Itty Bitty Joule Thief




About: I'm an electronic engineering student. I don't usually have much spare time but I like to work on random projects to keep myself entertained. I hope you like them!

A Joule Thief is a simple circuit that takes advantage of the inductive kickback produced by an inductive element (a wound toroid in this case) to step up the voltage. This extra voltage at the output of the circuit can be used to power devices which would not work with the input voltage alone. In this case that voltage is used to light an LED, which would not be able to light up with a single AA battery as the voltage required to do so is 3.3 volts, while the battery can only provide 1.5 volts at its best.

The joule thief can operate with very low voltages, this means it can work with batteries which would normally be considered "dead", taking energy from them in a seemingly impossible way, hence it's name.

In this instructable I'll show you how to build a tiny joule thief mounted on a piece of copper clad board.

Step 1: Materials:

  • 2N2222 transistor
  • 1k to 10k resistor (to select the brightness)
  • Copper clad board
  • 1x AA battery holder
  • 1x LED (I used white, but any color will work)
  • Solder
  • 1x small toroid* (less than 1cm of outer diameter)
  • Around 20-30 centimeters (8-12 inches) of thin copper wire
  • Epoxy

*Small toroids like the one I used can be easily obtained out of small CFLs, not all CFLs have them, but there's a high chance you'll find one inside. Not all toroids will work, it's wise to test the circuit in a breadboard before assembling it.

Step 2: Etch the Board

I first start by cutting a piece of copper clad board with the size of an AA battery, I will stick it to the back of the AA holder, so it must have the correct dimensions. In this case the dimensions are 1.2 x 5 centimeters.

To etch the board I stick some tape on both sides after scrubbing and cleaning it with alcohol, this will remove any oils left by my fingers which could ruin the process, it is also important to seal the edges too so the etchant doesn't eats them away.

Next I cut the shape of the circuit with an x-acto knife as seen in the pictures, the exposed areas will be eaten away. This disposition is fine as it is and I encourage you to use it, but you can always improve it's design.

To etch the board I use a mixture of Hydrochloric acid (20-30%) (also known as muriatic acid, pool cleaner or HCl) with hydrogen peroxide (3%) (also known as a common wound disinfectant) in a 1:2 ratio, 1 part of HCl per 2 parts of H202 . This solution can't be reused as it's unstable and will loose its power within some hours.

The etching process usually takes from 5 to 10 minutes.

After the board is etched I remove the sticky tape and I continue with the next step.

Step 3: Winding the Toroid

Winding the toroid can be a little frustrating, specially if your toroid and wires are very small. First you must take a piece of wire around 25cm long (depending on the size of your toroid) and bend it in half so you have two wires parallel to each other, then you start winding those two wires at the same time (from 10 to 20 times) until you end up with something like in the image. As you can see, the wires should should not cross each other.

Next, you have to connect the two inner wires as seen in the image, these will be connected to positive. The remaining two wires can be connected to the resistor that will be connected to the base or to the collector, it doesn't matter which way around.

Step 4: Completing the Circuit

The circuit is quite simple, but it can get confusing when soldering it onto the board.

I first tin all the pads on the board, this will avoid the copper from deteriorating and gives it a nice appearance, then I solder the toroid positive wire to the positive pad at the bottom, the two other wires will be connected
indistinctly to the pads at the right and the left.

I connect one end of a 2.2k resistor to the pad at the left and the other end to the "base" pad just above it. You can choose the brightness with this resistor. After finishing the circuit, if the current going in is higher than 30mA you can increase the value of the resistor in order to reduce it.

To connect the transistor I connect the collector to the upper right pad, the emitter to the upper left pad (the negative pad) and the base to the "base" pad just below it.

I finally solder a SMD LED taken from a LED bulb between the collector (positive) and the "emitter" (negative) pads. You can solder any kind of LED.

Once the circuit is complete I use acetone and a q-tip to remove most of the flux that was on the board, then I use epoxy to hold it to an AA battery holder, I use the remaining epoxy to seal the toroid and its wires, which can be easily damaged.

To finish it all I solder the positive wire coming from the holder to the positive pad at the bottom and the negative wire to the negative pad, connected to the emitter and to the negative pad of the LED.

IMPORTANT:I used a P2N2222A transistor, not a 2N2222A or a PN2222A, this means the collector and emitter are swapped, check your transistor's pinout before soldering it.

