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01 – Build your own Joule Thief

A Joule Thief is a small ingenious little electronic circuit used to drive a white super-bright LED from a single 1.5V battery cell and what makes it more remarkable is that it can drain a battery down to around 0.4 volts. Flat batteries still have a lot of energy in them but most battery operated devices require very specific voltages and as soon as a battery discharges to below their required voltage level which is normally around 1 volt they stop working. The Joule Thief can access the leftover energy of ‘dead’ batteries to run a white LED to make light. So now you can hook up your dead batteries and squeeze all the leftover power out of them and be the hero amongst your friends.

Z. Kaparnik published a circuit for a transformer feedback single transistor invertor in the November 1999 issue of Everyday Practical Electronics. He named the circuit a Joule Thief because it robs a battery of every last Joule of energy. The circuit is not a new invention as it is based on a blocking oscillator that was in use well before World War II. It was then build with a vacuum tube or valve but our Joule Thief uses a transistor.

The Joule Thief circuit diagram
The  Joule Thief circuit diagram is a very simple design and anybody can build it with only a handful of electronic components. The components can just be soldered together without a circuit board. If you don’t want to solder anything and you want keep it save for children, a breadbord can also be used for circuit construction. I prefer the breadbord option because it gives you more flexibility for experimentation in the classroom and mistakes can quickly be rectified. The list of components for the project are:

  1. 1k ohm resistor 
  2. a White or Blue LED, the ones that switches on at 3 volt
  3. a 2N2222 NPN transistor
  4. a ferrite toroid or a ferrite bead will also work
  5. a Breadbord for circuit construction (optional)
  6. Some connecting wires for the breadbord (optional)
  7. a piece of insulated copper wire (1 meter) to wound the toroid coil, 0.5mm diameter (25 SWG) or two pieces (30 cm each) of insulated solid core copper wire. The plastic insulated copper wire is easy to work with and any piece of wire can be used. Using two colours just makes it easier to keep track of the two coils on the toroid.

Basic Electronic Knowledge required
What basic electronic knowledge is required to successfully build a Joule Thief? If you aren’t a electronic nerd, don’t worry! I will quickly go over all the things you need to know and you can learn as you go along.

  • Toroid – A toroid is a doughnut-shaped object. In electronics a ferrite toroid core is used in a transformer or a coil especially in high frequency applications. The shape of the toroid concentrate the magnetic field uniformly inside the core making this type of transformer very efficient. Transformer cores are normally made of iron but high frequency transformers uses ferrite cores because iron cores saturates to quickly.
  • Coil – A coil is just a piece of insulated copper wire wound around a core, it can also be wound without a core. Another name for a coil is an inductor and it has an electric property called inductance measured in Henry. 
  • TransformerA transformer normally has two or more coils also called windings wounded on the same core. The function of a transformer is to transform an oscillating voltage on the primary winding into a lower or higher voltage on the secondary winding of the transformer by inductance. To generate a voltage in a coil (winding) the coil must move through a magnetic field, but a transformer is static so the magnetic field must move. The only way to achieve that is to increase the magnetic field from zero to a maximum and then decrease it again to zero.  To do that an oscillating voltage is supplied to the primary winding.
  • Back EMF – Back Electromotive Force is the voltage that is generated in an inductor when the magnetic field collapses and its polarity is the reverse of the voltage that generated the magnetic field in the inductor. 
  • Resistor – You must know how to read the colour code of a resistor, see Reading Resistor Colour Codes
  • Transistor – Identify the three legs of the 2N2222 NPN transistor in the data sheet on the right. The function of the transistor in the Joule Thief is that of a ON/OFF switch, see NPN Transistor for more information.
  • LED – Identify the Anode and Cathode of a LED to ensure correct wiring, see Light Emitting Diode for more information.

Step by step construction of the Joule Thief
Now we have done our homework and we have the knowledge to build our Joule Thief circuit.

Step 1 – Wound the Toroid






Take the two strands of plastic insulated wire and hold them together. Start off by sticking then through the middle of the toroid until you have 2 cm of wire left to make a connection with later. Keep the two wires together and wrap them around the toroid until you have covered the whole toroid. If you have wire left you can continue until you have about 2 cm of wire free on either side. Remove about a 0.5 cm of the plastic insulation on all four of the wires as in the photo. You have now wound your toroid and we are ready to continue.

