Make a Joule Thief




Introduction: Make a Joule Thief

Sometimes my Instructables are few and far between, but I try to make them as well as I can. Hope...

Yes, it's the infamous Joule Thief, in Instructable form! For those of you who don't know, the Joule Thief is a tiny little circuit that allows you to drive a white or blue LED from voltages as low as 0.5 volts. You think those batteries are dead? Don't throw them out yet! Hook them up to the Joule Thief to squeeze every last drop of energy out of them!

The idea and circuit came from this Make weekend project. Why don't you pay them a visit?

Step 1: Parts and Tools

For this project you will need very few tools and parts, as you will see in the picture below.
But for those of you who like it in text, here it is:

Helping Hands (Optional)
Soldering Iron
A Blue or White LED (Other colors are fine, too)
2N3904 Transistor or equivalent
1k Resistor (Brown-Black-Red)
Toroid Bead
Thin wire, two colors (magnet wire works)

You can get the toroid and transistor from a dead CFL; the transistor is usually labeled 13002.

Also, if you use a 2N4401 or BC337 transistor, your LED will be brighter because they can handle more amps.

Step 2: Wind the Toroid

The first step is to wind the toroid. I found mine in an old computer power supply, and it works fine for me. Toroids are donut-shaped objects like in the picture, and can be attracted by a magnet.

You can find toroids in a few places. Old computer motherboards, XBOX and X360 motherboards have them (don't take them unless it's dead!). You can find toroids in computer power supplies, or you could buy them at your nearest RadioShack.

Take your two strands of wire, and twist the ends together. You don't have to do this, but it makes winding a little easier.

Thread the twisted end through the toroid, then take the other two ends (Not twisted together) and wind it once around the toroid. Don't twist the wires; make sure that two wires of the same color are not right next to each other.

Keep winding, making sure you wind the coils tight. It will still work if they are kind of loose, but it is better to have them tight.

Ideally, you want about 8-11 turns on your toroid. Even if you can fit more, don't put more on. Make sure the turns are spaced evenly around the toroid.

Once you wind around the whole toroid, cut off the extra wire, making sure you leave a couple of inches for soldering.

Strip some insulation off the wires, then take a wire from each side, making sure they are of the OPPOSITE COLOR. Twist them together, and then you're done with the toroid.

Step 3: Solder It All Together

Now we are going to solder the whole thing together. You could put it on a board if you like, but in this Instructable, we're going to free-hand solder it. Or whatever you call it. You can follow these written instructions, or take a look at the pictures. They explain it very well.

First, take the two outer leads of the transistor and bend them outwards a little ways, and bend the middle lead backwards. Bend the LED's leads outwards, too. You don't have to, but it makes it easier to solder.

Take one of the wires coming from the toroid that is alone; that is, not twisted together with another wire. Solder it to one side of the resistor. Solder the other end of the resistor to the middle lead of the transistor.

Take the other single wire from the toroid, and solder it to the collector of the transistor. Solder the positive side of the LED to the collector as well, and solder the negative side to the emitter.

All that's left to do now is solder an extension wire to the negative side of the LED. Take a piece of that wire you had earlier and solder it to the transistor's emitter.

Step 4: Try It Out!

And that's it! You're finished with your Joule Thief. Touch the twisted wires coming from your toroid to the positive side of a battery, and the extension wire to the negative side. If all is working well, the LED will light up! If not, try using thinner wire on your toroid. I've included a schematic in the pictures if you prefer.

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734 Discussions

Hi.. this is not working to me,tryed everything,combinations but nothing.. even with full battery why is that so?
Tr is 2n3904 11 turns of double wire,just like you did but nothing happens.. even with new transistor?
Thank you..

Correction in the schematic of circuit:- The LED is supposed to be connected in reverse manner.(if you're using NPN transistor) i.e. anode to the emitter of transistor and cathode to the collector of transistor or vice versa if PNP transistor...

1 reply

Actually, it can work both ways. With mine i found it works better if i connect the Anode to the collector, and the cathode to the emitter.

Hi Can I connect this set up to a dc motor? will the effect be the same?

Genius! I am very impressed!

How big should the ferrite core must be and how many windings needed?

I hope this comment helps. Mine didn't worked at first but after some CRITICAL fixes: 1) Bigger toroidal and thinner wires. And MOST IMPORTANT 2) The transistor wired as shown will NOT work in many cases because Base, Collector and Emitter are NOT in the same place. You need to know where the BCE are and is recommended to see a diagram for this circuit. I used both ferrite core and transistor from a dead CFL and did the test with a "dead" 1.5v battery and a UV LED. My transistor was a 13002.

Mine is coming on but it is not getting brighter and the voltage is the same. I use a 13003 transistor. Please some help.

1 reply

Use a potentiometer 1k for adjusting brightness or even better 100ohms pot.

Hi, I tried making the same circuit but i don`t know what is wrong it`s not working i used the npn 2n2222 instead of a npn 2n3904 and i followed the same steps shown here but i can`t figure out why my circuit isn`t working? plz help i need it urgently!

2 replies

No importa el transistor, siempre y cuando sea NPN, lo que debe estar sucediendo es que una de las bobinas está en vertida, así que cambia el orden de solo una bobina

You need a wire from the LED's negative to the battery.

Would it be better to use a N channel MOSFET in place of the NPN bipolar transistor? There's usually a 5V reverse bias limit on the base-emitter junction, a FET could allow more negative voltages on its gate (e.g with a. lower turns ratio), plus it's more efficient because there's no saturation voltage drop. I guess you'd also have to watch that the steady state (DC) condition of the transistor being on permanently which could short the battery, but a nice little circuit!

hello , could a s9014 work ?


Does not work with 1.5V and 421 2DT nor with BC 337-40, only at 3-5V.

In my experience dead CFL's are not a reliable source for toroid's as they are often too small to be useful in a project like this.

Would a 2N3906B work i don't know about transiters