The Joule Thief

The “Joule Thief” is a simple voltage booster circuit that allows batteries that would otherwise be considered “dead” (with voltages as low as ~0.3 V) to power LEDs.

Make a Joule Thief:

In the November 1999 issue of EPE (Everyday Practical Electronics), a small and intriguing circuit was published in the Ingenuity Unlimited section by Z. Kaparnik. It was a very small implementation of a typical transformer feedback single transistor invertor. The transformer was a standard ferrite bead with two windings wound on it and the circuit was using the high voltage pulse generated when the transistor turns off to light an LED from a single 1.5V battery.

Here’s a PDF of the instructions that spawned the Joule Theif; look to Figure 1a.

It felt like everybody was building these 15–20 years ago, probably because it’s a great introductory electronics project. I want to catalog the Joule Thief here because it’s easy to make from scrounged parts (the NPN transistor that does most of the work is nearly ubiquitous, and you have a lot of wiggle room on the exact resistance used), and putting old alkaline batteries to use before they become toxic waste seems like a worthwhile endeavor.

A photo of a typical build