Running with joule thieves

What is a joule thief?

It is a simple circuit that goes between a single 1.5v battery and one or more 3 volt LEDs enabling them to light. It works with batteries down to 1/3 of a volt. A torch or radio sees less than about 1.25v as dead.

You can stack up the bulbs on them. I’ve put 46 3 volt LEDs on one joule thief. This will still run for days, especially off a DeadDuraCell.

The circuit is simple to assemble (it was my first go at electronics): a transistor, a resistor, a ferrite ring, some wire and the LED(s). The circuit can be twisted together and then—preferably—soldered, unless you’re happy just to treat it very carefully. I use croc clips with baby neodymium magnets in as battery clips that take AAA,AA,C or D cells.

joule thief diagram

The three joule thief wire sculptures shown in the top photograph each have a starship coil instead of a ferrite ring. This is made with 2 longer wires, but needs no ferrite ring. It is formed around pegs or nails. I make most of mine with these coils. When I measured them running with a frequency meter they pulsed the LEDs at over 300,000 hz (300khz) rather than around 30,000 for the ferrite ring. When you have a few running there’s a clear difference in how it feels being around them—plus they look great.

They make great night lights. I even converted a bike light that originally took two C cells and put four superbright LEDs in and a little JT. The switch and battery contacts work the same as before the conversion. This did its first winter on the used AA you see in the photo.

Joule thief bicycle light

Joule thief kits

I’ve decided to put together some kits as I have ended up buying bits in some kind of bulk. They include 2 each of the LED, transistor and resistor, one good quality ferrite ring and some wire. The spare bits allow for breakage (twist carefully) or can be a second joule thief if you try a starship coil or scavenge a ring from an old piece of electronics. If you wish to add a second ring I can do so for a small extra cost. Also included are 2 crocodile clips and small ND magnets that make a set of one-size-fits-all battery clips. Please contact me for details.

Joule thief kit assembled

Many people are experimenting with joule thieves and sharing online and there’s some interesting developments and variations including micro wind-generators and joule thief battery chargers, which I have tried in a 12 volt version as well with interesting results. Search Google and Youtube for inspiration.

Here’s a couple more pictures of mine. The strings you can see behind the buddha extend across the ceiling as well. All seven of the joule thieves run from one battery, a total of eighty-some lights. Any movement of the boat swings them about. The battery and the connections are all hooked by weight or twisted, so the lights pulse and ripple with movement as well.

buddha statue with joule thief lights

Below: three different JTs, including one using a 24 bulb ceiling light and another using a short string of red LEDs.


Shown below is my first little starship JT, on a board. Over 30 LEDs—I ran out—drawing only just over 20 milliamps. One LED lit conventionally with 3 volts would draw about 22 milliamps.

starship coil joule thief with approx 30 LEDs

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