Make a Joule Thief – Weekend Projects Video Podcast


MP4 | Hi-Def | Blip | Youtube | PDF | Subscribe

This week, Windell Oskay of Evil Mad Scientist Laboratories joins me to teach you how to make a super clever little circuit called the Joule Thief! The Joule Thief allows you to squeeze the life out of what most people think of as “dead” batteries!

We were inspired by Big Clive to make this project and it’s the perfect platform for a flashlight, book reading light, or really just something you should make to get more use out of your batteries!

Update: Windell posted more info and a detailed walk through of the toroid winding process. Check it out before you make it! – Link

76 thoughts on “Make a Joule Thief – Weekend Projects Video Podcast

  1. dragonphyre says:

    I saw a circuit for one of these on hack-a-day that you could build and put into a flashlight bulb. Honest, the entire circuit wasn’t much bigger than the jumbo-led they used in this podcast.

  2. Paolo* says:

    Could someone tell me where I can find the transistor and the ring inside of in regular household electronics I could have laying around… how can I know if it’s the right transistor.

    Basicly I have some kind of ipod/tv-b-gone hack I’ve been working on a bit and that would be the missing piece :)

  3. garethb2 says:

    The 2N3904 is nicknamed the “cockroach component” or the cockroach semiconductor, because they are EVERYWHERE. They’re in all sorts of consumer electronics: analog modems, answering machines, all sorts of consumer electronics devices. Of course, you’ll have better success looking in older techno-junk ’cause so much new stuff is surface-mount. And the 2N3904 (and its PNP sibling, the 2N3906) is one of the few electronics components that Radio Shack still actually sells.

  4. Drillbit97 says:

    Thank you !!! only request is to sell the pcb in the make store.

  5. karavshin says:

    This is a interesting project but a terrible writeup.

    1) There is no explanation WHY this works. That’s no fun to just blindly follow directions

    2) The directions aren’t that good. For instance, “Wind the toroid.” Huh? how many windings? If it doesn’t matter, say so.

    Writeup was hasty.

  6. n3rrd says:

    I’ve found Toroid style cores inside of older flatbed scanners, etc. Computer Power Supplies tend to have them, but they are larger as they are wound with heavier gauged wired.

    The 2n3409 can be found practically anywhere. If you can’t find it for some reason, the 2n4124 is an equivalent.

  7. trundlar says:

    Quick and dirty, but the part where he says how many turns to make is fuzzy–same question as karavshin just how many turns on the toroid?

  8. geobarefoot says:

    I agree wholeheartedly w/ karavshin and trundlar. I think most of us in the Maker community are pretty much always interested in why things work, not just how to put them together. My knowledge of transformers, winding toroids, etc. is limited. A lot of casual electronics tinkerers are probably familiar with the basic concepts of resistors, transistors, and LEDs, but we don’t bump into a lot of transformers (speaking for myself) in our blinky-blink projects. A basic understanding of what’s going on would be very helpful. I’ve also noticed this shortcoming in many Make articles. If magazine real estate is the issue, could you at least provide a few good links to resources? Thanks.

  9. bre3 says:

    Windell posted more details on winding the toroid. Make sure to check them out here! –

    It’s honestly a very simple circuit that’s hard to explain and he does it really well.

  10. n3rrd says:

    The difficulty in explaining this circuit is that the transformer is one of the most simple but most complex components around. I posted the hack-a-day link in the other Weekend Project post, but I forgot this other link. I find it to be incredibly in depth and he offers up possible improvements such as the addition of a single capacitor.

  11. w0j0 says:

    n3rrd, thank you for posting that link. I was about to try and post a poor explanation of what was going on since I’m in an EE program. I agree with others, that have posted here, with MAKE’s lack of deep explanations when it comes to cool stuff. I’m beginning to wonder if they are just publishing for a younger demographic and they are trying not to scare kids with too much technical talk — or worse if they just don’t have the time or ability to do so because the people putting these demo’s together don’t really know about circuit analysis. I love the mag regardless..just wish it was a little like popular mechanics a little more.

  12. volkemon says:

    @bre3 and n3rrd- both links are EXCELLENT and went the distance!

    @w0j0- I too subscribe, and have the same wishes. Well said. I think the mag does what it can, and the blog oftentimes fills in the gaps.

