Big Clive made a joule thief, you can read more about it here: http://www.bigclive.com/joule.htm. I thought this was a fun project to try myself. So that’s what I did.
I tried to make a coil myself. Clive recommended 20 windings, but due to the ferrite core and/or the wire I used I was only able to get five windings before I ran out of space. The joule thief worked nonetheless, although not down to 0.35V. It cut out somewhere before that.
Just to try it I used a little transformer that I had lying around, which works really well, too. When using this one, especially with a relatively full battery, I can hear the transistor quite well.
I didn’t have the resistors that Clive recommended, but I guess the way I did it is close enough.
It really works down to 0.35 V!
This is definitely a fun little project that I can fully recommend.
Have fun to try this yourself sometimes!