Ultra Low Power LED

Flashlight using a 1.5V cell

With this circuit it is possible to use batteries which are normally already used up for a long time as flashlight batteries. It is better known as "Joule Thief" and was published by Z. Kaparnik in 1999 in the journal EPE. The simple blocking oscillator consists of a coil, a resistor and a universal transistor. Two identical windings are wound on the ferrite core. One controls the base of the transistor, the other is used as a storage device to induce a voltage sufficient for the LED.

Picture: Test setup

The circuit operates at 0.35...1.5 volts and produces the voltage needed for a white light emitting diode, which is well above 1.5 volts. The battery in the test only has 0.6 volts, but the LED still reaches a usable brightness.

For simplicity, I pulled out a small ferrite core from the tinkering box. The number of turns of the enameled copper wire wound on it is relatively uncritical; it should not be too few in any case to keep the current low. As a test, I simply wound the core with about 1 to 1.5 meters of wire and the circuit worked fine straight away. By the way, the easiest way is to double the wire when winding.

Picture: Components
Picture: Coil

And finally here is the circuit diagram:

Picture: Circuit

Version 2 as mini flashlight: (coil core is made of an old ferrite for interference suppression of flat ribbon cables)

Picture: Mini flashlight
Picture: Mini flashlight
Picture: Mini flashlight
Rebuilds by visitors of my homepage

Picture: Construction by Nico

This picture was sent to me by Nicolas Jessen from Flensburg, who can now finally use his huge collection of drained batteries effectively.

Picture: Construction by Jens

Jens from Uelzen has (after I gave him some enameled copper wire) also finished his first test setup and is testing the runtime with a fresh battery.

Picture: Construction by Marcel

Marcel uses the circuit for a LED tea light. A great idea!