In this video I show my 20 years old joule thief, I explain how does it work and then I build another joule thief, so you can see the entire process step by step, easy to understand for beginners. Joule thief is the most simple type of a switching power supply or a boost inverter. In the simplest version it is made of only 3 components - a transistor, a transformer and a resistor. It can boost the voltage of a single AA or AAA battery (1.5V) to 3V for a white LED. It starts to work at 0.45V and once started, it runs down to 0.3V, so it can run even on a flat battery, which can no longer power up any other appliance.