Step 2 – Join the two coils of Toroid
The next step is to join the two coils or windings of the toroid. You may have noticed the two dots next to the toroid windings in the circuit diagram. These two dots indicate the polarity of the transformer and as you can see the two windings are opposite to each other. To make sure that the polarity of our toroid windings are correct we must do the following: 

  • If we look at the toroid we noticed that we have two wires of different colours sticking out at both ends of the toroid.
  • Take one wire of a specific colour (green in the example) on the one side of the toroid and one wire of the other colour (white in the example) on the other side of the toroid and connect (or solder) them together. These two wires forms the top pole of the toroid as in the circuit diagram and is connected to the positive terminal of the battery. 

Step 3 – Build the rest of the circuit
With the toroid sorted it is time to build the rest of the circuit. Using you transistor diagram and the LED configuration it should be easy to finish the circuit on breadbord or solder the components together. An easy way to remember the LED configuration is to remember that the flat side of the LED is the same as the cross bar in the LED symbol, so the arrow side is the round side of the LED.

Step 4 – Switch On
After you have completed the circuit it is time to switch on and if you have done every thing right you should have something like this.

How does the Joule Thief work?
It is important to understand how the Joule Thief works because we want to learn the principles involved and if don’t learn anything then it is just a monkey see monkey do exercise. To understand how the Joule Thief works we can do the following experiment:

  1. Remove the transistor from the circuit by pulling it out of the breadbord, if you have soldered everything together this might be more difficult. Looking at the circuit diagram we see that the secondary winding, the winding that is connected to the 1k resistor is now an open circuit. 
  2. We now take a connecting wire and connect it across the LED. If we quickly pull the one end of the wire out again, what do we see? The LED lights up in a brief flash. We can put the wire back again and repeat the process a couple of times and every time we disconnect the wire the LED flashes. 
  3. So what is happening here? When we connect the wire across the LED, current flow through the primary winding (the winding connected to the LED) and it generates a magnetic field. When we pull the wire out, we break the circuit and the magnetic field collapses. When the magnetic field collapses it generates a back-EMF voltage across the secondary winding with opposite polarity, that is the top of the winding is negative and the bottom of the winding connected to the LED is positive. But the current has nowhere to go and the voltage keeps rising until it reaches about 3 volt. At 3 volt the LED switches on and that creates a path for the current and the energy is released as a flash of light through the LED.
  4. So we now have proved that the LED is powered by the back-EMF voltage of the primary winding.

So what is the Transistor and secondary winding for? The transistor is just an ON/OFF switch that switches very fast on and off about 40 KHz that is 40 000 times per second. At that speed the LED appears to be permanently switched on.

I will try and give a simple explanation of how the rest of the circuit works. When the battery is connected current flow into the base of the transistor and the voltage across the base and emitter (Vbe) rises and the transistor is switched on. This means that a larger current can now flow through the primary winding and then through the transistor. This generates a magnetic field in the toroid opposite to the field created by the base current through the secondary winding. As soon as the primary current is greater than the secondary current the voltage on the secondary winding reverses switching the transistor off. The magnetic field collapses and the LED is switched on briefly and the whole process starts over again. The cycle repeats 40 000 times a second. Lastly the 1K resistor is there to limit the base current of the transistor.

That brings us to the end of the Joule Thief, I hope you have learned a lot and I am sure you will be having fun with your Joule Thief for quite awhile.

Technology Lesson Plan for Joule Thief
If you are a technology teacher or educator the following might be of interest to you: I have compiled a lesson plan for the Joule Thief that you can use in your preparation to present a technology lesson. I have put a few ideas down that you can incorporate in your own lesson plan or you can use this one as a framework or you may use it as it is. I would also appreciate any feedback or suggestions on how to improve the lesson plan.

The Technology Lesson Plan for the Joule Thief can be downloaded here, by right clicking on it and selecting ‘save link as’ from the popup menu.