Step 5: The End

This circuit uses around 20-30mA, this means it can last more than 50 hours on a fresh AA battery as calculated by an online battery life calculator. It's also quite bright, so it could be of some use in an emergency situation.

EDIT: I tried to see how much it would last on a fresh AA battery (a generic one), I connected it on Friday night at 24:00 hours and it's still running, without a very significant significant drop in brightness, two days later, Monday morning, almost 50 hours of continuous use. After the first day the voltage dropped from 1.48 volts to 1.20 volts, now it's at 1.05 volts.

EDIT 2: One day later it's still running, the brightness has dropped significantly, but it can still be used to see in the dark without problems (see 3rd picture) , the voltage is now at 0.78 volts. It's been running for three days straight.

EDIT3: The battery seems to have really died at some point during the Tuesday morning after 6 am, in total it's been running for almost 80 hours.

Thanks you for reading this instructable.



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37 Discussions


Reply 2 years ago

In this case is 44kHz, but it can depend on the toroid and windings used.


3 years ago

Could you put several of these in series to get, say, 30v, with suitably uprated transistors? That would allow floodlight diodes to be used, but the current would be pretty hefty.

2 replies

Reply 3 years ago

You can search "joule ringer by laser hacker" it's a joule thief
working at 12V that can light up a few home led light bulbs using
only 200mA.

I did some circuits similar and got a CFL tube to light up at 1.5V with an amp draw of around 40mA!


Reply 3 years ago

Yes, I guess you could have a bigger toroid with more turns with a bigger transistor, but at that point I think it would be more efficient and reliable to get a boost converter.


3 years ago

Instead of etching, have you thought about using wide Copper Tape and then cutting away the parts that are not needed? I was also looking at conductive paint to see if this would work as well. Paint it on a plastic sheet or even directly onto the back of the Battery holder?

Here is the link to Conductive Paint. I am not associated with the company that makes it or the store that sells it.

1 reply

Reply 3 years ago

I guess that would work too, conductive paint tends to have a big resistance, so I guess it wouldn't be as efficient.


Reply 3 years ago

Yes you could since the voltage drop would be the same, however it won't be as bright because this device usually outputs around 20mA.


3 years ago

Very nice! I have a question for you, You have one of the best images of how to wind the simple joule thief I've ever seen and I'm like to use it on my joule thief web page if you will allow me to do so. I'll also include a link to this instructable so show where it came from and to allow folks to get to your instructable from my page. My site is at: http:// cs. yrex. com / ke3fl / <>- click on the "Feature Article!" link and then click on the "Joule Thief Experiments" link.

6 replies

Reply 3 years ago


i noticed on your Joule thief site you wound the white and green wires separately on the toroid. if you wire in parallel so they interleave you can up the efficiency, sometimes by as much as 40%.

This is largely a result of separating each winding by the wire in the neighboring winding. this reduces the capacitance between each loop. it will allow faster oscillation frequency and higher magnetic triggering between the 2 windings.

just a bit of food for thought


Reply 3 years ago


Thanks for the information, I hadn't yet done any tests on parallel or seperate wiring so that's very good to know. Usually I wire them in parallel because it is easier to do & takes less time that way.


Reply 3 years ago

Oh, that image you were referencing was used with permission from a wiki article, as the comment below the image states: "To read the wiki article,
Click Here " I'm presently contacting Victor & the original author of the graphic, that Victor modified, for permission to use it on my site. Again, thanks much for the additional information about separate & parallel wiring.


Reply 3 years ago

I've been separating my windings just a bit but it seems it's actually better to put them parallel to each other touching each other. I've seen people who even prefer to strand the wires together and then wind them like that, it seems to work better, at least for gate drive transformers.


Reply 3 years ago

The image I used was a quick edit of this one:

doesn't seems totally fair to me to distribute that picture, even though it's an edit the
original belongs to him, but liked your page so much I made you these:

The second one has no background, so you can merge it with your page's background if you wish.

You don't have to quote me or add any link, I don't want recognition, seeing my work being shared is enough reward for me :)


Reply 3 years ago


Thanks again for the images. In this case it really doesn't matter if it is the RED or the GREEN wire that goes to the resistor and then the base of the transistor since it is the simple Joule Thief version. On the other hand, with the non-simple version it does matter which wire goes to the base of the transistor. It should be the wire with the fewer windings. You can read and see this on my Joule Thief Experiments page.