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Contents SILICON CHIP www.siliconchip.com.au Vol.25, No.3; March 2012 Features 16 Vertical Farms: Factory Farming In The 21st Century The earth has seven billion people to feed but arable land is decreasing. This Australian technology combines bioponics, LED illumination & computer control to grow food in vertically-stacked containers – by Leo Simpson 84 Review: Audio Precision APx525 Audio Analyser Audio Precision’s latest analyser relies heavily on digital signal processing via a host PC and features HDMI and Bluetooth interfaces – by Allan Linton-Smith & Nicholas Vinen 96 Q & A On The MPPT Solar Charger Detailed answers to five reader questions – by John Clarke Interplanetary Voice For Alien Sound Effects – Page 24. 98 The Story Of Atwater Kent Radios From The 1920s Atwater Kent was a legendary brand and their radios were sold in large numbers in the USA and also in Australia – by Kevin Poulter Pro jects To Build 24 Interplanetary Voice For Alien Sound Effects Be sure to blend in amongst diabolical characters at your next interplanetary meeting. Build this Interplanetary Voice alien sound effects unit and never be caught out of your galaxy again – by John Clarke 32 Ultra-LD Mk.3 135W/Channel Stereo Amplifier, Pt.1 Ultra-LD Mk.3 135W/Channel Stereo Amplifier, Pt.1 – Page 32. Based on the series of high-performance modules described last year, this superb stereo amplifier can deliver 135W RMS per channel into 8Ω and boasts extremely low noise and distortion figures – by Greg Swain 70 SemTest: A Discrete Semiconductor Test Set; Pt.2 Second article has the full circuit details of this versatile semiconductor test unit – by Jim Rowe 90 12/24V MPPT Solar Charge Controller Rev.1 The 3-stage MPPT Solar Charge Controller described in February 2011 has been a popular project. Here’s how to make some useful improvements to its efficiency and add some extra charging options – by John Clarke Special Columns SemTest Discrete Semiconductor Test Set, Pt.2 – Page 70. 44 Serviceman’s Log Lots of reasons for internet problems – by the Serviceman 65 Circuit Notebook (1) PICAXE-Based Garage Door Sentinel; (2) Improvements To The Milliohm Adaptor; (3) Measuring Short Intervals With The Maximite; (4) Numeracy Trainer For Young Children Departments   4 Publisher’s Letter   6 Mailbag 43 Order Form siliconchip.com.au 106 Ask Silicon Chip 109 Notes & Errata 111 Market Centre Efficiency Improvements For The MPPT Solar Charge Controller – Page 90 March 2012  1