An easy “Joule Thief” inverter circuit to power an electric torch with two LEDs using only one 1.2 or 1.5 volt (dead) battery

My handmade torch with a Joule Thief inverter design. The missing piece of the ferrite core can be found at the top of the battery. That’s the coil. The rubber band is used to hold down a momentary switch on the plastic tubing so that this photo could be taken.

These days, electric torches (flashlights) use LED bulbs. An LED bulb typically requires a minimum 1.85 volts to power on. Usually, a battery of two 1.5 volt cells (providing 3 volts) is used in torches to power them. If the voltage supplied by the cells in the battery falls below 1.85 volts, the LED will not light up. The “dead” cells in the battery will have to be replaced even though they still have considerable energy in them.

A “Joule thief” can be used to power an LED bulb using just one 1.5 (non-rechargeable) or 1.2 (rechargeable) cell. A “Joule thief” is so named because it can use just one cell. It can also power an LED with a cell that is deemed dead. The Joule thief circuit accomplishes this by using a coil to store energy and then generating spikes of voltages to turn the LED on and off several times a second. For human eyes, the LED appears to be lit up.

A Fritzing version of a Joule thief circuit

There are many videos on the Internet on how to build one. There are a number of misleading ideas of what is required to make it work. No need a circular ferrite core. I used a broken piece of ferrite core piece from a transformer salvaged from a failed phone charger. The number of windings on the coil does not increase the voltage. Instead, it affects the frequency of the voltage spikes. The voltage output is determined by the base voltage supplied by the battery and saturation level of the ferrite core. The windings need not be evenly spaced or anything like that. However, they need to be close to the ferrite core.

Parts required:

  • Two white LEDs
  • One non-polarized 0.1 uF capacitor
  • NPN transistor
  • 220 ohm resistor
  • jumper wire
  • ferrite core piece
  • winding (enameled) wire or any insulated wire

Construction:

  1. Take two strands of equal-length enameled wire and wind it around a piece of ferrite core, say 20 to 40 times. Remove the enamel from the four ends of the coil. Twist/solder two ends of different wires. Use a multimeter continuity test to ensure that you are not tying the ends of the same wire. You can also use a LED+button cell combo to do the continuity test. Connect this tied end to the battery’s positive terminal.
  2. Now, you have two ends of coil that are free. Connect one end to a resistor and the other end to the collector of the transistor.
  3. Connect the other end of the resistor to the base of the transistor.
  4. Connect the LEDs and capacitor parallel to the emitter and collector of the transistor. The emitter is on the negative side.
  5. Connect the negative lead from the emitter of the transistor to the negative terminal of the cell.

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