I'm still enamored with the Joule thief idea, but more importantly, I have a goal: to get the most out of this bucket of old batteries (pictured below) before the batteries start to leak (or before I amass any more of them).
I'm not too fond of flashlights anymore, because I can't make them small enough to be keyring friendly, and because most people don't use flashlights that frequently. So I've switched to mini LED trees (mini tree, not mini LED), like this one I gave to my sister to liven up her dorm room desk:
But making LED trees can get boring, so enter the brushbot:
The brushbot isn't a very good roomba substitute. He's useless on rugs and he tends to just circle in one place. But he could be useful in polishing stuff, and besides, he makes a mean zen garden.
By the way, the brushbot does NOT run on wheels! He just vibrates his way across a surface, like a cell phone or rumble-equipped controller left on a table. Brushbot is made of two red LEDs, a motor with an asymmetric thingamabob attached to it, and a joule thief per this schematic (Fig. 2) from "Waston's eBlog."