An article by Wired's Rhett Allain explains how to build a simple "joule thief" circuit that extracts extra usable energy from a supposedly "dead" battery. Batteries that stop powering a device aren't actually empty; they still hold chemical energy and voltage, just not enough to drive a current through a bulb or LED. The technique repurposes basic physics — specifically Faraday's law of induction, the same principle behind electric generators and induction hobs — to squeeze meaningfully more light and life out of otherwise-spent batteries.

The piece walks through building the circuit step by step, starting with a basic 1.5-volt AA battery powering an incandescent bulb via a tungsten filament that glows white-hot, before moving to the more efficient but higher-voltage LEDs, which typically need 3 volts (two AA batteries) to light up. The joule thief circuit combines a transformer and a transistor to boost a single 1.5-volt "dead" battery enough to power a 3-volt LED, using a hand-built transformer made of two insulated wire coils wound around a shared core.