the supply current of this is the clock of a digital section of a mixed analogue and of one subcircuit varies then the supply voltage drop and hence digital circuit getting into an analogue section, via the power the error voltage at this and other subcircuits fluctuates (this is supply lines or by capacitive coupling to high-impedance sometimes called ‘ground bounce’). This problem can be very significant, for example, when one subcircuit has a digital clock C ircuit 2 signal that is coupled via the supply into a sensitive amplifier. This situation can also occur in the mains wiring – along the C ircuit 1 equipment power cables and the wiring between supply outlets. S creened C ab le A solution to supply and ground noise is to wire connections Ground separately to a single point rather than using the same pointto-point wire for all the connections (see Fig.7). This approach applies equally to the wiring inside the cabinet of an instrument Fig.4. Screened differential signal. and to the supply connections on an integrated circuit. It may be more difficult to achieve with mains wiring, as it cannot be easily changed and doing so may be very dangerous. C ircuit 1 Magnetic f ield C ircuit 2 Rsupply2 Rsupply1 S upply C ircuit 1 C ircuit 1 C ircuit 2 C ircuit 2 Rground1 Fig.5. Large wiring loops (upper schematic) make a circuit susceptible to voltages generated by magnetic fields. Reducing loop size (lower schematic) helps combat the problem. Practical Electronics | January | 2021 Rground2 S upply Fig.6. Supply wiring resistance causes voltage shifts and noise due to supply currents. 55 Fig.7. Supply wiring to reduce noise. S upply C ircuit 1 S ignal Fig.9. Triaxial connector used with triaxial cables for guarded connections. Guard C ircuit 2 S upply Ground loops When two circuits, sub-circuits, instruments, or other equipment are grounded at two separate points on a ‘ground bus’ we have a situation know as a ground loop (or earth loop) (see Fig.8). The ground bus may be a circuit board track, the chassis of the equipment, point-to-point wiring, or the mains earth connected at different outlets – many people have suffered unnecessary levels of hum in their Hi-Fi systems due to earth loops! The ground loop will pick up magnetic interference, probably mains hum and may also act like an antenna picking up radio frequency interference (RFI). Large loops will make the problem worse. Ground loops are a particular problem when two or more mains-powered systems (such as lab instruments and sensor circuits) are separately earthed and connected together. It is also possible for mains leakage currents to cause currents to flow in the earth (eg, shields) of connections linking equipment. Leakage currents can flow through parasitic capacitances and equipment ground, for example in transformers and EMI filters. The interference causes a current IL to flow in the ground loop, which in turn causes an additional voltage drop (ILRG) across the resistance (RG) of the ground connection between the equipment or subcircuits. The solution to ground loops is to avoid them by using a single grounding point (Fig.8). Use of differential signals, only connecting screens at one end, use of very low resistance ground connections between circuits (reducing RG), and signal isolation using transformers or opto-isolators also help minimise ground loop problems. Power isolation transformers may also help with mains wiring. Again, it is worth pointing out that some potential ‘solutions’ related to mains wiring, such as disconnecting earths, could be lethal. O uter shield/ chassis/ ground/ signal return T riax ial C ab le S ignal S ensor Ground Fig.10. Guarded signal connection. of shielding. The inner shield is connected to a signal of equal voltage to the signal provided by a unity-gain amplifier (see Fig.10). This means that there is a zero-voltage difference between the signal and inner shield, so the leakage currents (and capacitance effects) are minimised. The outer shield is usually grounded and provides interference protection for the guard signal. IM – IL C ircuit 1 IM RS A ) IM–IL RG C ircuit 2 VM IL Signal guarding Signal guarding is concerned with getting the most out of screened cable connections, particularly when connecting very low-level signals from high-impedance sources to high-precision circuits. In such cases, effects such as leakage currents in the cables and cable capacitance can cause significant errors. Signal guarding uses triaxial cables and connectors (see Fig.9), which have an inner conductor, carrying the signal of interest and two layers x1 Guard RS IM IM RC VM IL B ) IM S ignal Guard RS x1 IM VM IL Ground IM C ircuit 1 IM RC 1 C ircuit 2 0 v RS IM I L2 VM Guard RC 2 x1 VM I L2 Ground Fig.8. Ground loops: currents induced in ground loops cause voltage drops which introduce noise (upper schematic). Using a common ground point can eliminate the loop (lower schematic). 56 Fig.11. Guarded resistance measurement: a) non-guarded setup, b) non-guarded equivalent circuit, c) guarded setup, d) guarded equivalent circuit. Practical Electronics | January | 2021 As an example of how guarding works, consider the schematic in Fig.11a, for which an equivalent circuit is shown in Fig.11b. Here we are trying to measure the resistance of a sensor (RS) which has a very high resistance value and therefore leakage through the cable insulation resistance RC is significant. We apply VM and measure IM – this should give the value of RS as VM/IM, but if actually gives us this parallel combination of RS and RC due to the leakage current IL. Using a guard (Fig.11c and Fig.11d) means that the voltage across RC1 between the inner conductor and guard is zero and hence no leakage current flows. The buffer amplifier has no difficulty in supplying the guard-to-ground leakage current IL2 and this does not disrupt the measurement. ESR Electronic Components Ltd All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. We can help and advise with your enquiry, from design to construction. Vibration and chemistry Systems processing low-level signals are also prone to a variety of forms of interference-based noise and errors other than electrically/magnetically coupled signals, including mechanical and electrochemical effects. Movement and vibration of cables can create electric current through the triboelectric effect – charges created due to friction between a conductor and an insulator. Low-noise cables are available for situations where this may be a particular problem. Making sure that cables are well supported and not subject to vibration or large temperature fluctuations helps reduce this effect for any cable. Movement can also generate unwanted signals through the piezoelectric effect, which occurs when mechanical stress is applied to insulators. Unwanted signals due to movement and mechanical stress are sometimes called microphonic effects, because if the signal is listened to, the movement of (for example) a cable will be audible. Batteries create electric current through electrochemical effects. Similar processes can occur if contaminants are present on PCBs and terminals. Variations in humidity can affect sensor systems with very high impedances. 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Our patented range of Plug-of-Nails™ spring-pin cables plug directly into a tiny footprint of pads and locating holes in your PCB, eliminating the need for a mating header. Save Cost & Space on Every PCB!! Solutions for: PIC . dsPIC . ARM . MSP430 . Atmel . Generic JTAG . Altera Xilinx . BDM . C2000 . SPY-BI-WIRE . SPI / IIC . Altium Mini-HDMI . & More www.PlugOfNails.com Tag-Connector footprints as small as 0.02 sq. inch (0.13 sq cm) Practical Electronics | January | 2021 Vi s i t o u r Sh o p , Ca l l o r B u y o n l i n e a t : w w w .c r i c k l e w o o d e l e c t r o n i c s .c o m 020 8452 0161 Vi s i t o u r s h o p a t : 40- 42 Cr i c k l e w o o d B r o a d w a y Lo n d o n NW2 3ET 57 Max’s Cool Beans By Max the Magnificent Flashing LEDs and drooling engineers – Part 11 G ood grief! I am bubbling over with excitement. I have so many things I want to talk about that I don’t have a clue where to begin. I know, I know... I need to sit down, take a deep breath, start at the beginning, work my way through the middle, and eventually stagger my way to the end. The problem is deciding what to talk about first. Ah, I can see in your eyes that you are desperate for me to commence with gamma correction. Well, if you insist... 0 ° P rim ary 330 ° T ertiary F F F F R ose 30 ° T ertiary 0 0 0 0 F lush O range 0 0 8 0 F F 30 0 ° S econdary F F 8 0 0 0 6 0 ° S econdary Magenta 0 0 F F 11 10 27 0 ° T ertiary Electric I ndigo Feeling off-colour R ed 9 0 Y ellow F F 1 2 F F 0 0 9 0 ° T ertiary C hartreuse 3 58 B rightness B rightness B rightness 8 0 F F 0 0 Way back in the mists of time when we started this mega-mini4 8 8 0 0 0 F F series (PE, March 2020), we introduced the concept of pulse7 5 6 width modulation (PWM). Since we can turn an LED on and 24 0 ° P rim ary 120 ° P rim ary off very quickly, the way we control its brightness is to vary B lue Green the proportion of on time to its off time. We refer to this as the 0 0 0 0 F F 0 0 F F 0 0 ‘duty cycle.’ A 20% duty cycle means the LED is on 20% of 210 ° T ertiary 150 ° T ertiary the time and off for the other 80%, while an 80% duty cycle A z ure 18 0 ° S econdary S pring Green means the LED is on 80% of the time and off the other 20%. 