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
| S.No | Component | Value | Qty |
|---|---|---|---|
| 1. | Breadboard | – | 1 |
| 2. | Battery | 1.5v | 1 |
| 3. | Connecting Wires | – | 1 |
| 4. | NPN Transistor | BC547 | 2 |
| 5. | LED | 5mm | 1 |
| 6. | Inductor | 100uH | 1 |
| 7. | Resistors | 22k ohm, 330 ohms | 1,1 |
| 8. | Ceramic Capacitor | 220pF | 1 |
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.
