Piggyaxe joule thief kit piggyaxe p a kit of parts to build the ultimate green circuit that uses the cells that other people throw away p p the joule thief is a simple circuit but it delivers great functionality its name is derived from the concept that it s stealing energy joules from the battery that powers it nothing special there except that this circuit will work down to absolute cell exhaustion at around 0 5 volts hence the idea that its stealing the last dregs of energ