Simple Joule Thief Circuit Using BC547 NPN Transistor

A joule thief is an electronic circuit that takes in a small voltage and turns it up to a higher voltage (for devices like LEDs, which require 2-3volts to run). This circuit can power low current circuits with a small voltage source, such as a 1.5V AA battery. In this project, we will design a simple joule thief circuit using a BC547 NPN transistor.

A joule thief circuit is basically an efficient, self-oscillating voltage booster circuit, using transistors, resistors, and an inductor, which can boost voltages as low as 0.4 V from any dead AA 1.5V battery, onto much higher levels.

Hardware Components

The following components are required to make Joule Thief Circuit

BC547 Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of BC547

Joule Thief Circuit

Working Explanation

The circuit used in this project is a modified Joule Thief. A Joule Thief takes a steady low voltage signal and converts it into a series of high-frequency pulses at a higher voltage. Initially, both transistors (Q1 & Q2) are switched off, On completing the circuit a small amount of electricity goes through the 22K resistor and the inductor, the induced electricity in the coil goes into the base of the transistor & opens up the collector-emitter channel of the first BC547 transistor, eventually creating a feedback loop by continuously draining power from the AA Battery.

The small-signal voltage obtained through the collector of Q1 serves as a control input for the Q2 Transistor connected as an amplifier and provides a bright light output through the LED.

Applications

• A joule thief circuit allows the power source to be much smaller allowing it to be used in small portable electronic devices and circuits such as miniature LED flashlights.
• Also used in devices such as a joule ringer & vampire torch.