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.

Research


positive:

  • as for the light, plus:
  • Very useful for turning low-voltage solar cells (which the lab has in abundance) into cell-phone chargers
    • The alternative is cutting the cells into pieces and attaching them in series, a messy process

negative:

  • an early circuit charged the battery excessively slowly, but the newer version should be higher current

why are you working on it?

Because a simple solar cell-phone charger would be exceptionally useful, and this would allow a wider range of panels.

theoretical cost breakdown (1 cedi = 100 pesawa = around 0.56 euros/0.71 US dollars)

(all prices more than expected)
Panel - around 1 Watt - 1 cedi
Circuit - 50 pesewas to 1 cedi
Enclosure - a few cedi for wood, clear acrylic, screws
Total: 5 cedi to make something that would take only 2-3 hours of sun to fully charge a phone

Work


See Joule Thief Light for the circuit: the only difference is that the lights are replaced by a reversed 4.7V zener diode (if charging a battery directly) or by a 5V regulator (through cell-phone: untested, maybe unncessary).

external image Circuit-Operation.jpgQS of quantsuff.com highlighted his Solar Powered Reading Light : the circuit (at right) will charge a cell-phone battery to 100% if the red LED is replaced with a 20mA blue or white one (and if R2 replaced with a 10-20 ohm resistor it'll light LED2 safely).

As the lab has about a dozen 100 uH inductors, this is definitely worth a shot.