Joule Thief — Squeeze the Last Drop from a Battery

Circuit Diagram

How to Build It

1. Wind a toroidal ferrite core with two identical windings of wire (the power inductor in the parts list can substitute for demonstration purposes).
2. Connect one end of each winding together and to the NPN transistor's base, through a 1K resistor.
3. Connect the other end of one winding to the transistor's collector.
4. Connect the other end of the second winding to +1.5V (a single AA battery).
5. Connect the transistor's emitter to ground (battery -).
6. Connect the LED between the transistor's collector and ground.
7. Even with a very weak battery, the LED should light at full brightness.

Bill of Materials