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Salvage It! BY JULIAN EDGAR Building a human-powered LED torch for next to nothing Would you like to have a torch where you wind a knob just a few times and a white LED stays on brightly for two minutes and then remains visible for hours? Well you can and the only parts that you’ll have to buy new are the LED and the box to mount everything in. Y OU’LL NEED A VARIETY of components from different salvaged goods to make this design, so it’s one to keep in mind as you collect bits and pieces over a period. First, you need the turntable motor from a microwave oven. This is an AC synchronous motor that’s about 20mm high and 50mm in diameter. In addition to the motor, inside the package is a system of plastic reduction gears that normally gives an output shaft speed of just 5 RPM (or thereabouts). By turning this shaft with a knob, it’s possible to easily generate up to several hundred volts AC output! There’s our power source. WARNING! Exercise extreme caution when salvaging parts from a microwave oven. The large capacitors in the EHT (extra high tension) power supply can retain a lethal charge, even after the power has been switched off. Although these capacitors should be discharged by bleeder resistors when power is removed, don’t take it for granted. Older microwave ovens may not be fitted with bleeder resistors, or the resistors may have gone open circuit. For this reason, always make sure that the capacitors in the EHT supply have been discharged before removing parts from a microwave oven. Plugpack bits Next, a small transformer is need to step that voltage down to something that can be rectified (ie, converted to DC) and used to drive a LED. This can be done using one or more diodes or a bridge rectifier. And guess what – inside any older plugpack you’ll find just those components, already wired up and ready to go! Don’t pick a recent lightweight plugpack, though – these use switchmode circuits that don’t work in this application. You can also recognise a switchmode design by the large number of internal components. We tried a variety of older 240V plugpacks with transformers and those with nominal outputs in the range of 6-12V DC all worked well. Energy storage The primary components needed are the turntable motor from a microwave oven, an old plugpack and some high-value, low-voltage capacitors. These salvaged parts shouldn’t total more than a few dollars but be careful when salvaging the turntable motor – the bite from the EHT circuitry in a microwave oven can be lethal, even with the power off (see warning in article). siliconchip.com.au To store the power you’ve generated, you need lots of capacitors. In addition to being small enough to fit inside your chosen box, these should have as much capacitance as possible, while having a voltage rating of about 10-16V. Several 10,000uF 10V capacitors are ideal, for example, but it doesn’t matter if you use 10,000uF 16V caps instead. Electrolytic capacitors are January 2006  109 While it looks as though the parts might cost a fortune, all you need to buy are the high-brightness LED, the box and possibly the trimpot. The alternator and gearbox come from a discarded microwave oven, the transformer and rectifier diode from a salvaged plugpack and the capacitors from a wide range of junked electronic equipment. This “optioned-up” version also includes a neon lamp (salvaged from a cook-top) and a lens from a discarded video camera. used in nearly every piece of discarded electronics equipment – always keep an eye out for large-value low-voltage units to salvage. A 10-100kW trimpot (preferably multi-turn) will make it easy to set the LED current. Again, these can be salvaged from lots of gear but failing that, are cheap to buy new. Another essential item is a suitable knob, so that the shaft of the motor/ gearbox can be turned by hand. This knob will need to fit a D-shaped shaft and if you can’t salvage one for nothing, you’re not really trying! White LED You also need a white LED and this will probably have to be purchased. A 5mm high-brightness white LED works well. However, if you want more light output and are prepared to turn the knob more often, higher-rated units can be used. For example, with this design, a 1W Luxeon LED can be strongly il- luminated, although not to full brightness. However, the more powerful the LED, the shorter the time the capacitor pack will keep it on after you’ve stopped turning the knob. With the 1W Luxeon, the capacitor pack will drive the LED for less than a second, so in many ways a lower rated LED is more practical. (Note: if you use a powerful LED, you should uprate the power rating of the current limiting resistor.) Finally, if you want to build the “optioned-up” model, you’ll need a lens (one salvaged from an old video camera is perfect) and