locally-made LED lantern
locally-made LED lantern
A "Joule Thief" is a simple circuit which 'boosts' a low DC voltage to a higher voltage in pulses. There's a lot of interesting articles on the web: just google "Joule Thief" for much more information on what it is and how it works. This project is specifically to design, build, and teach a JT cell-phone charger (with application as an LED driver) that is suited for a variety of inputs and gives a safe charging output.



  • By design Joule Thief circuits can handle a wide range of voltage and current inputs
  • Some circuits are very simple, requiring only a hollow wire coil (almost any wire will do), a basic transistor, and a resistor.
    • The circuit can be made more efficient and capable with a few capacitors.
    • Other circuits use an extra transistor in place of a hand-wound coil.
  • In general, the component requirements are very loose.
  • In the village of Kobina Andorh Krom, LEDs are hooked directly to three D-cells in series. As a result, the cells are thrown out with almost a volt of charge remaining: the reaction to a circuit powering lights with only 2 of these junked cells was quite enthusiastic.
  • A circuit with only a few components could be easily taught (less easily explained) and manufactured outside the Fab-Lab


  • We haven't been able to get air-core coils to work, greatly increasing the costs of our current circuit.
  • Without a simple circuit (or perhaps even with) this addition greatly complexifies electricity-use. If it's confusing even after quite a bit of poking at it with a 'scope, it might remain an black box despite the efforts of curious school children.
  • The lights of Kobina Andorh Krom don't have resistors; if adding that alone quadruples their efficiency, will the estimated doubling from a Joule Thief be of value? It's not worth it to charge a cell-phone from a dry cell (the D battery costs about the same as a charge), so might this end up not being so useful?

why are you working on it?

Because of the response at the village to this vs. that to resistors. Light from 'dead' cells is far more impressive than being shown a slightly dimmer light and assured it'll last longer. Because of that, this might be a far more effective way to encourage energy efficiency.


external image The-Reversed-layout.jpgSo far the only successful circuits have been variants on the previously linked Reverse Joule Thief (diagram at right). The current version has a few changes:
  • L1: 5 100-uH inductors in series (essentially a 500-uH inductor)
  • C1: 1 nF (1000 pF)
  • C2: 1000 uF
  • no battery to charge, so the LEDS at right are replaced by the light