Introduction: Easy Joule Thief Soldering Project

By Maker's Box

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.

    Notes:

    • 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.

    Notes:

    • 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.

    Notes:

    • 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.

    Notes:

    • 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.

    Notes:

    • 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.

    Notes:

    • 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.