a neon indicator from an old stove or cook-top. Building it To allow testing, roughly assemble the electronic circuitry for the torch on the bench before building the final version into a box. The first step is fit the knob to the shaft. In my case, I used a knob taken from the dashboard heater controls of an old car. That done, connect a multimeter set to “AC Volts” to the output of the motor (now working as an alternator!). Now turn the knob but make sure that you’re not touching the output terminals. You should get a no-load output of 100–200V, depending on how fast you turn the knob. If you come in contact with the output when you are turning the knob, you will give yourself a shock, so be careful! Note that you should never try to wind the knob flat out – you’ll strip the gears inside if you do. Instead, just turn the knob progressively and evenly at a slow speed (the optioned-up model has this aspect covered)! Before pulling the plugpack apart, follow this simple procedure to check that it is suitable. First, connect the plugpack’s mains input terminals to the alternator outputs (polarity doesn’t matter). That done, connect the plugpack’s output to the capacitor pack, Fig.1: a salvaged microwave turntable motor is used as an alternator to generate high-voltage AC. This is then fed to a plugpack transformer and rectified to produce 6-12V DC by the innards of a plugpack. The capacitors are used for energy storage, while the trimpot allows the current through the LED to be adjusted to its rated value (or less). 110  Silicon Chip siliconchip.com.au Here’s the non-optioned version. It’s pretty simple to look at but by turning the knob a few times, you can have the LED shining brightly for about two minutes and then remaining visible for hours. taking care to connect negative output lead (usually black or non-striped) to the negative side of the pack. Now connect your multimeter (set to Volts DC) across the capacitor pack and turn the knob. You should be able to read a voltage that gradually rises as you keep on turning. Any voltage from about 6-12V is fine. The trimpot (wired as a variable resistor) and the LED can now be added to the circuit. The circuit will look like Fig.1, except you have to add your multimeter to measure the LED current. To do this, simply wire the multimeter (set to milliamps DC) in series with the LED. Next, set the trimpot to its highest resistance and turn the alternator knob 10 times. It’s then just a matter of slowly adjusting the trimpot until the maximum current rating of the LED is reached. For example, if the maximum current rating of the LED is 100mA, set the trimpot to provide this current flow. Check that further turning the alternator knob doesn’t cause the required value to be exceeded. Alternatively, you may want to set the trimpot so that the LED operates at less than full brightness, so that it stays on longer after you stop turning the knob. Final assembly The plugpack can be opened to retrieve the parts by crushing the case slowly in a bench vice until it cracks. That done, you can build the unit into an off-the-shelf jiffy box. Seal the box siliconchip.com.au (eg, with silicone sealant) if the torch is to be used in the rain or in wet areas. And the optioned-up model? Well, it includes a neon lamp wired straight across the alternator. This lights at any voltage over 70-100V (the voltage depends on the neon lamp) and so it’s a good guide as to when the knob is being turned quickly enough to generate sufficient power. In practice, it should only just light. Neon indicators salvaged from stoves and cook-tops already have a series resistor built into their bodies, in which case you can just wire it straight in. The second option is to add a lens. Our prototype used a video camera lens, supported by a cut-down section from a Portaflood light. This is ideal if you want a long narrow beam – the prototype has a beam range of at least SC 100 metres! This is the “optioned” version. Again, it’s just a matter of turning the knob a few times to get the LED shining. Other Versions In the February 2004 issue of SILICON CHIP we covered a different design of human-powered torch. That approach used a direct-drive stepper motor as the power source. So what are the advantages of taking the approach shown here? Because of the built-in gearing of the microwave oven motor, you can generate much more power in a shorter time – just a few turns of the knob will keep the LED brightly lit for a reasonable period. However, the use of a gearbox also has downsides – when being wound, the torch is noisier than a stepper motor design and the plastic gears have a finite life. Rat It Before You Chuck It! Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you have a use for the highquality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! January 2006  111