Joule Thief

Posted on Feb 7, 2014

There are only 4 parts to the circuit, including the indicator LED. There is a hand-wound coil, a 1k resistor, a 2N4401 transistor and the LED. I quickly threw this together by just wiring the component leads together and it worked right away. The coil is wound with solid core telephone wire that was in my junk box. I wound 8 turns of wire (green)

Joule Thief
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, then connected another piece of wire (white) to the first and continued winding for 6 more turns. This makes a center tap when the two wire were joined. I would have wound 8 turns with both pieces of wire but the middle of the core was filled with wire and I could not get the last two turns in place. The battery powering this circuit is a depleted AAA that had less than 1v remaining in it. Since the forward voltage threshold of an LED is from 2v (red) to 3. 5v (white), a single 1. 5v cell will not be able to light up the indicator without some external circuitry. With this circuit, the coil and transistor form an oscillator that sends increased voltage pulses to the LED and this lights it up (quite brightly). The white light from the LED is bright enough to use as a flashlight in small spaces, and it makes use of discarded batteries that are too weak to use in other applications. The small circuit could be mounted inside a pedal that was using 1. 5v power, to drive the LED indicator. A red, blue or green colored LED could be used in place of the white shown here. The circuit can be improved by using smaller gauge wire and winding more turns on the core. I plan on rewiring the core with some 38 ga. solid wire that I had used for a wah inductor. I should be able to get 20 or more turns on each coil section, and this will improve performance.

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