Example Micro-torch Circuit
Running an LED off just 0.75 volts to make a Micro-torch
An Ultrabright yellow LED usually needs at least 2 volts across it before it will light. This simple circuit by Z. Kaparnik was published in "Everyday Practical Electronics" magazine (page 804 November 1999). It uses a single transistor and a transformer comprising just a few turns of wire on a tiny ferrite bead to form an oscillator. The circuit will operate off an almost dead battery (just 0.75 volts) and produces around 30 volts with the LED disconnected.
With the LED connected, the voltage is clamped and the circuit produces short duration high-current pulses which drive the LED to full brightness without destroying it.
The result is an extraordinarily bright beam of light. The circuit is so small that it can be built into a key fob, a walking stick knob and many other common items into which a torch would normally not fit. The beam is bright enough to allow you to find your way along a path in complete darkness.