0 0 8 0 F F 0 0 F F 8 0 C yan 0 0 F F F F In the case of the microcontroller unit (MCU) we are working with – the Seeedunio XIAO – we can use values from 0 (0x00 Fig.1. The colour wheel we’ve been using in our experiments. in hexadecimal) being fully off (0% brightness) to 255 (0xFF in hexadecimal) being fully on (100% brightness). Using this I created a quick test sketch (program) to see how this would PWM technique, a value of 128 (0x80 in hexadecimal) means affect the colours on my 12×12 array (the full sketch is presented the LED will be on half of the time and off the other half, rein file CB-Jan21-01.txt – it and the other files associated with this sulting in 50% brightness. article, are available on the January 2021 page of the PE website). In a later column (PE, September 2020), we introduced the Since the folks at Adafruit were conscious that some of their users colour wheel (Fig.1) that we decided to use for our 12×12 pingwould be using low-end Arduinos with limited SRAM, they store pong array experiments (Fig.2). If you have built one of these their table in PROGMEM (the Flash program memory). By comarrays yourself, or if you are playing with tricolour LEDs in parison, my Seeedunio XIAO has so much memory that I can general, you may have noticed that some of the colours seem afford to flaunt it, so I dispatched the butler to fetch my flauntto be a tad ‘off’. For example, the rose may appear very close ing trousers and stored my table in SRAM (Fig.3.). to magenta, while the flush orange may appear more yellow Previously (PE, October 2020), we met the GetRed(), Getthat one might expect. Green(), and GetBlue() functions that extract and return the On the bright side (no pun intended), the red, green, and 8-bit red, green, and blue components from a 32-bit colour value. blue elements in our tricolour LEDs work as expected and Also, we introduced the BuildColor() function that accepts 8-bit provide a linear response such that a 50% duty cycle does red, green, and blue components and returns a 32-bit colour value. indeed result in 50% brightness (Fig.2a). The problem is that The thing is, we have to apply our gamma correction to each our eyes have evolved to accommodate a huge dynamic range, of the colour channels individually. Thus, in our new sketch, from moonlight to sunlight and – as part of this – they have a we’ve added a GetGammaCorrectedColor() function that sort of built-in non-linearity (Fig.2b). accepts a 32-bit colour value, splits it into its red, green and Although not immediately obvious from my diagram, the blue components, uses our GammaXref[] look-up-table to curve of this non-linearity is defined by a somewhat tricky apply gamma correction to each component, and then returns power-law function. In order to address this, we need to drive the gamma-corrected 32-bit result. the red, green, and blue LEDs using the inverse of this function, which results in our eyes per10 0 % ceiving what we were hoping for in the first W hat our eyes perceiv e T he LED place (Fig.2c). We call the process of applying W hat our work s as eyes perceiv e ex pected this inverse function, ‘gamma correction.’ 50 % There’s a great article on Adafruit’s website covering all of this in depth (https://bit.ly/31zaSLK). H ow the LED H ow the LED is driv en is driv en As part of this, they provide what they call ‘The H ow the LED is driv en Quick Fix’ in the form of a cross-reference lookP W M P W M P W M 0 % up table that we can use to remap the linear 0 x 0 0 0 x 8 0 0 x F F 0 x 0 0 0 x 8 0 0 x F F 0 x 0 0 0 x 8 0 0 x F F values we would like to use into their gamma( a) W hat we ex pect to see ( b ) W hat we actually see ( c) A pplying gam m a correction corrected counterparts that will provide us with the colours we want to see. Fig.2. Gamma correction. Practical Electronics | January | 2021 gammaCorrectedColor = BuildColor(tmpRed, tmpGreen, tmpBlue); The idea is that you have two programs fighting each other in a virtual machine known as the Memory Array Redcode Simulator (MARS). The objective is to be the last program standing. To that end, each program can try to sabotage the other one and/or try to defend itself by self-repairing. You can get a really good feel as to what this is all about by reading the Beginner’s Guide to the Redcode pseudo assembly language that is used to create the warrior programs (https://bit.ly/34m5YUo). As Ken said in his email, ‘I figure this can be made visually appealing by presenting the memory array on a screen (or ping-pong ball array) and colour-coding each cell either ‘Neutral,’ ‘Last written for or by Program A,’ or ‘Last written for or by Program B’ — using green, blue, and red respectively, for example — and running the programs at only a few steps per second so progress can be followed.’ Initially, I was a tad skeptical that a 144-element MARS would suffice but – having looked at the Redcode Beginner’s Guide – I’ve changed my mind. Now I’m thinking about creating a MARS simulator to run on the Seeeduino XIAO that I’m using to power my 12×12 array. I’m also thinking about creating a Redcode assembler utility that can generate the warrior programs to run on the simulator. But wait, there’s more! Do you remember me talking about the NeoPixel Simulator that you can use to test your own programs to run on my 12×12 array (PE, November 2020)? Well, if I manage to find the time to get a MARS simulator up and running, we could combine it with our NeoPixel Simulator, thereby allowing you to create and test your own Code War warrior programs and then send them to me to be run on the real array. return gammaCorrectedColor; Keep your balance const uint8_t GammaXref[] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 7, 7, 8, 8, 8, 9, 9, 9, 10, 5, 6, 6, 6, 6, 7, 7, 10, 10, 11, 11, 11, 12, 12, 13, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, 23, 24, 24, 25, 25, 26, 27, 27, 28, 29, 29, 30, 31, 32, 32, 33, 34, 35, 35, 36, 37, 38, 39, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 50, 51, 52, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 66, 67, 68, 69, 70, 72, 73, 74, 75, 77, 78, 79, 81, 82, 83, 85, 86, 87, 89, 90, 92, 93, 95, 96, 98, 99, 101, 102, 104, 105, 107, 109, 110, 112, 114, 115, 117, 119, 120, 122, 124, 126, 127, 129, 131, 133, 135, 137, 138, 140, 142, 144, 146, 148, 150, 152, 154, 156, 158, 160, 162, 164, 167, 169, 171, 173, 175, 177, 180, 182, 184, 186, 189, 191, 193, 196, 198, 200, 203, 205, 208, 210, 213, 215, 218, 220, 223, 225, 228, 231, 233, 236, 239, 241, 244, 247, 249, 252, 255 }; Fig.3. Gamma-correction cross-reference look-up table. uint32_t GetGammaCorrectedColor (uint32_t uncorrectedColor) { uint8_t tmpRed; uint8_t tmpGreen; uint8_t tmpBlue; uint32_t gammaCorrectedColor; tmpRed = GammaXref[ GetRed(uncorrectedColor) ]; tmpGreen = GammaXref[ GetGreen(uncorrectedColor) ]; tmpBlue = GammaXref[ GetBlue(uncorrectedColor) ]; } The rest of the sketch is used to load the left-hand side of the array with uncorrected colours directly from our colour wheel and the right-hand side with their gamma-corrected counterparts, starting with red at the bottom and ending with rose at the top (Fig.4). Although it’s not easy to see from this image, in the real world the gamma-corrected values do present what appears to be a richer colour palette. For example, the gamma-corrected orange (second row from the bottom) looks more orange and the gamma-corrected rose (top row) appears more vibrant. War, what is it good for? According to Edwin Starr in his 1970 hit single ‘War’ – and Frankie Goes to Hollywood more than a decade later – the answer is ‘Absolutely nothing.’ Of course, it may be that neither of these luminaries were familiar with the concept of ‘Code War’. Actually, if the truth be told, neither was I until my chum, Ken Wood, who has been following these columns, sent me an email telling me all about this idea. In 1984, Alexander Dewdney wrote a column in Scientific American magazine about a programming game called Core War that he had created with DG Jones. A scan of this original article, along with a lot of supporting material, can be found on the CoreWars.org website and Wikipedia. Fig.4. Uncorrected colours (left) vs. gamma-corrected colours (right). Fig.5. 9DOF BoB (Image source: Adafruit.com) Practical Electronics | January | 2021 When I was a kid, my parents bought me a wooden marble maze toy. I just found something quite similar on Amazon (https://amzn.to/2HwZg4M), although the one I owned had larger mazes and used smaller ball bearings. The reason I mention this here is that a reader emailed me to suggest I attach a sensor to my 12×12 array such that, if the array is held in a horizontal plane, I could control the ‘rolling’ of a lit pixel by detecting the tilt of the array. By some strange quirk of fate, I just happened to have one of Adafruit’s BNO055-based 9DOF (nine degrees of freedom) Fusion breakout boards (BOBs) in my treasure chest (junk box) of spare parts (Fig.5) (https://bit.ly/3dP8EwU). This little beauty is based on a BNO055 microelectromechanical system (MEMS) sensor from Bosch. In turn, the BNO055 contains a 3-axis accelerometer, a 3-axis gyroscope, and a 3-axis magnetometer (they also throw in a temperature sensor for good measure). The really cool thing about this device is that it also contains a 32-bit Arm Cortex M0+ processor, which performs all sorts of mindbogglingly complicated sensor algorithms for you and provides you with data in a form you can use without your brains leaking out of your ears. As usual, the folks at Adafruit provide a wealth of information on this sensor, including pinouts, wiring, and how to download the required libraries (https://bit.ly/35sVvpz). Also included is some sample Arduino Code, which I used to create my first test program. The purpose of this initial sketch was to make sure I could get my XAIO microcontroller to talk to the BNO055. All we do is loop around reading the x, y, and z orientation values from the BNO055 and display them as floating-point values on the Arduino’s Serial Monitor. Note that the XIAO communicates with the BNO055 via an I2C bus, which uses pins 5 and 6 on the XIAO, but we don’t declare these pins in our sketch because Adafruit’s libraries handle all of this for us (file CB-Jan21-02.txt). The next step involved some mental gymnastics to visualize how I was going to mount my breadboard in the 12×12 array case, and which (sensor) values corresponded to what (left-right and forward-backward) tilts. Eventually, I determined that the 59 Size does matter! y values from the sensor would reflect To paraphrase an old saying, ‘It’s not tilting the array to the left or right, while the size of your array, it’s what you do the z values from the sensor would rewith it that counts.’ In reality, of course, flect tilting the array to the front or back. we all know in our heart of hearts that Thus, my second test program involved size does indeed matter. If you have two my taking the floating-point y and z values people standing next to each other, one from the sensor and converting them into clutching a paltry array and the other integer tilt values for use in my sketch. staggering under the weight of a garganWith a little manipulation, I ended up tuan offering, which one do you think with an integer tiltLeftRight value the punters are going to be looking at? that ranges from −90 (tilted left so much ‘Great minds think alike,’ as they say. Of as to be vertical), through 0 (horizontal), course, they also say that ‘Fools seldom to +90 (tilted right so much as to be verdiffer,’ but I’m sure that doesn’t apply to tical). Similarly, I ended up with an inus. For example, a guy who goes under teger tiltFrontBack value that ranges the moniker ‘Bitluni’ created a 20×15 = from −90 (tilted down to the front – that Fig.6. The Prognostication Engine’s main 300 ping-pong ball display using tricois, to the user – so much as to be verticontrol panel. lour LEDs (https://bit.ly/3jmltjj). Somecal), through 0 (horizontal), to +90 (tilted time later, the attention-grabbing Bitluni to the back so much as to be vertical). In We’ve discussed some of the effects followed up with a 40×30 = 1200 ball this case, all we do is loop around readwe could employ with the switches and array (https://bit.ly/31Bv8g0). ing the y and z orientation values from pushbuttons in previous columns, so In an earlier column, when I was buildthe BNO055 and display them as my malet’s now turn our attention to the LEDs ing my 12×12 array, I noted that if I ever nipulated integer values on the Arduino’s associated with the pots. did this again, I would build 8×8 sub-arSerial Monitor (file CB-Jan21-03.txt). As an aside, the pots are motorised, rays and then link them together to form a Now things start to get more interestso if some unauthorised person were to larger array. Well, the over-achieving Biting. In our next sketch, we commence attempt to change the engine’s settings, luni is of like mind, because he recently by setting one of the pixels – our ‘ball’ as soon as they go ‘hands-off,’ the pots created a 48×40 = 1920 ball array using this – in the center of the array to white. We will automatically return to their offivery technique (https://bit.ly/3jrwKyV). then loop around using the readings cially designated positions under proI can think of so many things we could from the sensor to make this pixel ‘roll gram control. do with such an array. For example, can around’ in response to tilting the array. Remember that we are using 16-element you imagine using a machine vision system The ‘ball’ stops ‘rolling’ when it hits one NeoPixel rings from Adafruit (https://bit. based on artificial intelligence (AI) to of the sides or gets trapped in one of the ly/37RtFpQ). In front of each ring (Fig.7a) detect and recognize gestures as we wave corners until we tilt the array in the opwe have a brass bezel and an antique our arms around controlling a wall-size posite direction (file CB-Jan21-04.txt). Bakelite knob (Fig.7b). Mounted in the version of the game Tetris? All I can say For your delectation and delight, I just bezel, in front of each pixel, we have a is that I hope to meet up with the nefaricaptured a short video showing this prosmall pseudo-mother-of-pearl ‘dot’ which ous Bitluni one day (and appropriate his gram in action (https://bit.ly/3mXHZS3). adds to the steampunk look-and-feel. wall-sized array while he’s not looking). Of course, this is only the beginning. Since we have 16 pixels, we can considCurrently, the ‘ball’ simply ‘rolls’ at a coner each as spanning (or representing) an stant rate once the tilt has passed a certain arc of 360°/16 = 22.5°. Now, different pots Prognostication revisited threshold. The next step will be to add have different physical ranges for how far I don’t know if you recall (I can barely some physics into the mix such that the they can rotate. The ones I’m using support remember myself), but this entire saga speed of the ‘roll’ varies as a function of rotations of 290°, which means there are commenced with my wishing to illuthe angle of the tilt. three pixels to which the pointer on the minate the controls on my PedagogiAfter that, we are limited, as usual, only knob cannot, in fact, point (Fig.8). cal and Phantasmagorical Inamorata by our own imaginations. For example, we Of course, although these pixels are Prognostication Engine. In addition to could make green ‘food’ pixels randomly shown as being dark gray in the figure, the lighting the furnace in the upper conappear on the array and then try to guide fact that we can’t point to them doesn’t trol panel and the ginormous vacuum our rolling pixel to hit them and ‘eat’ them mean we can’t light them up. One option tubes sitting on top of the engine, the before they randomly disappear again. The would be to simply paint the white line main control panel features eight toggle more you ‘eat,’ the more points you get. and dot on the knob black, and then map switches and six pushbuttons, each of Also, we could add red ‘hole’ pixels into (translate) the 290° rotation of the knob which is accompanied by two tricolour which our rolling pixel would ‘fall’ if they into a 360° range on the ring. LEDs. Also, there are five potentiometers were to come into contact (these ‘hole’ However, I like the white line and dot (pots), each surrounded by a ring of 16 pixels could be stationary, or we could on the knob. We could just turn the three tricolour LEDs (Fig.6). make them randomly appear and disappear like the ‘food’ pixels). We could come up with all sorts of cool games based on this technology if we put out heads together. Speaking of which, if you think of anything, please ( a) Neopix el ring f rom A daf ruit ( b ) W ith b ez el and k nob ( a) K nob rotated f ully anticlock wise ( b ) K nob rotated f ully clock wise drop me an email and Fig.7. 16-pixel rings. Fig.8. The rotation of the knob. share the good word. 60 Practical Electronics | January | 2021 6 1 pixels at the bottom off, but where would 5 7 2 0 W hat we’ v e got be the fun in that? My solution will be to 8 4 3 15 W hat we want use a separate colour for these three pixels. 3 9 4 14 New P ix el O ld P ix el C alculation Initially this will just be a steady colour, but 0 4 ( 0 + 4 ) % 5= 4 it may be that later we use different colours, 13 2 10 5 1 0 (1+ 4 ) % 5= 1 fades, and flashes on these three pixels to 2 1 (2+ 4 ) % 5= 1 3 2 (3+ 4 ) % 5= 2 provide us with additional information 6 12 1 11 4 3 (4 + 4 ) % 5= 3 about the state of the machine. 11 0 12 7 W hat we’ v e got The way I want to visualise things is 15 13 8 10 Num b er of pix els - 1 9 14 with pixels 0 and 12 corresponding to Modulo operator ( a) W hat I want ( b ) W hat I got the maximum anticlockwise and clockNum b er of pix els wise rotations of the knob, respectively Fig.9. You can’t always get what you want. W hat we want (Fig.9a). However, the way in which I creFig.10. Thought experiment with 5-pixel ring. ated my prototype resulted in the pixels being presented in a different manner (Fig.9b). Still prognosticating furiously To be honest, I don’t recall how the rings are oriented on Although lighting the pixels the way we’ve just done won’t form the main Prognostication Engine, but it really doesn’t matter part of the Prognostication Engine’s primary function, it helps because we are going to perform a simple cross-reference opus to wrap our brains around some of the nitty-gritty details, eration, and we can easily modify the cross-reference values and we will be able to use all of this stuff as part of a flamboylater. In the case of our prototype, we will declare our crossant power-up display. reference values as follows: In this vein, suppose we want to modify our previous program such that we have only a single pixel lit at any one time. int RingXref[NUM_NEOS_RING] = In this case, when we turn our new pixel on, we also want to {7,6,5,4,3,2,1,0,15,14,13,12,11,10,9,8}; turn the previous pixel off. Remembering that we are cycling the pointer to our pixels, iNeo, from 0 to 15, we would ideThe way this works is really simple, if we want to light what ally like to modify our core for() loop do something like we like to think of as pixel i in our imaginary world, we will the following (the new code is shown in bold): access RingXref[i], which will return the number of the corresponding pixel in the real world; for example, RingXref[0] for (int iNeos = 0; iNeos < NUM_NEOS_RING; iNeos++) will return 7 (tra-la!). { So, just to illustrate where we’re at thus far, what we are // Turn the new pixel on going to do is create a simple sketch that lights our pixels tmpNeo = RingXref[iNeos]; from 0 to 15, first red, then blue, then green, then start all NeosRing.setPixelColor(tmpNeo, tmpColor); over again. Since these LEDs are so bright, we’re going to add a ModifyBrightness() function that will dim them down // Turn the old pixel off to a specified percentage of their full value, which will allow tmpNeo = iNeos – 1; us to keep our original colour definitions as-is. The heart of tmpNeo = RingXref[tmpNeo]; this program is a function called LightMultiple(), which NeosRing.setPixelColor(tmpNeo, COLOR_BLACK); we will call from our loop() function (file CB-Jan21-05.txt). NeosRing.show(); void LightMultiple (uint32_t thisColor) delay(InterPixelPadDelay); { } int tmpNeo; uint32_t tmpColor; This would work great for every value of iNeo