Made my joule thief at York Hack Space last night. Most tricky part was winding the coils around the ferrite bead. I was supposed to get 20 turns onto it but I think the beads I brought where a little on the small side. I managed to get 13 or 14 turns on in the end and it works a treat. The final version is running on my desk right now and I have upgraded the bluetak to an elastic band. I will put printing a case for it on my reprap list.

It looks to be only oscillating at 2khz rather than the 50khz I was told to expect but is working great for a battery that was showing less the 1v.

Reading how the circuit works it looks like you could drive any voltage from the coil with a diminishing return in current. I wonder if there would be an optimal number given the components I have.