Squeeze More Juice Out of a Dead Battery!
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.
- Wired explains a DIY "joule thief" circuit for draining dead batteries further.
- It uses Faraday's law via a transformer and transistor.
- Lets a single 1.5V battery power a 3V LED.
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A “dead” battery has run too low to power a device, but it can still contain chemical energy. The problem is that its voltage may no longer be high enough to push the current needed by a bulb or LED.
A joule thief is a small circuit that uses a transistor and a transformer to raise the voltage from a single AA battery. The transformer has two coils of wire wound around a shared core; changing current in one coil induces a voltage in the other, the principle behind electric generators.
This can let a low-voltage battery power an LED that would usually need more voltage. The circuit does not create energy: it makes some of the battery’s remaining energy usable for a device that could not run directly from it.