A “Joule Thief” is a simple voltage booster circuit. It can increase the voltage of a power source by changing the constant low voltage signal into a series of rapid pulses at a higher voltage. You most commonly see this kind of circuit used to power LEDs with a “dead” battery, but there are many more potential applications for a circuit like this.

In this project, I am going to show you how you can use a Joule Thief to charge batteries with low-voltage power sources. Because the Joule Thief is able to boost the voltage of a signal, you are able to charge a battery with a power source whose output voltage is actually lower than the battery itself. This lets you take advantage of low voltage power sources such as thermoelectric generators, small turbines, and individual solar cells.