This is a photo of a Joule Thief I built more than a year ago using the core from a dead CFL light bulb. The windings were only 9 turns each of solid insulated telephone wire. That was all I could get on the core. This core is the typical size for the CFLs where there is limited space in the base of the light. I’ve seen smaller ones, too. They all seem to be high permeability so that it does not take a lot of turns to get enough inductance (100 uH or more) for a Joule Thief.
Have you used UV LEDs? I think they are probably interesting.
I’ve tried a few, but the ones I got were from eBay, and they were included in a lot with other cheap (meaning inferior) LEDs of various colors. I guess I can blame the low output and poor beam quality on this. IR LEDs are a lot more interesting and fun.
In what way? I have controlled some and sent signals with them to dedicated receiver chips and seen invisible beams in alarm circuits. In what way do you mean?.
Me, too. But what’s fun is to adjust the Joule Thief so that it oscillates at 38 kHz. Then let the IR LED poke through your shirt button hole and go over to someone’s house while they’re watching TV. Watch their face as they find that their remote doesn’t work or acts weird.
I see. That is indeed mischievous.