joule-thief

Designed back in January 2013, this joule thief lights a white LED from the dregs of a 1.5 V battery. Its trick: a standard two-terminal inductor, so you skip winding your own transformer. Empty AA cells still held enough energy for over a week of light.

The build

• An LTSpice schematic and waveform, plus photos of the working PCB and a detail shot
• Two 2N2222s, a 470 uH inductor, resistors and a white LED form the whole circuit

Why it lights

The pair of transistors oscillates the inductor, storing energy and releasing it as a voltage spike on each cycle. That spike exceeds the white LED's forward voltage, drawing power the cell can barely supply.