except 0 because 0 – 1 = –1, which will cause our program to do sometmpColor = ModifyBrightness(thisColor, BRIGHTNESS); thing excruciatingly painful when we attempt to use this value as an index into RingXref[]. It’s always the ‘end conditions’ for (int iNeos = 0; iNeos < NUM_NEOS_RING; iNeos++) that bite you when you are least expecting it. One alternative { would be to add a test for this end condition as shown below: tmpNeo = RingXref[iNeos]; NeosRing.setPixelColor(tmpNeo, tmpColor); for (int iNeos = 0; iNeos < NUM_NEOS_RING; iNeos++) NeosRing.show(); { delay(InterPixelPadDelay); // Turn the new pixel on } tmpNeo = RingXref[iNeos]; } NeosRing.setPixelColor(tmpNeo, tmpColor); As you can see, we first modify the brightness of the colour (I’ve cut things down to only 20% for this experiment, and they are still bright). Next, we cycle round lighting each of our pixels, using our RingXref[] to cross-reference the pixel numbers, upload the new values, and pause for a short delay that we’ve called InterPixelPadDelay. If you look at the code, you’ll see that I’ve defined a CYCLE_TIME of one second. Also, I’m assuming that it takes only 1ms (one millisecond, represented by CALC_UPLOAD_DELAY) to perform all my calculations and upload the new values. If you look at the setup() function, you’ll see how we used these values, along with the number of pixels in the ring, to calculate the InterPixelPadDelay. Practical Electronics | January | 2021 // Turn the old pixel off if (iNeos == 0) tmpNeo = 15; else tmpNeo = iNeos – 1; tmpNeo = RingXref[tmpNeo]; NeosRing.setPixelColor(tmpNeo, COLOR_BLACK); NeosRing.show(); delay(InterPixelPadDelay); } Now, although this would be a perfectly acceptable solution, it still feels a bit ‘graunchy,’ if you know what I mean. What I usually do in this sort of case is perform thought experiments 61 5 6 5 7 10 2 11 1 12 0 15 14 13 ( a) Using a single pix el 7 8 9 3 9 3 6 4 8 4 10 2 11 1 12 0 15 14 13 ( b ) Using m ultiple pix els Fig.11. Two ways to reflect the position of the knob. with pencil and paper. The first thing is to reduce the scope of the problem to a smaller number of pixels. Our original ring contains 16 pixels, which is an even number and a power of two (2^4 = 24). Given a choice, we want to come up with a generic solution that can work for any number of pixels, so we will play with a non-power of two. Furthermore, for this sort of thing, I prefer an odd number and a prime number on the basis that, if our solution works for this, it will work for anything. So, purely for this thought experiment, let’s assume that we have a 5-pixel ring. I start by drawing out a table (Fig.10). My first column lists what we’ve got (ie, what we know in the form of the information we have to hand), which is the number of the new pixel we’re going to turn on. The next column lists what we want, which is the number of the old pixel we want to turn off. I then dork around with various equations until I come up with a calculation that satisfies my needs. In this case, a simple calculation using the modulo operator % satisfies our requirements (this operator returns the integer remainder from an integer division). At this point, I transferred the equivalent over to my real-world code (file CB-Jan21-06.txt). Now, our programs thus far have involved lighting our pixels in a clockwise fashion. Just for giggles and grins, let’s suppose we want to modify our most recent sketch such that it lights the pixels in a widdershins (anticlockwise) pattern. Can you create your own table to determine the necessary calculation? – you can check out my solution in file CB-Jan21-07.txt Turning the knob Finally, for the moment, let’s consider how we are going to translate the turning of the knob, which we read via our potentiometer, into the lighting of the pixels in our ring. Remember that we’ve decided not to access the bottom three pixels, which we will light magenta. Let’s assume that our On colour will be yellow and our Off colour will be red. Two main possibilities immediately spring to mind. The first is that we could light a single pixel to indicate the position of 62 Fig.12. Steve Manley’s array of ten 21-segment displays. the pot (Fig.11a). The second is that we could light multiple pixels to indicate the position of the pot (Fig.11b). You can take a look at my programs for both of these implementations in files CB-Jan21-08.txt and CB-Jan21-09.txt, respectively. You will observe that I’ve also added the gamma correction we introduced earlier into these final two sketches. Also, I just created a short video that shows all of the effects we’ve discussed here in action (https://bit.ly/360O2hw). 21-Segment Victorian Displays Do you remember watching Monty Python when they said, ‘And now for something completely different’? Well, hold onto your hat because here we go. Way back in 1898 (123 years ago as I pen these words), someone called George Lafayette Mason filed a patent for a 21-segment display. These devices used 21 small incandescent bulbs (one per segment) to create letters, numbers, and symbols. A complicated electromechanical switch could be used to activate various groups of segments as required to represent the different characters. The way I heard about this is that I have some friends (stop laughing; it’s true) called Steve Manley and Paul Parry, where Paul is the owner of Bad Dog Designs (https://bit.ly/2FTSBBk). Paul and Steve ran across a group called Smartsockets (https://bit.ly/2HOeFhw), which was founded by Chris Barron and is moderated by Chris and John Smout. In fact, it was John who came up with the idea of resurrecting these displays, but John and Chris are focused on their Smartsocket implementations in which each display has its own PIC microcontroller. By comparison, Paul, Steve, and myself prefer to drive all of our displays with a single Arduino-compatible microcontroller. As a result, the two sub-groups are heading in slightly different implementation directions, although they still communicate what they are doing with each other. Steve has done a tremendous amount of work creating the circuit boards and 3D printed shells for these displays (Fig.12). Steve, Paul, and I each have ten of these displays, although I’ve not had the time to do anything with mine thus far. In fact, Steve has created a fantastic video that illustrates everything he’s done in exquisite detail (https://bit.ly/3oJwZcm). As we see in this image, Steve has used his 3D printer to make a pseudo-brass faceplate for his display. In my case, Paul introduced me to one of his friends, Kevin McIntosh. Kevin’s company, the Laser Hut (https://bit.ly/2Gf8qmg) offers laser cutting and engraving services, and he’s going to laser my pseudo-brass faceplates for me. I will, of course, be documenting all of this in excruciating detail in future columns. Coming soon As I’ve been known to say on occasion, ‘Show me a flashing LED and I’ll show you a man drooling’ (hence the title of this miniseries). When we first started discussing the topic of flashing LEDs and drooling engineers (PE, March 2020), I really had no idea as to the myriad topics into which we were going to stick our snouts. In reality, of course, we’ve barely scratched the surface of this multifaceted topic, but I hope that the stuff we have covered has sparked your imagination and tempted you to toy with your own creations. In the not-so-distant future, we’re going to take a break from LEDs and turn our attention to other topics, but turn that frown upside down and turn it into a smile, because that time is not yet come. Next month, for example, we will be using our 12×12 array to implement a version of Conway’s Game of Life – see: https://bit.ly/pe-jan21-cgol Until then, as always, I welcome your questions, comments, and suggestions. Cool bean Max Maxfield (Hawaiian shirt, on the right) is emperor of all he surveys at CliveMaxfield.com – the go-to site for the latest and greatest in technological geekdom. Comments or questions? Email Max at: max<at>CliveMaxfield.com Practical Electronics | January | 2021 Visual programming with XOD By Julian Edgar Light Column Thermometer P icture the scene. You have some guests at your house and their eyes are attracted to a small device sitting on a shelf. The base is clear, and an electronics board can be seen, together with two projecting tubes, each pointing upwards. Sometimes the tubes fill with light, their entire columns illuminated. There seems to be a pattern in the way the tubes flash – but what is it? First the two tubes light up together, one white and one blue. They flash for a few moments and then the white tube goes dark, just the blue tube continues to flash. Both then turn off, and the cycle starts again. It’s obviously not random flashing, but what is it? This time the guests count the flashes, and realise the white and blue tubes flash twice, and then the blue tube flashes three times. But no – this time around, the blue tube flashed four times! So, what is it? Realisation dawns – it’s a thermometer, in this example reading 23°C and 24°C, respectively. This is a fun project – an indoor thermometer that displays the temperature in a unique way. It’s cheap, very easy to construct and almost as easy to customise. Cheap? The Light Column Thermometer uses an Arduino Uno board (clones now cost under £5 delivered), two clear plastic Uno enclosures (£2 each) and a handful of low-cost components. The light columns are recycled ballpoint pens illuminated by bright LEDs. Easy to construct? Soldering will take you only moments and other than that, it’s just a case of drilling some holes and screwing and gluing parts together. Customisable? The program (sketch) for the Uno is written in XOD visual programming language. XOD (pronounced ‘Zod’) was introduced in the March 2020 issue of PE and is completely free to use. Unlike lines of code that often appear impenetrable, XOD is easy to understand and hence is easy to modify. If you wish, it will take you literally moments to alter the flash duration, flash fade speed – or even how often the cycle repeats. And you don’t need to have white and blue flashing columns – just use whatever colour LEDs you prefer. Building The Light Column Thermometer uses the following parts, with the specific ones I used shown in brackets.  