Easy Joule Thief Soldering Project





Introduction: Easy Joule Thief Soldering Project

The Joule Thief is a clever little circuit that can light a LED with a battery that is nearly dead. It does this with a pair of opposing magnetic fields. I like this circuit because it is simple to build, yet demonstrates some very complex electrical behavior.

I've designed a printed circuit board to help make this easier to build for beginning Makers. I used open source KiCAD software to lay it out (a future Instructable?), and OSHPark to manufacture it. If you want to just wire one up without the board, check out Angelo's Instructable which uses essentially the same circuit.

If you want to learn a bit more about the principles that make this circuit work, check out this video: http://youtu.be/0GVLnyTdqkg. And, if you want to see some serious research in to the operation and efficiency of the circuit, check out http://cs.yrex.com/ke3fl/htm/JouleThief/JouleThief.htm.

Step 1: Gather the Parts and Tools

    I've seen a number of different circuits using an array of parts, so it is certainly possible to do this with what you have on hand. Here is what I'm using:

    • 1 - PCB, https://oshpark.com/shared_projects/OpQUM5it ($8.10 for 3 copies, free shipping)
    • 2 - AA Battery Clip, Keystone Electronics #92
    • 1 - Ferrite Tubular Bead, Digi-Key 240-2301-ND or similar
    • 1 - SPDT slide switch, E-Switch EG1218
    • 1 - 1K resistor 1/8 - 1/4 W
    • 1 - NPN Transistor like a 2N3094, 2N2222, or 2N4401
    • 2 nanoseconds (about 24") of insulated wire. Two different colors are helpful.
    • 1 - LED, 5 or 10mm, choice of color

    Remember, I've done all the hard work if you just want to buy the kit at Tindie.com.

    For tools, you will need:

    • Soldering iron
    • Solder
    • Wire nippers
    • Wire stripper
    • 1 used AA battery (at 0.5V or higher)

    Step 2: Resistor

    1. Bend the leads of the 1K ohm resistor.
    2. Solder both leads.
    3. Cut the leads flush with the solder joint.


    • There is no polarity to resistors so it can go in either direction.
    • The color code for 1K ohm is brown (1), black (0), red (x 100).
    • I like to start with resistors because they are fairly heat tolerant and sit lowest on the board.

    If you have never soldered before, check out SparkFun's tutorial at www.sparkfun.com/tutorials/213 and watch youtu.be/QKbJxytERvg or youtu.be/P5L4Gl6Q4Xo.

    Step 3: Power Switch

    1. Insert the power switch in the lower left corner.
    2. Solder the three leads.
    3. Clip the leads close to the solder joints.


    • This is a simple single-pole, single-throw switch (SPST).
    • Like the resistor, it has no polarity and can go in either direction.

    Step 4: Transistor

    1. With the flat face oriented away from you, bend the leads to match the holes.
    2. Insert the transistor into the PCB being careful to note orientation.
    3. Solder the leads.
    4. Cut the leads flush with the solder joints.


    • Each lead of the transistor is special, so orientation is critical for the circuit to work.
    • The silkscreens shows the job of each lead: C- collector, B- base, E- emitter.
    • Applying voltage to the base allows current to flow from the collector to the emitter (NPN type transistor).
    • This is fairly generic application, so there are a number of transistor variants that will work.

    Step 5: Light Emitting Diode

    1. Insert the LED with the longer leg in the hole marked "+".
    2. Solder the leads.
    3. Cut the leads flush with the solder joints.


    • The LED will not work if installed backwards.
    • Any type of LED will work.
    • The silk screen on the back also shows which length of leg goes where.

    Step 6: Wind the Coil

    1. Insert both wires about half-way through the coil.
    2. Wrap the wire around the coil.
    3. Continue both side of the wire until you have 7 - 9 loops.
    4. Pull the wires as tight as possible.
    5. Cut the leads so they have and even 1/2" to 1" remaining.


    • The wires can cross each other, but keeping them flat to the coil looks nicer.

