The Joule Thief (Wikipedia) is a well-known “instant gratification” hobby circuit that uses just a handful of components to pull off a pretty impressive parlor trick — using a single 1.5V battery, the Joule Thief can light a high-voltage blue or white LED that normally requires 3.5V or greater to turn on. Even more impressive, it can do so using a battery that is so drained of energy as to be counted “dead” for almost all other purposes. I have not measured this value myself, but it is commonly claimed that a Joule Thief can light a white LED from a battery with an open-circuit voltage as low as 0.6.
We published a miniaturized flashlight build based on the Joule Thief circuit as Zombie Flashlight in MAKE Vol 37. That build uses an empty lip balm tube to house a AAA battery and the Joule Thief electronics, which are assembled on a tiny slab of perfboard. We had a great time building our test prototype in the MAKE labs, and got interested in the idea of designing a free-form Joule Thief flashlight that could be soldered together from individual components with no prototyping board or PCB. We were playing around with the components and had a batch of those new(ish) “Piranha-pin” low-profile LEDs lying around at the time. The discovery that a TO-92 transistor package nestled perfectly back-to-back into the empty space between the four LED pins kind of sealed the deal — we needed to do a free-form build. The Vampire Flashlight is the result.
The design is supercompact and minimal, and the end product is a usable flashlight only just slightly bigger than the battery itself. While the Joule Thief is a very simple circuit to build, this form factor demands a bit of soldering skill and patience. If you don’t enjoy the process of meticulous electronics craftsmanship, it could easily be adapted for construction by mounting battery pack, switch, toroid, and everything else on a small rectangle of perfboard, then completing the circuit connections with 22AWG solid-core jumper wires.
How it Works
There is of course no magic here. The Joule Thief does not produce more energy than it consumes — rather, it uses the energy that is available to it in very clever ways. Just as it is possible to design a hydraulic system (like Heron’s Fountain) that uses a relatively large volume of water at lower pressure to transport a relatively small volume of water at higher pressure, it is possible to design an electrical circuit that “steps up” a relatively large current at lower voltage to a relatively small current at higher voltage. There are in fact several tricks for doing this, and the Joule Thief uses more than one of them.
The most straightforward to understand is that the circuit oscillates to produce short bursts of current at higher power than the battery could continuously sustain. These bursts of power come tens or even hundreds of thousands of times per second, which is much faster than our eyes can detect; the LED appears to be continuously lit in spite of the fact that it is actually powered less than half the time.
Voltage from the battery (A) trickles through the transformer’s secondary winding (B) to the transistor’s base (C), opening the transistor’s collector path (D) partway. This energizes the primary winding (E) partway, storing up energy in a magnetic field which in turn induces current in the secondary, boosting the voltage back to the base, opening the transistor further … around and around. This feedback loop rapidly slams the transistor fully open. The transformer’s tiny ferrite core (F) rapidly saturates, at which point induction ceases, the base voltage drops again, the transistor slams shut, the magnetic field collapses and its stored energy is dumped to the LED (G). The cycle starts again, switching on and off about 50,000 times a second (50kHz) — more than fast enough to make the LED appear steadily lit.