A “Joule Thief” is a simple voltage booster circuit. It can increase the voltage of a power source by changing the constant low voltage signal into a series of rapid pulses at a higher voltage. You most commonly see this kind of circuit used to power LEDs with a “dead” battery, but there are many more potential applications for a circuit like this.
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Step 1: Gather Your Components
BUY transistor 2N3904:
BUY 1K Resistor:
Ferrite toroid core
NPN transistor 2N2222, 2N3904, or similar
1k ohm resistor
A used AA battery (if you don't have one than you can use new AA too)
Components buying link (affiliate):-
Toroid ferite core -
Resistor set -
Step 2: Circuit & Working Explanation
A Joule Thief is a self-oscillating voltage booster. It takes a steady low voltage signal and converts it into a series of high frequency pulses at a higher voltage. Here is how a basic Joule Thief works, step by step:
1. Initially the transistor is off.
2. A small amount of electricity goes through the resistor and the first coil to the base of the transistor. This partially opens up the collector-emitter channel. Electricity is now able to travel through the second coil and through the collector-emitter channel of the transistor.
3. The increasing amount of electricity through the second coil generates a magnetic field that induces a greater amount of electricity in the first coil.
4. The induced electricity in the first coil goes into the base of the transistor and opens up the collector-emitter channel even more. This lets even more electricity travel through the second coil and through the collector-emitter channel of the transistor.
5. Steps 3 and 4 repeat in a feedback loop until the base of the transistor is saturated and the collector-emitter channel is fully open. The electricity traveling through the second coil and through the transistor are now at a maximum. There is a lot of energy built up in the magnetic field of the second coil.
6. Since the electricity in the second coil is no longer increasing, it stops inducing electricity in the first coil. This causes less electricity to go into the base of the transistor.
7. With less electricity going into the base of the transistor, the collector-emitter channel begins to close. This allows less electricity to travel through the second coil.
8. A drop in the amount of electricity in the second coil induces a negative amount of electricity in the first coil. This causes even less electricity to go into the base of the transistor.
9. Steps 7 and 8 repeat in a feedback loop until there is almost no electricity going through the transistor.
10. Part of the energy that was stored in the magnetic field of the second coil has drained out. However there is still a lot of energy stored up. This energy needs to go somewhere. This causes the voltage at the output of the coil to spike.
11. The built up electricity can't go through the transistor, so it has to go through the load (usually an LED). The voltage at the output of the coil builds up until it reaches a voltage where is can go through the load and be dissipated.
12. The built up energy goes through the load in a big spike. Once the energy is dissipated, the circuit is effectively reset and starts the whole process all over again. In a typical Joule Thief circuit this process happens 50,000 times per second.
Step 3: Wind the Toroid
The transformer in the circuit is made by winding wire around a ferrite toroid. These toroids can be purchased from electronics suppliers or they can be salvaged from old electronic equipment such as power supplies.
Take two pieces of thin insulated wire and wrap them around the toroid 8-10 times. Be careful not to overlap any of the wires. Make the wires as evenly spaced as possible. To hold the wires in place while I was prototyping, I wrapped the toroid in tape.
And after that join two opposite color wires from both end together as shown in image & refer video for better understanding.
Step 4: Connections
follow the above circuit and solder the positvive of the led to collector of transistor & negative to the emitter & 1 k ohm to base then one of the single wire of toroid to the collector & other one to the 1k resistor as shown in image & video and connect a wire to the emitter then connect +ve of the battery to the two together joined wires of toroid & -ve of battery to the wire connected to emitter.
Step 5: Final Step
After this make this permanent on a pcb along with the switch to turn it on or off and reuse your old used AA battery in your mini torch made with joule thief circuit.
If having issue with circuit etc. Then refer vudeo for better understanding.
Enjoy making your own joule thief & reuse your old AA batteries.