The Joule thief is a really fascinating circuit, simple yet very intricate. Basically, it’s a step-up converted in its most elementary expression. I will spare you the theory since there is plenty of information on it on the web; rustybolt.info is a good place to start.
Joule thieves in all sorts of forms have been featured countless time on DIY websites and I felt it was time I build one. However, I did not want to leave the circuit at the breadboard stage because as it stands, the joule thief has characteristics that make it very attractive for all sorts of low power applications and I figured a flash light would be a very good home for a joule thief, where having the option of using dead batteries is certainly a big plus not to mention using less cells because the circuit steps the voltage up. Why dead batteries? Because a battery is never really dead, its voltage just falls down logarithmically until it hits a point where the device it was powering up stops functioning, which does not mean the battery is totally drained but rather that its voltage has fallen below a usable level. Since joule thieves are step-up converters, they can take that “dead” battery, and give it a new life by stepping up its output voltage to usable levels again.
For my flashlight, I opted for a maglite body for its sturdiness and simplicity. I have been using those for years and they have served me well, but with traditional incandescent lamp bulbs (I do know they make LED versions now), they eat through batteries like crazy. So the challenge was to convert a 2 AA battery maglite so it could run off a joule thief circuit and a single AA but could easily get converted back to using a lamps(As I will tell later, the joule thief’s light output is not so strong … sufficient in most cases but not strong).
For more detail: Maglite Joule thief