Sunday, January 15, 2012
Super Tech Lab Joule Thief
One of my pet peeves is throwing out AA batteries because most gadgets consider a 1.5V battery dead by the time it degrades to 1.3V or below. It seems like 86% of its energy is wasted. My digital camera is the worst offender; understandably, the flash requies a lot of current. Rather than sending these 'spent' batteries to a land fill, I have been saving them for some energy harvesting/joule thief circuit experiments. I did not design the joule thief circuit pictured above (there are dozens of variations across the net), but I redrew it in tinycad and built a demonstration circuit. It is simply an oscillator that will run on very low power, providing an inductive 'kick' to keep an LED illuminated well below a straight DC supply voltage. The induction comes from a toroid wound with a bifilar winding (opposing coils). The easiest way to make it is to get a twisted pair of wire (like black and white so you can tell them apart), wind about 10 turns around the toroid, and then connect the black wire from one end to the white of the other (this gives us opposing coils). The 2N3904 could be replaced with just about any NPN transistor (2N2222, etc.)
The circuit I built oscillated at 10KHz, and it illuminated a LED for 108 hours on a 'bead' battery (1.3V).
It made a nice night light. The toroid could have been much smaller, I just used the first one I could scrounge.
A good green project would be to retrofit some battery operated device to use a circuit like this to get more life out of a set of batteries.
Larger Circuit Diagram [Link]
My other interests [Link]
TinyCad free schematic capture [Link]