    Step 7: Solder the Coil

    1. The silkscreen shows the positioning of the coil leads. Wires of the same color must go in the pads connect by the coil diagram (see picture).
    2. Solder the leads.
    3. Clip the leads flush with the solder joint.

    Step 8: Battery Clips

    1. Insert the battery clips in the correct pads for the type of battery you intend to use.
    2. Solder clips in place.
    3. Insert a battery and bask in the glow of your own thievery!
    4. If it doesn't light, remove the battery immediately, and start troubleshooting with the next step.


    • Being a big piece of metal, these take a bit of heat to solder. Be patient.
    • You could install these on back side of the board if you wanted.
    • You could solder the one side in, and leave the other loose so you can switch between battery types.

    Step 9: Trouble Shooting

    If it doesn't light, don't loose heart! Here are some things to check:

    • Look at your solder joints. Any missing or bridged connections should be fixed.
    • Try a fresh battery. This circuit become unstable at a low voltage and may not start if the battery is completely dead.
    • Check the orientation of components with polarity or specific orientation:
      • LED
      • Transistor
      • Coil leads
    • If the transistor is getting hot, there is likely a short or bridged solder joint.

    If all else fails, ask for help. I've tried to make this as easy and fool-proof as possible, but if you are a true beginner, you may need the help of someone more experienced. Learning how and when to ask questions is a key Maker talent. Don't be afraid to ask for help.



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    24 Discussions

    I agree, this is a great learn-to-solder kit. The components are nicely spaced and the instructions lead you through in a logical way where already-placed components don't get in the way of later components.

    I'm not a beginner, but I assembled a couple of these from your kit just to show off to my kids that I could use a dead battery for something. A couple of things that might be worth noting if you do a revision of the instructions:

    1. It could be specific to the PCB batch mine came from, but the smaller through-holes were closed on one side by the purple layer. Not a big deal for me. I poked through them (from the pad side) with the resister lead.

    2. The battery clips were pretty hard to seat on the board. I understand the idea, that the leads have shoulders and you need to push them in until the neck holds them in place. I just had to bear down pretty hard on some of them to get them in place. Maybe there is some foolproof way to easily put them, but I used muscle.

    2 replies

    I've updated the board so the current kits have a good fit.

    Great little kit. I built mine quickly, but the coil took the longest to get right. This is the best kit I have found. I am good at electronics, but have had trouble making a joule thief work. I have tried all kinds of coils and the most I could get was a blink when I touched the battery leads to the battery. There really is a trick to the coil. It has to have the right amount of inductance to start the circuit oscillating. The wrong amount and no reaction what so ever. Thanks for building such a nice little kit. Great for kids trying to learn and adults as well. I highly recommend this kit.

    While your instructable is very well detailed and sounds easy for those with limited electronic experience, Could you better explain the science behind the project? What exactly it does and how it works? Thank you!

    5 replies

    A Joule Thief circuit is a more efficient way of using the energy stored in a battery to light an LED, but a description of how it works is beyond the scope of an Instructable.

    I disagree completely. Instructables is a place of learning, and there are countless people that thirst for the details that surround projects such as these.

    It's a common fallacy among producers of these types of kits and projects to believe that builders gain electronic knowledge from assembly alone. But, in actuality, the benefits are limited to soldering practice and a lesson in following printed instructions.

    I believe that several paragraphs describing the operation of the circuit are certainly warranted, it would be a valuable resource for those who are not interested in the kit, but ARE interested in how it works and to possibly make their own version of the circuit. Kit builders who are interested in the final product alone can simply skip over this information.

    I'm not suggesting that a description or Theory of Operation be as detailed to include electrons and holes and everything after it, just a layman's description of the components and how they work would go a long way to help understand the circuit function. Following that, an overview of how the transistor switches the magnetic field on and off, sending the collapsing magnetic field's back EMF peaks through the diode to the LED as the system oscillates.

    How hard is that?

