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Is this one of Stan’s wind-ups? Emergency power, when all else fails... B ack in the 20th century, it used to be said that you really knew you were a parent when battery costs for your kid’s toys exceeded monthly power bills. Fortunately, recent spectacular improvements in rechargeable technology now offer cost-effective secondary batteries and chargers – and at keen prices. NiMH “AA” cell energy capacities have near tripled from 750mAh to well over 2000mAh since the year 2000. It’s assumed you’ll have a nearby mains outlet for recharging – but many occasions arise (commuting in a peakhour train maybe) when you’re away from such facilities but with hi tech toys crying for a top up. Such an occasion arose recently, when bad weather meant a mate on a weekend walkabout, hunting in a nearby but isolated NZ mountain region became hut-bound for nearly a week – trapped by a flooded river. Although warm and dry indoors, the batteries in his mobile phone, radio and torch all progressively ran flat. Even his digital camera, intended for the 12-pointer deer he’d hoped to shoot, gave up! Perhaps a solar panel would have helped, although the scudding winter rain clouds, which caused the flood in the first place, rather hinted otherwise. In despair at his “Joule less” plight, he even took to the old trick of warming batteries in the oven and chewing their outer casings to persuade a few more electrons to flow… At least he had an ultra bright white LED torch! Let’s face it – these efficient lighting devices have been one of the most benevolent developments in 94  Silicon Chip by Stan Swan Above: the Benex Dynamo Torch, from Jaycar, which forms the basis of our emergency power plant. Below, winding the handle produces the power to charge the batteries. siliconchip.com.au The opened-up torch, showing where we “tapped in” to the rectified dynamo supply (blue arrows). It’s not difficult to do and the full functions of the torch are retained. Of course any neat output socket could be used but simple red and black banana types were found most versatile – space inside the torch just allows for these – and likely to be the most useful with cold hands or broken connectors. The torch’s existing lighting circuitry remains quite unchanged, so naturally it still operates as a very bright and ergonomic LED torch. Performance? decades. But even they typically only offer 80 hours on “lite” mode – perhaps a week of evening use if half charged. Yes, all very inconvenient – but fortunately just as Search and Rescue were being briefed the lost hunter reappeared. Improvise . . . or starve! Back at civilisation, the challenge was put to me: “Fix me up a reliable charger, or no venison next trip”. A past SILICON CHIP article on modified disk drive generators sprang to mind but their output looked limited to LED lighting only. Additionally, bush tramping demands near-bullet-proof devices, otherwise rain and mud may rule over fine engineering. Marine emergencies further throw their own brand of cruelty and inconvenience… Short of carrying in a generator, PV panel and SLA gel cell, rigging a thermocouple to the fireplace or poking a wind turbine above the bush line, just what other ready approaches exist to generate a few crucial watts? Digital smoke signals? output of a good half amp. Impressive, although perhaps rather optimistic unless the LEDs are pulsed and actually draw less current! Disassembly of the torch revealed a sturdy brushed geared electric motor, measured as providing about 9V DC at some 200-300mA to a simple 12V lamp dummy load. While not 0.5A, this output (say 6V <at> 0.25A) hence satisfies the magical 1W energy budget our emergency quest demands. A 4-diode rectifier bridge on the lamp’s PC board ensures unidirectional generator output (along with two 0.6V voltage drops), and soldering across two diodes readily allows external connections to be made. For even more efficiency you could replace the four diodes with suitably rated Schottky diodes, with their much lower (~0.1V) forward voltage drop. However, these devices are fairly expensive. Many mobile phones and UHF transceivers now idle at just a few milliamps on squelched receive, so almost a 50:1 energy benefit could result. Typically, a minute’s winding (about all you’d get away with on a peakhour train before dirty looks develop!) should extend reception by about half an hour. Outgoing transmissions will be power hogs, so text messages may be the best energy investment (although a “minutes winding for a minutes talk” may be tolerable in an emergency). As a bonus the 5V or so delivered by this modified “1 Watt Wonder” can also charge up to four normal AA or AAA NiMH/NiCds. You need to appreciate charging maths – dead flat cells, of say 1200mAh capacity, will need hours of unrealistic winding to bring to full charge. But even five minutes may be enough to persuade your digital camera to snap that trophy shot, find your dropped car keys, phone your mates or – phew – call the rescue chopper. SC I see the light! Here’s where I got lucky: a call to Jaycar Electronics revealed a Swiss-designed BENEX Dynamo LED torch – Cat ST3337 – amongst their superb lighting range, priced around $AU30. Aside from the torch’s normal multiple 15,000mCd lighting options, a sturdy handle folds out to hand charge the unit’s internal 3.6V Li-Ion battery. Claims that “1 minute wind-up = 30 minutes lighting” imply a 30:1 charge/ discharge ratio, so that a 20-30mA drain LED may indicate a dynamo siliconchip.com.au We found a pair of banana sockets (polarised of course) with matching banana plugs gave the best result. There’s room inside the torch for a variety of small sockets. November 2004  95