* My 4 year old daughter loves flashlights, but often forgets to turn them off, and all of our flashlights use D cells or multiple AAs.
* We have a large (20+) collection of “mostly dead” batteries that I feel kind of bad just tossing or recycling
Both these problems are nicely solved by the Joule Thief circuit (http://www.emanator.demon.co.uk/bigclive/joule.htm) – an LED driver that can drain the very last bit of power from a battery and still function for days on end. It has an additional advantage of only using a few parts, and I happened to have them all on hand… a rarity for me!
I have no idea what the “ferrite” material is. I stole this massive coil out of a broken car stereo, cracked open the plastic and found that it was actually some sort of tightly wound metal tape. So I put the plastic back on since it was prone to damage otherwise and just ran a bunch of untwisted cat5e wires around it. Works great. The transistor is NPN… I assume the values of pretty much everything here really don’t matter much. I did have problems getting the lights until I realized I had connected the wrong end of one transformer lead to the battery: the two coils must oppose one another, not run in the same direction.
Of course a bare circuit is fun but not all that useful. So I stuffed it in a little elephant toy I got off Freecycle and gave him a shiny trunk. For now the battery is soldered straight to the leads. One day I’ll get a real AA battery holder and some velcro to finish this up. In the meantime he’s a good nighttime buddy for my 4 year old daughter.