Joule Thief
Made a “Joule Thief” with my son this morning. Able (barely) to light a 3V white LED from a very depleted AA cell with 0.66V remaining. He insisted on using a relatively fresh D cell. Since that is pulling about 50mA now and will pull less as the battery wears out, and Duracell’s datasheet indicates 50 hour life at 250mA constant current, I figure he has at least 250 hours of illumination available. Since it has only a momentary pushbutton switch, I have my doubts that he’ll have the patience to use up even the one battery—at least unless he replaces the switch.
When we started the project, I wasn’t sure whether I needed a ferrite toroid or could get away with a steel toroid. I didn’t have a ferrite toroid handy, so to test I made a steel toroid from fence wire I had handy. But that saturated far too quickly and didn’t work at all. Then I found out that the old CFLs that I hadn’t disposed of have ferrite toroids in them for their power supplies, so I carefully harvested one. Also other potentially useful components in there: high-voltage capacitors (one electrolytic, five polypropylene), a PTC (no visible rating), a fusible resistor, several 1N4007 diodes, some sort of transformer, and a few unmarked parts.
Took ideas from http://makezine.com/projects/vampire-flashlight/ (like finding a toroid in a CFL) and http://makezine.com/projects/joule-thief-battery-charger/ (protecting the 2N2222 with a Zener diode) and built it on a 2cm x 8cm strip of perfboard. Stuffed the D cell and perfboard into a used toilet paper roll to be the body of the flashlight. ☺
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