I used a pen style flashlight from duracell.
It has a 2.2 volt lamp and two AAA batteries.
One of the penlites is going to be replaced by the joule thief.
And of course the lamp is being replaced by a 5 mm white LED.
Thanks to all the great ideas on instructables to make this my first instructable.
Step 1: Schematic
I is almost the same as http://www.instructables.com/id/Joule-Thief-LED-Night-Light/
The instructable from ledartist what inspired me to make a flashlight joule thief.
On the second photo is the prototype I made to replace a battery and the lamp.