Arduino Uno (eBay cheap clone – *see note below)  Plastic Uno box (laser cut – I used two, eBay)  Two pre-wired 5mm LEDs (blue and white, already fitted with dropping resistors – Banggood)  LM35 DZ temperature sensor (eBay)  Two discarded clear ballpoint pen barrels (salvaged)  Assorted screws, nuts and spacers (in my parts box) The Light Column Thermometer displays the temperature by using two illuminated plastic tubes. One tube shows ‘tens’ and the other ‘units’, with the number of flashes indicating the appropriate values. Practical Electronics | January | 2021 *Note: Some low-cost Uno modules do have one ‘wrinkle’. Many use a non-standard USB communications chip, which if you are to communicate with it, needs a new PC driver. Some users report that Windows can find the driver by an automatic on-line search, but I had to download the driver from: www.wch.cn/downloads/ CH341SER_EXE.html 63 If you are new to using Arduinos, perhaps initially stick with ‘official’ products – they don’t need new drivers. The enclosure uses the baseplates of two laser-cut boxes widely sold for use with the Uno. I used two baseplates (as opposed to the normal top and bottom parts) so that the top plate didn’t have cut-outs in it. These cutouts are provided to gain access to the pins, but I spaced the top and bottom of the enclosure further apart than usual (thus giving internal access to the pins) and so I didn’t need the cut-outs. Of course, you can use just one box if you don’t mind the slots in the top panel. The top and bottom panels are spaced 20mm apart using plastic stand-offs, and the Uno is bolted to the baseplate via screws, plastic washers and nuts. All the holes in the enclosure panels for mounting are drilled out to 3mm diameter, allowing the use of normal-sized spacers and screws. (As standard, these holes appear to be 2.5mm) The side and end plates of the enclosure are not used. The light columns are, as described, salvaged ballpoint pen barrels. Pick a transparent design that has an interesting shape, preferably without writing on it. Different barrels will give different lighting effects; test the result by shining a 5mm LED down the end of the tube. LED SCL SDA AREF GND 13 12 11 10 9 8 Anode (a) LED Use prewired LEDs that include dropping resistors, or add your own resistors (approx 470Ω to 1kΩ) to standard LEDs. 7 6 5 4 3 2 1 0 LED Cathode (k) An Arduino Uno and two low-cost commercially available enclosures form the main components of the project. The upright columns are salvaged ballpoint pen bodies. DIGITAL UNO ANALOG IN A0 A1 A2 A3 A4 A5 5V RES 3.3V 5V GND GND VIN POWER LM35DZ LM35DZ 1 2 1 2 VCC VOUT 3 GND 3 Fig.1. Connection diagram for the Arduino Uno, together with the pin-outs for the two LEDs and the LM35DZ temperature sensor. Its 5V supply is taken from the Arduino. Note: you can use LEDs prewired with dropper resistors, or ordinary LEDs and choose your own resistor. 64 The end of the pen barrels I used had a short length of exposed plastic thread. Two holes just undersize of this thread diameter were drilled in the top panel and then the plastic pen barrels could be screwed into the holes. A little cyanoacrylate glue (‘superglue’) was used to secure them into place. The pre-wired 5mm LEDs were glued into the ends of the tubes. Wiring Refer to Fig.1 for the pinout of the LM35 temperature sensor. Port A0 is used for the signal, and the sensor’s 5V and ground connections can be made close by on the Uno (all Uno ports are labelled). I used header pins and soldered the LM35 straight to these. Ensure that the signal wire cannot touch the ground connection. The LEDs were wired between ground and ports D3 and D5, again using cut-off header pins. Remember the correct polarity for the LEDs – positives/anodes, to the ports; negatives/cathodes to ground. Power to the board can be supplied via the DC socket (5-12V) or USB input. Software To upload the program (sketch) to the Uno you will have first needed to install XOD on your PC (see https:// xod.io/downloads/ – remember, it’s free after you register). You will also need to download the Light Column Practical Electronics | January | 2021 Thermometer sketch from the January 2021 page of the PE website. (Note that depending on whether you have used XOD previously, the software might prompt you to do some further downloading of extra libraries.) Refer now to Fig.2. The beauty of XOD is that’s it very easy to understand. In the red box (top of diagram) we have the input from the temperature sensor, constantly read through Analog Port A0. This value is multiplied by 500, averaged and then rounded. (The ‘live’ temperature is shown in the green ‘watch’ node, that operates when the sketch is uploaded to the Uno in ‘debug’ mode. We now need to extract from this number the ‘tens’ and ‘units’ – see the green box. Dividing the value by 10 and then using a ‘floor’ node does this for the ‘tens’. Now, what about the ‘units’? The ‘modulo’ node does this by calculating the remainder of (again) dividing our temperature value by ten. Two ‘watch’ nodes allow us to see these outputs live. Let’s do the white box next – the flashing shows the number of ‘units’. Our ‘units’ number is fed to a ‘flip-n-times’ node. This node flashes the LED output the required number of times, and also sets the flash rate and duty cycle – in this case, 0.2 seconds ‘on’ and 0.5 seconds ‘off’. The ‘gate’, ‘not’ and ‘equal’ nodes then prevent an output if the ‘units’ number is zero. (Otherwise, the ‘flip-n-times’ node outputs one flash, even with a 0 input.) We then feed the output through an ‘or’ node (more on this in a moment) and then through a ‘fade’ node. The fade node gives a gradual (although still pretty fast) rise and fall in LED brightness with each flash. Now, what about the ‘tens’? The tricky part here is twofold: first, the ‘units’ column needs to flash at the same time as the ‘tens’ column when ‘tens’ are being shown, and second, the ‘units’ can’t start to flash until the ‘tens’ have finished their sequence. Flashing the ‘units’ LED when the ‘tens’ LED is flashing is achieved by the ‘or’ node. But what about not starting the ‘units’ until the ‘tens’ are done? This is done Fig.2. The Arduino sketch for the Light Column Thermometer is written in XOD visual programming language. The sketch is fully explained in the main text, but in brief, Practical Electronics | January | 2021 the red box shows the temperature input nodes, the yellow box extracts the ‘tens’ and ‘units’ from the reading, the white box flashes the LED for the ‘units’ reading, the brown box flashes the LED for the ‘tens’ reading, the yellow box starts the ‘units’ only after the ‘tens’ have finished, and the blue box sets the cycle time. 65 ‘tens’ flashes and four ‘units’ flashes). Each flash takes 0.7 seconds (0.5 off and 0.2 on), giving a total of 4.2 seconds. Add to that the one-second delay between the ‘tens’ finishing flashing and the ‘units’ starting, and we have 5.2 seconds for the sequence. Since the clock resets at (in this example) six seconds, we have a 0.8 second delay at the end of the cycle before it starts again. Taking this approach means the cycle time adapts to the required number of flashes. As with all XOD sketches, this one is easily customised. For example, if you wanted the cycle time to be longer, you could put another ‘add’ node in above the ‘clock’ and add a further (say) five seconds to the cycle time. Note that as configured, the thermometer cannot show negative temperatures, nor degrees Fahrenheit. (Both of these are quite possible, though. For example, XOD even has available a direct units conversion node for Celsius/ Fahrenheit – you might want to try adding it to the sketch.) Spacers are used to hold the two parts of the enclosure 20mm apart. This gives enough room for the wiring of the temperature sensor and two LEDs to be placed within the enclosure. by the contents of the yellow box – that registers when the ‘tens’ have finished, adds a delay of one second, and then starts the ‘units’ flashing. The brown box comprises the logic for the ‘tens’ flasher; it uses the same approach as for the ‘units’. The blue box starts the cycle. It does this by adding the ‘tens’ and ‘units’ numbers together to gain the total required number of flashes. In this example that is six (two Conclusion I find the Light Column Thermometer fascinating to watch. It can be seen from across the other side of the room, and reading it is oddly relaxing – perhaps because you have to pause and watch, rather than just glance and run. It’s also an ideal project with which to discover the fascinating world of XOD. XOD files The XOD file discussed in this article can be downloaded from the January 2021 page of the PE website. 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Practical Electronics | January | 2021 67 Practical Electronics PCB SERVICE PROJECT JANUARY 2021 CODE utube Valve Preamplifier ................................................. 01112191 Arduino DCC Controller ..................................................... 09207181 PRICE £12.95 £10.95 DECEMBER 2020 Pseudo-Random Sequence Generator ............................. 16106191 £7.95 Clever Charger .................................................................. 14107191 £11.95 PE heremin Amplifier ....................................................... 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AO-0920-01 PE Theremin PSU transformer .......................................... AO-0920-02 Micromite Explore-28......................................................... 07108191 Ultrabrite LED Driver ......................................................... 