How to make a Joule Thief

What is a Joule thief?

A Joule Thief is a simple, low-cost, booster circuit that can even start working from as low as 0.3V. It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where other circuits consider the battery fully discharged, hence the name joule thief.

So, how this circuit works?

Here you can see the schematic, there is a 1.5v battery, a NPN transistor with base collector and emitter, a varistor (variable resistor), a transformer with a centre tap and a LED. When you power up the circuit, the current will flow through L1 to the resistor and then to the base of the transistor. The transistor will then open, and it will allow current to flow from the Collector to the Emitter of the transistor, which will theenergize L2.

Now the voltage of the battery is connected to L2 and a voltage also gets induced into L1. Now the voltage of L1 adds up to the voltage of the battery, and it supplies even more current into the base of the transistor, so the transistor will stay fully open until the core of the transformer will saturate. When the core of the transformer is saturates, the voltage of L1 slowly starts to fall, so it no longer supplies enough current into the base of the transistor, so this transistor will close.

At this moment the voltage of the transformer will invert, and the voltage of L2 adds up to the voltage of the battery, and it can boost the voltage of the battery enough to light up the LED. Now the current is flowing through L2 to the LED until the magnetic field of the transformer collapses and the whole cycle repeats.

So what do you Need to make a Joule thief?

For this project you will need very few tools and parts, But for those of you who like it in text, here it is:
1) Soldering Iron
2) Solder
3) A LED
4) Flux
5) BC547 Transistor or equivalent
6) 1k potentiometer that we will use as a varistor.
7) Toroidal ferrite cores
8) Insulated copper wire
9) Switch

Construction of the Joule thief

Step 1: Winding the transformer.

You need to wind the transformer on the toroid core with 1:1 of the windings. I recommend about 30 to 40 turns on the core.

Step 2: Solder it all together

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First, bend the transistor like this ^.

Connect the middle pin of the potentiometer to one of the other legs to make it a varistor.

Solder the base of the transistor to the varistor like this.^

Twist the end of one winding with the start of the other winding and solder it to form a centre tap. Now solder the other two ends to the varistor and the collector of the transistor.

Now connect the cathode of the LED to the emitter and anode to the collector of the transistor.

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Now, connect the centre tap to the positive of the battery through a switch and emitter of the transistor to the negative. And your joule thief is ready! For aesthetics and stability, you could add tape.

Best of luck!
Happy making :)
The Terminator, over and out!