A Joule Thief (JT) is a step-up voltage transformer based on the working mode of a PWM (Pulse Width Modulation), it produce a oscillation in a inductor with the help of a transistor (2N3904, 2N2222, ...) then the output of the inductor is your new voltage. The result is that you can light a white, blue or even a ultraviolet LED with a single 1.5V cell (AA, AAA or any type of cell, i have used a watch cell too). This "voltage booster" circuit is not very efficient but it can use batterys that are near depleted. I whud not power up a microcontroler are anything that whud be disturbed by the voltage and power oscillation of the output, there are some dedicated step-up voltage boosters like the IC from Maxim with a 90% efficiency. With a bigger inductor and another transistor it is possible to get some very high voltage, i have see on the web some JT that can light up a neon tube from a 12v bike lead acid battery.

Step 1: Tools

Solder iron, Dremel, Hot Glue, Cutter or Exacto Knive and ... a pair of hands .

Step 2: Parts

- 1 toroid made from ferrite (it can be buyed or recovered from a old PC mainboard or a PC power supply)

- 1 meter of enamel coated copper wire with a diameter of 0.5mm (i got my from a old transfomer)

- 1 transistor (any NPN transistor like 2N3904, 2N2222, ... read the datasheets) i will use a 2N3904

- 1 resistor 1kohm brown-black-red (a 1k variable resistor whud make it easy to "tune" the JT for lower voltages)

- 1 pcb of 1cm X 1cm is the smallest (my is 1cm x 2,5cm for the 2 screws to fix it, and is recovered from junk)

Step 3: Winding the Toroid (transformer)

The circuit runs at up to 30 to 50,000 cycles per second, so the transformer needs to be made from a material suitable for those frequencies. Fortunately, small ferrite beads used for electrical noise suppression etc are readily available and work well for this task.

To wind this, you take the thin enamel coated copper wire and fold it in half. Now, pass the folded end through the core most of the way so that the two free ends stick out about 40mm from the core. Glue these to the core side-by-side and let the glue set. Hot melt glue makes this simple and fast, but make sure you only use a tiny dot of glue. Now, wind 20 turns onto the ferrite core in a bifilar fassion (well, you have to really as you are using two-core wire!) and once finished, spread the turns evenly around the core and glue the other end (the folded end) of the wire to the core to stop it unwinding. The best way to do this is to put a tiny squirt of hot melt glue into the hole of the ferrit core. This stops the windings moving or unwinding. Try to keep the turns as tight as possible against the core, but small gaps between the wire and the core are ok. Now you need to cut the folded end of the wire so that you have two separate windings on the core. Trim the wire ends
to about 20mm in length and remove the enamel coating. Once the wire ends are stripped, use a
multimeter to find the two ends of each winding. They become windings A and B. Wire ends A1 and B1 start at the same place on the transformer, as do A2 and B2.
Another way to wind this http://www.flickr.com/photos/oskay/1830118932/in/photostream/ .
2N3904 Datasheet (pdf) http://www.fairchildsemi.com/ds/2N/2N3904.pdf.

Step 4: Observations

1. My JT is made for the purpose of light a single white LED, thats why i have experimented with different sizes of the toroid and different types of wire until i got the smallest battery drain, 17mA. (my first toroid i made got a healthy 45mA).

2. JT can drive multiple LED in series, puting them in parallel is a bad idea, it will only light a small number of them and the drain will increase unnecessary.

3. Read the datasheet of the transistor you use and do not go over the max power and intensity specified there or it will burn the transistor.

4. Adding a ceramic capacitor (104 or smaller) in parallel will drop the LED drain and it can then light up more LEDs. Putting a small ceramic capacitor in parallel with the resistor will make it work with lower input voltages. Lowering the resistor value from 1kohm to less can make it work with lower voltages too. Be aware not to go over the max values after you lower the resistor.

5. The pictures are made with a infrared camera (is the only i have that has a acceptable macro mode) and that is why the colors are strange.

6. Efficiency calculation : see the picture ... (i hate math :P )