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# How to build a joule thief? Answered

I'm building a voltaic cell and a joule thief. I searched on some instructables and followed the one on youtube by MAKE:. When I hook up a 9V for a few seconds (To not burn the LED out), it works. However, a AA doesn't work. Not even a brand new one. So, I presume that I haven't wrapped the toroid around enough to create more voltage and lessen the amperage (If that's what joule thieves do)? I posted some pictures below. Also, I'm looking to buy a voltmeter, I'm around basic level and just need to measure if the joule thief is even increasing voltage (For now at least). Here are some cheap ones I'm looking at on amazon:
http://goo.gl/gVvo6P
http://goo.gl/hKKtGu
http://goo.gl/Q2rrKm
http://goo.gl/MQZeeS
http://goo.gl/if04Es
Some of those have autoranging, is this to automatically find the voltage/amperage?

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## Discussions

Great try on your first attempt at the JT circuit. What transistor are you using? If it is the 2n3904 with a 1k resistor, then you have a good starting point. I see the toroid you are using is powdered iron and not ferrite. This is ok and you can still make it work but, it changes the parameters a bit and you will have to experiment. It appears that you are using 22 ga. wire which is good. As the commenter before mentioned, double check that you paired the correct ends together after winding the toroid. You will need to wind as many turns as you can fit, especially if you keep using the toroid you have. You probably will have to adjust your resistance if you have a different transistor, maybe even with your toroid as well. If you have to do this, just get a 2k variable resistor and temporarily hook that into the circuit in place of the 1k resistor. Use a meter to set the vr to about 1k, energize the circuit and slowly increase the resistance a bit, or decrease it. When you get your led to be the way you want it, remove the vr from the circuit and measure the resistance. Then, get a regular resistor of this value to place in the circuit. Yes, do NOT use a 9 volt battery with this circuit. A DMM (digital multi meter) is good to have around as in my resistance checking example above, however, they will not work when checking the output of the JT circuit as the frequency (30 kHz and up) is way above what the meter can read reliably.

Best of luck to you.

You are welcome. But, just because you have a red and black connected does not mean they are correct. As in the make How To Make A JT video, you need to take one from each end. Basically what it is, is this: The wire is being joined as one long wire, if you can picture this. In the middle of this long wire is where the red and black are joined. You have your two wires as the start, and the end, and you need to connect one of each color together fro m these. (I hope I made this understandable) Watch their video again and you will see what I mean.

If you use the variable resistance method that I suggested, you don't need your meter until you find the sweet spot by tuning the circuit with the VR temporarily in the circuit. (Using clip leads or jumpers) Then, without changing the VR, remove it from the circuit and measure the resistance that you have it set on. That is where you use your meter, on the VR, after it is removed from the circuit. Or, you could build a resistance substitution box, (available in a kit) and dial in the resistance that makes it work well. Or, and this is what most folks end up doing, you can try various resistors in the circuit until you find what works but, this is very tedious and time consuming and I do not suggest this. I don't know if I am allowed to say this here or not but I have many videos on the JT circuit on my channel on that big video site. (Tube) Just check Pirate Labs. I have other JT circuit that put out 400 volts and can light 400 led from a single AA battery. It is a very interesting circuit and I have been making them for some years now. Also check Rimstar.org as he has some great tutorials on the JT circuit.

Bill

By variable resistance, you mean a potentiometer, right? I will also check those sites later today.

Yes, exactly. You need one to be at least the 1K that you are using and about 2k would be good. That way, you can dial in something below your 1k and also above it to see what works best. You can also leave that vr in the circuit in place of your 1k so you can adjust your circuit as the input battery dies. I have done this also and sometimes it is handy.

Bill

1. A common mistake people make in joule thief is that wrong ends of toroid wire are attached together. I'm not 100% sure but it seems on your pictures that where red and black wire are attached together they are the ends that come out from toroid together. Now you should switch them around so that black wire from one side (one end of parallel wrapped wire pair) is attached to red wire from the other side (other end of the wrapped wire pair).
2. Double check your transistor pin order.
3. Seven windings around toroid can be too few. I've seen recommendation to make 20 windings for a joule thief.

The 9V battery proves that there is a connection through one of the toroid wires and LED, but don't do that any more, it will burn out the LED pretty quickly, 9V is way too much.