Bring “dead” batteries back to life with a toroid and the Joule Thief circuit.
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.