  13. yaddayaddayoda says:

    Okay, I seem to be having a bit of trouble making this work. I hit up the junk box at work and came up with all of the necessary parts, a 2N3904, a white LED that runs well at 3.2 volts, a 1k ohm 1/4 watt resistor, and a ferrite toroid that is maybe a little smaller than the podcast (I wound 8 double turns of wire on it). The ferrite torroid was in a pack that looked older than I am… it was labeled “powdered iron toroid inductor” so I am pretty sure it should work.
    When I fire the thing up with a new AA battery the output voltage is actually LOWER than the battery voltage… ie 1.5 volts in; 1.3 volts out to the LED. Any ideas????

  14. philliptorrone says:

    @volkemon – yup, the magazine and the site work together – most/all our articles have pages dedicated to them on the site and we cover things on the site that couldn’t or didn’t make it to print. the site is also a two-way entity, folks here asked for more information and it was then linked up almost immediately.

  15. ElfLord says:

    Cool, I made one :) Something is wierd about it though. My Joule Thief makes a soft, very high-pitched noise whenever it is on. Has anyone else noticed that before, and could there be any reason for it?

  16. yaddayaddayoda says:

    ElfLord, you are hearing the high-pitched noise from the transformer. The operating frequency happens to be in the high audible range. You can solve that by adding or removing a few turns, or by painting the windings with clear nail polish to dampen the noise (or you can dip the whole thing in nail polish or varnish). This is an old trick that TV repair guys use on the flyback transformers in TV sets and CRT monitors.

    You can tell the old guys who have been around TV stations for a few years. The 15.734 kHz frequency that a TV monitor makes has worn out their high frequency hearing in a notch just a few hundred Hz wide, and they can’t hear that noise anymore.

    You can actually perceive audio that is “above” the normal limit of about 20 kHz. I think that some people can tell if there is a high switching frequency up to about 40 kHz.

  17. guyfrom7up says:

    I made one of these and it didn’t work and I’m very disapointed. I think whats wrong is my toroid, but I dunno. I think I might have to go buy better stuff.

  18. n3rrd says:

    The toroid is the least likely problem. The LED could be hooked up backwards, or dead. The transistor could be toast, you could have the leads on the transistor mixed up. If you didn’t use different colored wires for the primary and secondary windings, maybe you shorted out one winding to itself?

  19. yaddayaddayoda says:

    Nope n3rrd, I am thinking that it may indeed be the toroid. The LED and transistor are good. (and hooked up in the right direction, LOL.) I looked around and found that several variations on this circuit recommend more like 20-40 turns of extremely thin varnished wire, instead of using CAT5 wire or telephone wire.

    Looks like that’s what I’ll try next. Good thing I built it on a breadboard!

    There are also a few variations that add a capacitor, etc. I had been thinking that the beauty of this design was the low parts count… sigh…

  20. n3rrd says:

    What kind of transistor are you using? Is it the “recommended” 3904 or 4124? If not, as is stated elsewhere, it could be a very high gain transistor and those tend to be very slow. If it’s slow, it would prevent oscillation.

  21. c0redump says:

    I’m using a BC548B transistor, and at first the circuit failed to oscillate. I reduced the base resistor to 150 Ohm, and it started working. I’ve subsequently found that the circuit is quite sensitive to some unknown property of the power supply, and will actually work with a 390 Ohm resistor when connected to an actual battery (I was using a bench PSU for testing).

    Mine oscillates at 250kHz, using a toroid from a dead PC/ATX power supply.

  22. Clavis says:

    This is awesome! I had the day off and decided to try making this, and it worked on the first try! The only bit I bought was the transistor — 79 cents at RS! Everything else I found lying around my bits bins — even the toroid! I can’t believe it worked right off! I’m going to build another one and jam it inside the body of an old AA maglite! Thanks, MAKE! You rock!

  23. Clavis says:

    Oh, I should add that I used wire-wrapping wire and did 16 turns around the toroid.

    It seems that it hasn’t been sufficiently specified that the pair of wires needs to end up forming alternating rings — that is, if the two wires are red and blue, then the wires should alternate “red, blue, red, blue, red, blue…” as you go around the toroid. People having problems might want to make sure that they don’t get out of order as you go around.

    I assume that’s important, anyway…

  24. jim_f says:

    I made a little Joule Thief printed circuit board using my cnc mill.

    Here’s a short video I made of the process.


  25. Moddy says:

    Hi, I’m new to transistors, and had a 2n4401. Only problem is, I don’t know why it doesn’t work. I have no probably getting a before mentioned transistor to make it work, I just don’t know why the 2n4401 doesn’t, it has closely related parameters. Thanks for any input. Moddy

  26. Tjeiken says:

    Why does it matter, if the LED is white or blue? can’t a green one be used..?