16109191 AUGUST 2020 Micromite LCD BackPack V3 ............................................ 07106191 Steering Wheel Audio Button to Infrared Adaptor .............. 05105191 £8.95 £6.95 £5.95 £7.95 £5.95 £5.95 £7.95 £7.95 JULY 2020 AM/FM/CW Scanning HF/VHF RF Signal Generator ........ 04106191 £11.95 Speech Synthesiser with the Raspberry Pi Zero ............... 01106191 £5.95 PE Mini-organ PCB ........................................................... AO-0720-01 £14.95 PE Mini-organ selected parts ............................................ 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UPC0320 CODE Motion-Sensing 12V Power Switch ................................... 05102191 USB Keyboard / Mouse Adaptor........................................ 24311181 DSP Active Crossover (ADC) ............................................ 01106191 DSP Active Crossover (DAC) ×2 ...................................... 01106192 DSP Active Crossover (CPU) ............................................ 01106193 DSP Active Crossover (Power/routing).............................. 01106194 DSP Active Crossover (Front panel).................................. 01106195 DSP Active Crossover (LCD)............................................. 01106196 JANUARY 2020 Isolated Serial Link ............................................................ 24107181 DECEMBER 2019 Extremely Sensitive Magnetometer ................................... 04101011 Four-channel High-current DC Fan and Pump Controller ... 05108181 Useless Box ....................................................................... 08111181 NOVEMBER 2019 Tinnitus & Insomnia Killer (Jaycar case – see text) ........... 01110181 Tinnitus & Insomnia Killer (Altronics case – see text) ........ 01110182 OCTOBER 2019 Programmable GPS-synced Frequency Reference .......... 04107181 Digital Command Control Programmer for Decoders ........ 09107181 Opto-isolated Mains Relay (main board) ........................... 10107181 Opto-isolated Mains Relay (2 × terminal extension board)...10107182 JULY 2019 Full-wave 10A Universal Motor Speed Controller .............. 10102181 Recurring Event Reminder ................................................ 19107181 Temperature Switch Mk2 ................................................... 05105181 JUNE 2019 Arduino-based LC Meter ................................................... 04106181 USB Flexitimer................................................................... 19106181 MAY 2019 2× 12V Battery Balancer ................................................... 14106181 Deluxe Frequency Switch .................................................. 05104181 USB Port Protector ............................................................ 07105181 APRIL 2019 Heater Controller ............................................................... 10104181 MARCH 2019 10-LED Bargraph Main Board ........................................... 04101181 +Processing Board ............................................. 04101182 NOVEMBER 2018 Super-7 AM Radio Receiver .............................................. 06111171 £10.95 £8.50 £12.50 £5.95 £8.50 £29.95 £8.50 £16.75 £8.75 £11.50 £8.75 £8.75 £11.50 £8.75 £11.50 Brainwave Monitor ............................................................. 25108181 £12.90 Super Digital Sound Effects Module .................................. 01107181 £5.60 Watchdog Alarm ................................................................ 03107181 £8.00 PE Theremin (three boards: pitch, volume, VCA) ............. PETX0819 £19.50 PE Theremin component pack (see p.56, August 2019) ... PETY0819 £15.00 1.5kW Induction Motor Speed Controller........................... 10105122 £14.95 PRICE AUGUST 2019 FEBRUARY 2019 APRIL 2020 Flip-dot Display black coil board................................................. 19111181 Flip-dot Display black pixels ....................................................... 19111182 Flip-dot Display black frame ....................................................... 19111183 Flip-dot Display green driver board ............................................ 19111184 PROJECT FEBRUARY 2020 OCTOBER 2018 H ouchscreen requency Counter .......................... 04110171 Two 230VAC MainsTimers ................................................ 10108161 10108162 £12.90 £8.00 £10.45 £8.00 £10.45 £5.60 £10.45 £5.60 £14.00 £11.25 £8.60 £35.00 £27.50 £12.88 £12.88 PCBs for most recent PE/EPE constructional projects are available. From the July 2013 issue onwards, PCBs with eight-digit codes have silk screen overlays and, where applicable, are double-sided, have plated-through holes, and solder mask. They are similar to photos in the project articles. Earlier PCBs are likely to be more basic and may not include silk screen overlay, be single-sided, lack plated-through holes and solder mask. Always check price and availability in the latest issue or online. A large number of older boards are listed for ordering on our website. In most cases we do not supply kits or components for our projects. For older projects it is important to check the availability of all components before purchasing PCBs. Back issues of articles are available – see Back Issues page for details. 68 Practical Electronics | January | 2021 Double-sided | plated-through holes | solder mask PROJECT SEPTEMBER 2018 CODE 3-Way Active Crossover .................................................... 01108171 Ultra-low-voltage Mini LED Flasher ................................... 16110161 AUGUST 2018 PRICE £22.60 £5.60 Universal Temperature Alarm ............................................ 03105161 Power Supply For Battery-Operated Valve Radios ........... 18108171 18108172 18108173 18108174 £27.50 Touchscreen Appliance Energy Meter – Part 1 ................. 04116061 Automotive Sensor Modifier .............................................. 05111161 £17.75 £12.88 JULY 2018 JUNE 2018 High Performance 10-Octave Stereo Graphic Equaliser ... 01105171 MAY 2018 High Performance RF Prescaler........................................ 04112162 Micromite BackPack V2..................................................... 07104171 Microbridge ........................................................................ 24104171 APRIL 2018 Spring Reverberation Unit ................................................. 01104171 DDS Sig Gen Lid ............................................................... Black DDS Sig Gen Lid ............................................................... Blue DDS Sig Gen Lid ............................................................... Clear £7.05 £15.30 £10.45 £10.45 £5.60 £15.30 £8.05 £7.05 £8.05 MARCH 2018 Stationmaster Main Board ................................................. 09103171 + Controller Board .............................................. 09103172 SC200 Amplifier Module – Power Supply .......................... 01109111 FEBRUARY 2018 GPS-Synchronised Analogue Clock Driver ....................... 04202171 High-Power DC Motor Speed Controller – Part 2 + Control Board ................................................... 11112161 + Power Board .................................................... 11112162 JANUARY 2018 High-Power DC Motor Speed Controller – Part 1 .............. 11112161 Build the SC200 Amplifier Module ..................................... 01108161 DECEMBER 2017 Precision Voltage and Current Reference – Part 2............ 04110161 NOVEMBER 2017 50A Battery Charger Controller ......................................... 11111161 Micropower LED Flasher (45 × 47mm) ......................... 16109161 (36 × 13mm) ......................... 16109162 Phono Input Converter ...................................................... 01111161 SEPTEMBER 2017 Compact 8-Digit Frequency Meter..................................... 04105161 AUGUST 2017 Micromite-Based Touch-screen Boat Computer GPS ....... 07102122 Fridge/Freezer Alarm ......................................................... 03104161 JULY 2017 Micromite-Based Super Clock ........................................... 07102122 Brownout Protector for Induction Motors ........................... 10107161 JUNE 2017 Ultrasonic Garage Parking Assistant ................................. 07102122 Hotel Safe Alarm................................................................ 03106161 100dB Stereo LED Audio Level/VU Meter ......................... 01104161 £17.75 £16.45 PROJECT MAY 2017 CODE PRICE The Micromite LCD BackPack........................................... 07102122 Precision 230V/115V 50/60Hz Turntable Driver ................ 04104161 APRIL 2017 Microwave Leakage Detector ............................................ 04103161 Arduino Multifunctional 24-bit Measuring Shield ............... 04116011 + RF Head Board ................................................ 04116012 Battery Pack Cell Balancer ................................................ 11111151 MARCH 2017 Speech Timer for Contests & Debates .............................. 19111151 FEBRUARY 2017 Solar MPPT Charger/Lighting Controller ........................... 16101161 Turntable LED Strobe ........................................................ 04101161 JANUARY 2017 High-performance Stereo Valve Preamplifier .................... 01101161 High Visibility 6-Digit LED Clock ........................................ 19110151 £11.25 £19.35 £8.00 £17.75 £9.00 £16.42 £17.75 £7.60 £17.75 £16.42 For the many pre-2017 PCBs that we stock please see the PE website: www.electronpublishing.com PE/EPE PCB SERVICE Order Code Project Quantity Price ......................................................... £12.88 ......................................................... £12.88 £15.30 ......................................................... £12.88 £12.88 ......................................................... £15.35 £12.88 £8.00 £5.60 £8.00 ......................................................... Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......................................................... Tel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Email . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I enclose payment of £ . . . . . . . . . . . . . . (cheque/PO in £ sterling only) £12.88 £10.45 £8.00 £10.45 £12.90 £10.45 £8.00 £17.75 payable to: Practical Electronics Card No . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Valid From . . . . . . . . . . . . . . . . . Expiry Date . . . . . . . . . . . . . . . . Card Security No . . . . . . . . . . You can also order PCBs by phone, email or via the shop on our website: www.electronpublishing.com No need to cut your issue – a copy of this form is just as good! All prices include VAT and UK p&p. Add £4 per project for post to Europe; £5 per project outside Europe. Orders and payment should be sent to: Practical Electronics, Electron Publishing Ltd 113 Lynwood Drive, Merley, Wimborne, Dorset BH21 1UU Tel 01202 880299 Email: shop<at>electronpublishing.com On-line Shop: www.epemag.com Cheques should be made payable to ‘Practical Electronics’ (Payment in £ sterling only). NOTE: Most boards are in stock and sent within seven days of receipt of order, please allow up to 28 days delivery if we need to restock. Practical Electronics | January | 2021 69 DIRECT BOOK SERVICE Teach-In 2017 The books listed here have been selected by the Practical Electronics editorial staff as being of special interest to everyone involved in electronics and computing. They are supplied by mail order direct to your door. Introducing the BBC micro:bit FOR A FULL DESCRIPTION OF THESE BOOKS AND CD-ROMS SEE THE SHOP ON OUR WEBSITE PYTHON CODING ON THE BBC MICRO:BIT Jim Gatenby www.electronpublishing.com Python is the leading programming language, easy to learn and widely used by professional programmers. This book uses MicroPython, a version of Python adapted for the BBC Micro:bit. All prices include UK postage Among the many topics covered are: main features of the BBC micro:bit including a simulation in a web browser screen; various levels of programming languages; Mu Editor for writing, saving and retrieving programs, with sample programs and practice exercises; REPL, an interactive program for quickly testing lines of code; scrolling messages, creating and animating images on the micro bit’s E s playing and creating music, sounds and synthesized speech; using the on-board accelerometer to detect movement of the micro:bit on three axes; glossary of computing terms. This book is written using plain English, avoids technical jargon wherever possible and covers many of the coding instructions and methods which are common to most programming languages. It should be helpful to beginners of any age, whether planning a career in computing or writing code as an enjoyable hobby. 118 Pages Order code PYTH MBIT £7.99 Not just an educational resource for teaching youngsters coding, the BBC micro:bit is a tiny low cost, low-profile A -based single-board computer. The board measures mm mm but despite its diminutive footprint it has all the features of a fully fledged microcontroller together with a simple LED matrix display, two buttons, an accelerometer and a magnetometer. ike Tooley’s book will show you how the micro bit can be used in a wide range of applications from simple domestic gadgets to more complex control systems such as those used for lighting, central heating and security applications. Using Microsoft Code Blocks, the book provides a progressive introduction to coding as well as interfacing with sensors and transducers. Each chapter concludes with a simple practical project that puts into practice what the reader has learned. The featured projects include an electronic direction finder, frost alarm, reaction tester, battery checker, thermostatic controller and a passive infrared (PIR) security alarm. No previous coding experience is assumed, making this book ideal for complete beginners as well as those with some previous knowledge. Self-test questions are provided at the end of each chapter, together with answers at the end of the book. So whatever your starting point, this book will take you further along the road to developing and coding your own real-world applications. 108 Pages PRACTICAL ELECTRONICS HANDBOOK – 6th Ed Ian Sinclair 440 pages Order code NE21 £33.99 STARTING ELECTRONICS – 4th Ed Keith Brindley 296 pages Order code ELSEV100 Order code TF43 Order code TF47 £32.99 £21.99 A BEGINNER’S GUIDE TO TTL DIGITAL ICs Robert Penfold 142 pages OUT OF PRINT BP332 £5.45 UNDERSTANDING ELECTRONIC CONTROL SYSTEMS Owen Bishop 228 pages 70 Order code NE35 Order code NE48 £34.99 £7.99 496 pages + CD-ROM Order code NE45 £38.00 INTRODUCTION TO MICROPROCESSORS AND MICROCONTROLLERS – 2nd Ed John Crisp 222 pages Order code NE31 £29.99 THE PIC MICROCONTROLLER YOUR PERSONAL INTRODUCTORY COURSE – 3rd Ed John Morton 270 pages Order code NE36 £25.00 PIC IN PRACTICE – 2nd Ed David W. Smith 308 pages Order code NE39 £24.99 MICROCONTROLLER COOKBOOK Mike James 240 pages Order code NE26 £36.99 B O O K O R D E R I N G D E TA I L S For postage, add £3 per book to Europe, £4 for rest of the world per book. CD-ROM prices include VAT and/or postage to anywhere in the world. FUNDAMENTAL ELECTRICAL AND ELECTRONIC PRINCIPLES – 3rd Ed C.R. Robertson 368 pages 298 pages All prices include UK postage. £18.99 ELECTRONIC CIRCUITS – FUNDAMENTALS & APPLICATIONS – Updated version Mike Tooley 400 pages Order code BBC MBIT INTERFACING PIC MICROCONTROLLERS – 2nd Ed Martin Bates PROGRAMMING 16-BIT PIC MICROCONTROLLERS IN C – LEARNING TO FLY THE PIC24 Lucio Di Jasio (Application Segments Manager, Microchip, USA) GETTING STARTED WITH THE BBC MICRO:BIT Mike Tooley THEORY AND REFERENCE MICROPROCESSORS Send a cheque, (£ sterling only) made payable to: Practical Electronics or credit card details (Visa or Mastercard) to: Electron Publishing Limited, 113 Lynwood Drive, Wimborne, Dorset BH21 1UU Books are normally sent within seven days of receipt of order. Please check price (see latest issue of Practical Electronics or website) before ordering from old lists. For a full description of these books please see the shop on our website. Tel: 01202 880299 – Email: shop<at>electronpublishing.com Order from our online shop at: www.electronpublishing.com £36.99 Practical Electronics | January | 2021 ARDUINO COMPUTING AND ROBOTICS ARDUINO FOR DUMMIES NEWNES INTERFACING COMPANION Tony Fischer-Cripps John Nussey Arduino is no ordinary circuit board. Whether you’re an artist, a designer, a programmer, or a hobbyist, Arduino lets you learn about and play with electronics. You’ll discover how to build a variety of circuits that can sense or control real-world objects, prototype your own product, and even create interactive artwork. This handy guide is exactly what you need to build your own Arduino project – what you make is up to you! n Learn by doing – start building circuits and programming your Arduino with a few easy examples – right away! n Easy does it – work through Arduino sketches line by line, and learn how they work and how to write your own. n Solder on! – don’t know a soldering iron from a curling iron? No problem! You’ll learn the basics and be prototyping in no time. n Kitted out – discover new and interesting hardware to turn your Arduino into anything from a mobile phone to a Geiger counter. n Become an Arduino savant – find out about functions, arrays, libraries, shields and other tools that let you take your Arduino project to the next level 295 pages Order code NE38 120 pages 128 pages Order code BP542 Order code BP722 Arduino can take you anywhere. This book is the roadmap. Exploring Arduino shows how to use the world’s most popular microcontroller to create cool, practical, artistic and educational projects. Through lessons in electrical engineering, programming and human-computer interaction, this book walks you through specific, increasingly complex projects, all the while providing best practices that you can apply to your own projects once you’ve mastered these. You’ll acquire valuable skills – and have a whole lot of fun. n Explore the features of commonly used Arduino boards n Use Arduino to control simple tasks or complex electronics n Learn principles of system design, programming and electrical engineering n Discover code snippets, best practices and system schematics you can apply to your original projects n Master skills you can use for engineering endeavours in other fields and with different platforms 357 Pages Order code EXPARD01 £7.99 Order code BP721 £7.99 £7.50 Order code BP705 £8.49 Order code BP703 £8.49 £8.49 WINDOWS 8.1 EXPLAINED 180 Pages Order code BP747 £10.99 AN INTRODUCTION TO THE NEXUS 7 118 Pages Order code BP744 Order code NE46 £26.00 288 pages + Order code BP901 £14.99 298 pages Order code BP902 £14.99 128 pages Order code BP705 £8.49 120 pages Order code BP708 £8.49 120 pages Order code BP704 £8.49 WINDOWS 8.1 EXPLAINED Noel Kantaris AN INTRODUCTION TO WINDOWS VISTA P.R.M. Oliver and N. Kantarris 120 pages 366 pages GETTING STARTED IN COMPUTING FOR THE OLDER GENERATION Jim Gatenby HOW TO FIX YOUR PC PROBLEMS Robert Penfold 128 pages £16.99 WINDOWS 7 – TWEAKS, TIPS AND TRICKS Andrew Edney Order code BP716 Order code BP709 Order code MGH1 HOW TO FIX YOUR PC PROBLEMS Robert Penfold AN INTRODUCTION TO eBAY FOR THE OLDER GENERATION Cherry Nixon 120 pages 224 pages MORE ADVANCED ROBOTICS WITH LEGO MINDSTORMS Robert Penfold THE INTERNET – TWEAKS, TIPS AND TRICKS Robert Penfold 128 pages £8.99 INTRODUCING ROBOTICS WITH LEGO MINDSTORMS Robert Penfold £7.99 Order code BP514 eBAY – TWEAKS, TIPS AND TRICKS Robert Penfold Jeremy Blum £8.99 WINDOWS XP EXPLAINED N. Kantaris and P.R.M. Oliver 264 pages Order code BP601 ROBOT BUILDERS COOKBOOK Owen Bishop FREE DOWNLOADS TO PEP-UP AND PROTECT YOUR PC Robert Penfold 438 Pages EXPLORING ARDUINO £8.49 EASY PC CASE MODDING Robert Penfold 192 pages + CD-ROM 