The Joule Thief is an incredible bit of electronics. Using just a coil, resistor and transistor it can light a 4v LED using a flat battery.
I thought I’d have a go, and sure enough, it’s as amazing as it sounds. The LED here is 4v, and won’t normally even light on a good 1.5v battery, let alone a dead one.
Total cost of the components (including LED) is about 60p. I scavenged the enamelled copper wire from an old transformer.
This battery is for all intents and purposes completely flat. At less than 1.1v, it would normally be consigned straight to the bin. But here is is running a 4v ultrabright LED.
It’s been lit for the last three nights continuously, and is showing no sign of stopping yet. It’s perfect as a mini night light to save having to switch on the normal lamp.