    And how much does it matter, if it is the correct transistor? i have a BE547A NPN transistor – that wouldn’t work? i’m gonna try when i get home from school, anyways =)

  27. Anonymous says:

    I am also having problems with a BC548B tranny, it’s all I could find at Map**n Electronics. I found one that had similar charicteristics to the 3904 but was unable to source the actual component. Any ideas? Brill ‘cast BTW.

  28. nitroburn says:

    I used a BC337B and it worked great. didn’t even rewind the coil, used it as I pulled it from a Power Supply.

  29. warpcat says:

    Experiencing trouble ;) I’ve now made two of these, neither of which work. I’ve tried two different toroids, I know my LED will fire with 3v.
    I’ve posted a pic of it here:
    The toroids I got didn’t seem to have any magnetic fields (didn’t know if that was bad), so I used one from my sons ‘magnet experiment kit’ that I KNEW was magnetic (pictured). But it still won’t work. I can hook a 1.5v battery to it, two 1.5v batteries (3v) to it, and nothing. But if I short the led with the 3v the led works fine. Any thoughts folks? I really want to get this working! :)

  30. largejunglecat says:

    I was having a lot of trouble getting my joule thief to work. I was prowling the web for advice and I found this:

    He has a small cap (I used ~1uF) running parallel with the 1k resistor. I threw the cap onto the breadboard and BAM! it worked like a charm. I don’t have an explanation, but you might try it if you’re having trouble.

  31. Bill Sherman says:

    I made mine into an Artbot for Robogames 08. If you connect a CDS photo resistor from the base to the emitter, the led lights when dark or a shadow is cast upon CDS cell. Below is a link to view the artbot. The current drops from 30 ma to 1 ma when the led is out. Mine also has two leds in series. Open circuit voltage can be as high as 40 volts with a diode and capacitor instead of a led. Now that bit of info can give you new ideas for the circuit.

  32. Bill Sherman says:

    My joule thief is now blinking! The battery voltage is about .6 volts. I see the voltage drops a bit when it blinks, then it will come back up and blink again. The rate started at 1 per second and has slowed down now. I started a chart of the voltage with a data logging system at work and will run it over the weekend (its still been blinking for days) till it dies. I have a movie of the blinking effect at my youtube site.

    The led is a resistor type, NOT a blinking led. has anyone seen this effect?

    1. Nibbels says:

      Dear Bill, if your joule thief is blinking, its becouse there is no “extra” energy generated, also you CANNOT measure the voltage with a voltage meter (the meter sucks down your performance) What happens is the voltage difference generated by the resistor makes the transistor flicker (inside) doubeling the voltage, then the LED will suck out as many it needs and the rest is burned up in your resistor. *try to play with the resistor, higher or lower ohmage will do the trick.*

  33. Bill Sherman says:

    I wound two sets of joule thief’s on one ferrite core. I was hoping I could get some beat effects from each other. There is some interaction between the two circuits. I’ll have to check with a scope to see what it is. Photo of the circuit on a proto board at:

  34. me says:

    that bloke says solder so wierdly like soder or sumin.. lol and doesn’t say why this works at all booo!

  35. gccradioscience says:

    1. What is the ferrite material required because there are many different types of material for the make this project work? The type of material for this project is not the type I have in my parts box. What sources that I can find this online so I can place an order to get these type? When you say that it can be out of any power supply or radio, what do you mean by that? Also where do you find these ferrite donuts?

    2. What different transistors required for this project to
    work effectively? What if I don’t have a 2N3904 or 2N2222
    could I substitute another transistor such as a C1740 small voltage type?

    3. What type of wire were you using and how do you wind
    this coil? When you shown how you wound it you went too

    4. Is it really necessary to use the expensive blue and
    white LEDs can there be substitutions since this is only
    a weekend experiment?

  36. G33K says:

    I just made one, it is incredibly simple! It took me like 5 minutes, and very practical too.
    What is the lowest practical voltage?
    Thanks for the great project,

  37. G33K says:

    I just made another one, I put it in a housing with a switch, it is brighter then my other 1 LED flashlights… and it runs of 2 supposedly “dead” AA’s! It has a bigger toroid then my last one and it makes a very high pitched wine;)

Comments are closed.

Discuss this article with the rest of the community on our Discord server!


Maker Faire Bay Area 2023 - Mare Island, CA

Escape to an island of imagination + innovation as Maker Faire Bay Area returns for its 15th iteration!

Buy Tickets today! SAVE 15% and lock-in your preferred date(s).