308 pages ANDROIDS, ROBOTS AND ANIMATRONS Second Edition – John Iovine Order code BP707 128 pages £19.99 £41.00 HOW TO BUILD A COMPUTER MADE EASY Robert Penfold n Get social – teach your Arduino to communicate with software running on a computer to link the physical world with the virtual world Order code ARDDUM01 COMPUTING FOR THE OLDER GENERATION Jim Gatenby £8.99 £26.99 BOOK ORDER FORM Full name: ....................................................................................................................................... Address: .......................................................................................................................................... ......................................................................................................................................................... .............................................. Post code: ........................... Telephone No: .................................... Email: .............................................................................................................................................. ¨ I enclose cheque/PO payable to Practical Electronics for £ ......................................................... ¨ Please charge my card £ ....................................... Card expiry date......................................... Card Number ..................................................................................... Valid From Date ................ Card Security Code ............... (The last three digits on or just below the signature strip) Please send book order codes: ....................................................................................................... ......................................................................................................................................................... .......................................................................................................................................................... Please continue on separate sheet of paper if necessary 180 Pages Order code BP747 £10.99 COMPUTING WITH A LAPTOP FOR THE OLDER GENERATION Robert Penfold 120 pages Order code BP702 £8.49 AN INTRODUCTION TO EXCEL SPREADSHEETS Jim Gatenby 18 pages Order code BP701 £8.49 KINDLE FIRE HDX EXPLAINED 118 Pages Order code BP743 £8.99 THE BASIC SOLDERING GUIDE LEARN TO SOLDER SUCCESSFULLY! ALAN WINSTANLEY The No.1 resource for learning all the basic aspects of electronics soldering by hand. With more than 80 high quality colour photographs, this book explains the correct choice of soldering irons, solder, fluxes and tools. The techniques of how to solder and desolder electronic components are then explained in a clear, friendly and nontechnical fashion so you’ll be soldering successfully in next to no time! The book also includes sections on reflow soldering and desoldering techniques, potential hazards, useful resources and a very useful troubleshooting guide. Also ideal for those approaching electronics from other industries, the Basic Soldering Guide Handbook is the best resource of its type, and thanks to its excellent colour photography and crystal clear text, the art of soldering can now be learned by everyone! 86 Pages Order code AW1 £9.99 VISIT OUR WEBSITE FOR MORE BOOKS AND FAST, EASY ONLINE ORDERING: www.electronpublishing.com Practical Electronics | January | 2021 71 Next Month – in the February issue Remote Monitoring Station If you have an expensive car, boat, caravan, holiday house, farm… you need to know what’s going on when you are away. Is the battery going flat? Is your boat taking on water? All you need to do this is a couple of Arduino shields and a little software. You can even remotely trigger actions, such as switching off misbehaving equipment! Using Cheap Electronic Modules Next month, we’re looking at an 8-channel USB logic analyser. It is essentially a clone of the original version of the well-known and respected Saleae Logic unit. It’s completely compatible with the Saleae design, but you can get this one for as little as £6 – much less than one-twentieth the cost! Low Distortion DDS Signal Generator This two-channel audio signal generator produces very low distortion sinewaves, triangle waves, square waves, pulse trains and noise. It has adjustable output frequency, amplitude and phase, plus sweep and pulse modes. It’s ideal for testing amplifiers, loudspeakers and all sorts of audio equipment, as well as for general purpose use. Air Quality Monitor Measure air quality with a volatile organic compound (VOC) meter. The mighty Micromite BackPack and a cheap module make building one of these dead easy! PLUS! All your favourite regular columns from Audio Out, Cool Beans and Circuit Surgery, to Make it with Micromite, Practically Speaking and Net Work. On sale 7 January 2021 Content may be subject to change Welcome to JPG Electronics Selling Electronics in Chesterfield for 29 Years Open Monday to Friday 9am to 5:30pm And Saturday 9:30am to 5pm • Aerials, Satellite Dishes & LCD Brackets • Audio Adaptors, Connectors & Leads • BT, Broadband, Network & USB Leads • Computer Memory, Hard Drives & Parts • DJ Equipment, Lighting & Supplies • Extensive Electronic Components - ICs, Project Boxes, Relays & Resistors • Raspberry Pi & Arduino Products • Replacement Laptop Power Supplies • Batteries, Fuses, Glue, Tools & Lots more... Shaw’s Row T: 01246 211 202 E: sales<at>jpgelectronics.com JPG Electronics, Shaw’s Row, Old Road, Chesterfield, S40 2RB W: www.jpgelectronics.com Britannia Inn JPG Electronics Maison Mes Amis Old H all Ro ad Old Road Rose & Crown orth tsw Cha Johnsons d Roa Morrisons Sparks Retail & Trade Welcome • Free Parking • Google St View Tour: S40 2RB NEW subscriptions hotline! Practical Electronics We have changed the way we sell and renew subscriptions. We now use ‘Select Publisher Services’ for all print subscriptions – to start a new subscription or renew an existing one you have three choices: 1. Call our NEW print subscription hotline: 01202 087631, or email: pesubs<at>selectps.com 2. Visit our shop at: www.electronpublishing.com 3. Send a cheque (payable to: ‘Practical Electronics’) with your details to: Practical Electronics Subscriptions, PO Box 6337, Bournemouth BH1 9EH, United Kingdom Remember, we print the date of the last issue of your current subscription in a box on the address sheet that comes with your copy. Digital subscribers, please call 01202 880299 or visit: www.electronpublishing.com Published on approximately the first Thursday of each month by Electron Publishing Limited, 1 Buckingham Road, Brighton, East Sussex BN1 3RA. Printed in England by Acorn Web Offset Ltd., Normanton WF6 1TW. Distributed by Seymour, 86 Newman St., London W1T 3EX. Subscriptions UK: £26.99 (6 months); £49.85 (12 months); £94.99 (2 years). EUROPE: airmail service, £30.99 (6 months); £57.99 (12 months); £109.99 (2 years). REST OF THE WORLD: airmail service, £37.99 (6 months); £70.99 (12 months); £135.99 (2 years). Payments payable to ‘Practical Electronics’, Practical Electronics Subscriptions, PO Box 6337, Bournemouth BH1 9EH, United Kingdom. Email: pesubs<at>selectps.com. PRACTICAL ELECTRONICS is sold subject to the following conditions, namely that it shall not, without the written consent of the Publishers first having been given, be lent, resold, hired out or otherwise disposed of by way of Trade at more than the recommended selling price shown on the cover, and that it shall not be lent, resold, hired out or otherwise disposed of in a mutilated condition or in any unauthorised cover by way of Trade or affixed to or as part of any publication or advertising, literary or pictorial matter whatsoever. 72 Practical Electronics | January | 2021 Did you know our online shop now sells the current issue of PE for £4.99 inc. p&p? Practical Electronics Prac Electro tical nics The UK’s premier electronics and computing maker magazine The K PIC n’ Mix Make it with Micromite Audio U Out’s pCircuit remierSurgery electro PIC18F development Counting pulses and Germanium Interference PIC n’ nics an Mix er and noise board rotary encoders PBJT IC18amplifi d com Fd Make puting board evelopmen it with M t Co maker unting icromit magaz Toot toot! p e u ls ro es and A tary e ine WIN! u d io ncod Out T ers oot to Microchip SAM IoT WG Comp ot! Development DCC Clete ArdBoard uino ontrol WIN! ler NeoPixel gamma correction Complete Arduino DCC Controller WIN! Microc SAM Io hip T Develo WG pme Board nt WIN! N eo P i x gamm el correc a ti o n 7 6 5 4 3 2 1 0 Amazing Nutube miniature valve stereo preamplifiAermazing Nu valve tube m Fascinating stere o pream iniature XOD/Arduino plifier Fun LED thermometer Christmas Fun LE Tree offer! Christ D Tree o mas PLUS! ffer!Jan 2021 £4.99 P LU S ! Techno Talk – Lights out for dangerous bulbs? SCL SDA AREF GND 13 12 11 10 9 8 Germa Circu n BJT am ium Interf it Surgery ere plifier and n nce oise DIGITAL POWER ANALOG IN 5V RES 3.3V 5V GND GND VIN A0 A1 A2 A3 A4 A5 UNO UNO Techn o Talk MOSFET KickStart – NEW series! – using the 2N7000 –L KickSt art – N TVights out fo9 772632 573016 Net Work – New electric cars and Android r dang EW se Net W ries! – erous ork – N bulbs? u s ing e w www.electronpublishing.com <at>practicalelecelect practicalelectronics ric cars the 2N7000 www.e MOSF lectro a n d npubli ET Andro shing id TV .com 01 IN A1 A2 A3 A4 A5 ANALOG Fascin XOD/ ating A therm rduino omete r <at>prac ticalele c practi Jan 20 21 £4.99 01 9 772 632 5 73016 calele ctronic s You read that right! We now sell the current issue of your favourite electronics magazine for exactly the same price as in the High Street, but we deliver it straight to your door – and for UK addresses we pay the postage. No need to journey into town to queue outside the newsagent. Just go to our website, set up an account in 30 seconds, order your magazine and we’ll do the rest. www.electronpublishing.com