Joule Thief

v1.3

This circuit powers a bright white LED with nothing more than your "dead" 1.5V batteries. The great thing about this circuit is that it can be made from parts you can find in scrap electronics, including the power source. The idea for this circuit has been on the web for awhile, the page I used was at Evilmadscientist Labs. Their writeup has important instructions for building the toroid coil since the wiring has to be correct for the circuit to work.

A battery is typically considered dead when the voltage falls below some threshold of a circuit it was powering and will no longer function. But it still contain plenty of useful power for a joule thief!

Warning: Be careful not to use high voltage batteries (i.e. 9V or 12V) to power this circuit as you can either blow the transistor with large voltage spikes or burn out the LED with excessive current

I found a nearly infinite supply of batteries from battery recycling boxes and the family business. A local business may be happy to throw their batteries in a box and let you get rid of them for free. You can find the little ferrite coils in scrap electronics so there should be no need to buy them. You want small ferrites because they work without adding a lot of size/weight to your circuit. Larger ferrites will work (it's only the inductance you're after) but they don't require many windings to achieve the same effect. When winding the toroid don't try to pack the maximum number of windings on the coil (unless your ferrite is very small). The oscillating frequency will decrease with increasing inductance and if it gets too low it will oscillate in the audible range (2Hz-20kHz) producing an irritating whine. Keep the oscillations above the range where humans can hear and it will only annoy your dog. If the circuit is whining just take off the coil and remove a few turns until it stops.

A variety of salvaged ferrite cores can be used to power a joule thief. All of these early coil designs were functional but several had too many windings (too much inductance) causing them to oscillate in the audible range

More recent Joule Thief v1.3 circuits showing alternating wire colors. Battery connections are made with conductive epoxy and magnets

I've been using one of my joule thief circuits continuously as a night light for over a year. To take advantage of all battery types I made two joule thief circuits, one with a AA battery holder and one with neodymium magnets attached to wires with conductive epoxy. The magnetic battery contacts work well for larger batteries and the holder works well for AA and AAA batteries.

Two new and improved joule thief circuits used as night lights. One board uses a AA battery holder and the other uses neodymium magnets and conductive epoxy to make contact to large batteries

This circuit is deceptively complex to understand. My interpretation of its operation is that current initially flows through one toroid winding into the transistor base. This small current turns on the NPN transistor and allows the second toroid winding to flow a relatively large current straight to ground. A large current through the toroid creates a magnetic field that opposes the current flow in the first winding to the transistor base (since they're wound in opposite directions), shutting off the transistor. After the transistor turns off and current flow stops, the magnetic field collapses producing a voltage spike (similar to the collapse of the magnetic field when an inductive load like a motor turns off). Since the LED is wired in parallel, the voltage spike across the transistor is also applied to the LED which exceeds the diode forward voltage and lights up the LED. This is different from a DC-DC converter where a DC voltage drives an oscillator and the AC voltage is then transformed. There is no transformation in this circuit, we're taking advantage of the toroid's magnetic field to automatically turn the transistor off and generate a voltage spike. Although the <1V power from the battery isn't sufficient to light the LED directly, the voltage spikes are several volts and will turn the LED on for some percentage of time.

Oscilloscope trace showing a Joule Thief oscillating at 5.5kHz (audible noise). Voltage spikes reached 5V with an input voltage <1V dc

In my experience, a joule thief can use a dead battery for:

After a battery can no longer produce a reasonable brightness or the circuit turns off completely you can be fairly sure that it really is dead. Final voltage will likely be <0.5V and you can finally send the battery off for recycling.

Eagle Design Files

Cadsoft Eagle pcb schematic [ .PNG | .SCH ]

Cadsoft Eagle pcb layout [ .PNG | .BRD ]

References

  1. Evilmadscientist Labs page with a good description of building the critical toroid coil