Summary of Bring “dead” batteries back to life with a toroid and the Joule Thief circuit.
The article describes a compact free-form Joule Thief flashlight called the Vampire Flashlight that lights high-voltage LEDs from nearly dead 1.5V cells. It’s built without a PCB by carefully soldering components—LEDs, a TO-92 transistor, toroidal inductor, switch, and battery—into a very small, usable flashlight slightly larger than the battery. The build requires patience and soldering skill, though it can be adapted to perfboard mounting for easier assembly.
Parts used in the Vampire Flashlight:
- 1.5V AAA battery (or similar single cell)
- High-voltage blue or white Piranha-pin LED(s)
- TO-92 bipolar transistor
- Toroidal inductor (toroid) for the Joule Thief transformer
- Slide or push switch
- Thin perfboard (optional alternative construction)
- 22 AWG solid-core jumper wire (optional)
- Solder and soldering tools
- Small slab of perfboard (optional, from Zombie Flashlight reference)
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.
For more detail: Bring “dead” batteries back to life with a toroid and the Joule Thief circuit.
- What is the Vampire Flashlight?
A supercompact free-form Joule Thief flashlight built by soldering components together with no PCB, only slightly larger than the battery. - Can it light a white LED from a nearly dead battery?
Yes; the Joule Thief can light high-voltage LEDs from a 1.5V cell even when the battery is considered dead. - What main components are required?
The project uses a battery, high-voltage LED(s), a TO-92 transistor, a toroidal inductor, and a switch, with optional perfboard and jumper wire for easier assembly. - Does the design require a PCB?
No; it is a free-form soldered build, though it can be adapted to perfboard mounting. - Is soldering skill necessary?
Yes; the compact free-form form factor demands patience and careful soldering. - Can the design be adapted for easier construction?
Yes; you can mount battery pack, switch, toroid, and other parts on a small perfboard and use 22 AWG solid-core jumper wires to complete connections. - How small is the final flashlight?
It is only slightly bigger than the battery itself. - What inspired this free-form build?
The discovery that a TO-92 transistor fits between the pins of Piranha-pin LEDs led to the compact free-form design.
