How to make a Joule Thief

Often, we have to dispose of batteries that have not been fully depleted, resulting in the loss of the energy remaining in the battery. One way to get the most out of batteries is to build a simple assembly that allows you to extract all the remaining energy from the battery. Today we will look at how to make this assembly, called Joule Thiefto light LED's with as little as 0.5 volts.

Assembly of a simple Joul Thief


- High brightness LED (white or blue).

- A 2N3904 transistor.

- A 1k resistor (brown/black/red).

- Cables (preferably two different colours).

- A (ferro-magnetic) toroid.

Materials needed to create a Joule Thief

We start the assembly with the toroidIf the toroid is not in use, we take a long extension of a couple of wires, join them together and wind them around the toroid.

We must make a number of turns in the range of 7 to 10, around the toroid to create a kind of transformer, and with the help of a pair of pliers we strip the four ends of the wires, in this way, we will have the four contacts of our transformer.

ferro-magnetic toroid winding

Once the transformer has been created, it is time to assemble it. Using a soldering iron and wires, we connect a couple of connections from the newly created transformer to a wire that will go to the positive pole of the worn battery to be used.

Joule Thief wiring diagram

The remaining pair of connections of the transformer shall be connected as follows:

- One will go to the 1k resistor, then we connect the free end of this resistor to one of the transistor contacts (negative pole of the transistor).

- The other connection of the transformer will go directly to the other contact of the transistor (positive pole).

Our assembly is almost ready, now we must connect the positive pole of the LED to the positive pole of the transistor with the help of the soldering iron, in the same way we do with the negative pole of the LED.

Assembly diagram of a Joule Thief

Now, we just need to make a wire connection from the soldering point of the negative pole of the transistor and the LED, to a wire that we must connect to the negative pole of the battery to be used. Once this is done, we look for an old or used battery, we make the connections with the wires to its poles and we will see how our LED lights up.

0 thoughts on “Cómo hacer un Joule Thief”

  1. Because the transistor receives positive feedback (double feedback reversal, one in the transformer and one in the transistor) through the transformer, and the DC current is chopped up into AC current. The secondary of the transformer acts as a coil that acquires an inductive current through the transistor when it is conducting, and that current has to come out somewhere when the transistor is cut, so it tends to go towards the LED, increasing the voltage sufficiently for the LED to consume that current.

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