I made a Joule Thief recently and it worked -on a protoboard- but now it does not work, it gives the same voltage from the batteries, I apply like 2V and the white LED does not light up, I have no idea of what might be the problem, the coil is about 12 turns, the LED is 3.5V 100 mA. Also I have used a 3.2V 20mA LED and it´s the same.