    Every one is at different levels. This project is geared toward beginners learning how to solder (as in 10 year olds), and the main discussions generally include polarity of an LED and how a transistor works. I've included a link to a youtube video that explains the rest better than I can. I am not an EE, I am a hobbyist.

    @MakersBox - Sorry for the confusion. My comment was directed at pavium where he stated that a description of how an electronic circuit works "is beyond the scope of an Instructable". I disagreed and opined that many people would want to learn from such a description (as seen within these comments), and that an Instructable is exactly the place to do it.

    I was not suggesting that you include this within *your* present Instructable, I was merely providing a quick sample of what might be written in a basic description of "how it works", using your Joule Thief circuit as a relevant example.

    P.S. - Don't be fooled by correlating age with abilities, you'd be surprised. The son of a buddy of mine is really good with micro-controllers and having them control various motors and such. As a project, I paid him to write the code for a small application I had to save myself some time, and he had it done the next day. This kid just turned 10.

    The best explanation I've come across is: http://youtu.be/0GVLnyTdqkg

    G'day, Thanks for an interesting project. I've made a number of 300mAh 1W LED torches for the family, running on 3 NiMH AA's with a resistor. My question is, Would your project run one of these LED's? and, if so, on one or two AA's? I dislike using my current set up as I need (for best usability) a two way switch, one way through the resistor for when the batteries are freshly charged (which seems like a waste of power), then the other way by-passing the resistor once it's got too dull. Your project sounds much better :) I can solder and follow instructions, but my electronics knowhow's fairly limited. I've attached a photo of one of the LEDs I'm using.
    Thank you very much for your time.

    1W 300mAh LED Warm White.jpg
    2 replies

    If you can send a part number or link to a data sheet, I can take a look. Does it need to be heat sinked?

    G'day M.B., Found these details, hope it's what you need. It covers 1 - 3w (Mine are 1w) and the lumen output on mine is closer to 110lm, not the 180-200lm on the data. I'm guessing it's talking about the 3w in that case. Yes I do put it on a heatsink, and sometimes on a length of aluminium tube, all with heat transferring grease between . Operating voltage is 3.4 - 3.8v. At the bottom end (down to 3.2v) they don't heat up much, but at the top end they can get quite hot, and that 0.6v difference equals more than double the brightness. I should shut up and post this :) Thanks for your time.

    Screen Shot 2016-05-10 at 5.46.58 pm.pngScreen Shot 2016-05-10 at 5.48.52 pm.png

    Any idea what the efficiency is?

    Nice little board.

    5 replies

    I know it is pretty low, but have never measured it. It will last weeks on a mostly-dead battery, and that is efficient enough for me.

    I have a small LED torch (Ring CyberLight) that aledgedly has a DC/DC convertor circuit in it to run at almost dead battery voltages, but it is so tightly constructed that I haven't been able to investigate it. However, it gives impressive light output for a very long time and I have never had to change a cell (AA 1.5-volt) because of cell failure. I do change it just to see if there is a real difference in output

    Better than to throw that dead battery away :)

    I like it, also to measure the efficiency I simply measure the average current being used at 1.1 Volts. You can see my Joule Thief page on my website where I went through all sorts of tests to figure out what I wanted to measure and what I finally settled on, which was the current at a specific voltage. However, this does not mean that any our #1 circuit at 1.1 V is better than all other circuits because things change as the voltage decreases and it just may be that a different circuit out performs our #1 at 1.1 Volts when the voltages change. So, if you have time have fun measuring the current at say 1.1, 1.0, 0.9, etc down to 0.4Volts where these circuits generally fail. Oh, one word of additional information, you can not start at lower than 0.4 volts and get the circuit to start working at 0.4 volts, it turns out that the circuit will keep the LED lighted as the voltage decrease but the starting voltage may be ~ 0.5 - 0.6 Volts. These may also be a measure of efficiency, the starting voltage, and/or the lowest voltage that keeps the LED lighted. My Joule Thief Page:


    Wow, that is some serious research. Thanks for posting it! I'm going to add your link to the introduction.

    NIce!!! =D