The Joule Thief is a simple circuit that takes a low voltage source and, through induction, turns it into a higher voltage. Voltage inputs can be as low as a considered “dead” AA battery. The circuit works by rapidly switching the transistor causing the current in the coil of wire to stop and go at high speeds. When a current flows through a coil of wire, an electromagnetic force (EMF) is generated and creates a magnetic field. When the current flow stops, that magnetic field collapses and the EMF induced in the coil reaches a much higher voltage than its primary input. The transistor is doing this well over 10s of thousands of times per second that the human eye cannot tell the LED is turned on and off so quickly.
Note, however, power is not being created, but is being transformed—the power input from the battery is the same power outputted from the circuit minus the power being absorbed by the transistor and light-emitting diode (LED), except at a much higher voltage.
Why is it necessary to use the Joule Thief to run the LED? Double A batteries are typically 1.5 volts and LEDs need at least 2-3.5 volts to work. However, LEDs do not need a significant amount of current. Since the Joule Thief uses the majority of the current to induce a higher voltage the smaller current left over can be used to run the LED. Go ahead, try and light one of the LEDs with just the AA battery—it won’t work.
See the attached schematic for the Joule Thief circuit:
Step 1: Parts & Step 1
If you are not familiar with schematics, the following step-by-step instructions will aid you in your construction of the Joule Thief. Ω
Make sure that the following components are available:
1. Solder-less Breadboard
2. AA battery holder
3. Copper magnet wire
4. Ferrite torroid
5. Transistor (2N3904)
6. 1 kOhm resistor (1000 ohms Ω)
8. Some extra jumper wires
Step 2: Step 2
Connect the battery holder on one side of the breadboard. Be sure to observe polarity— Black wire is negative (-), Red wire is positive (+).
Step 3: Step 3
Begin with the ferrite torroid and loop one of the magnet wires on one side of the torroid. Try not to overlap the wire. *Also, be sure to count how many times you wind the wire because you will want to wind it that many times on the other side of the torroid. These instructions have the torroid wound 9 times on each side.
Step 4: Step 4
Begin the other side with the other strand of magnet wire. Try to keep the end of the wire the same length as the first.
Step 5: Step 5
If one side has longer wire than the other, then begin twisting the wires together. If they are the same length, it does not matter which side you twist together. Just make sure that the shorter end does not get twisted (it makes connecting the torroid on the breadboard easier). Do not twist the wire all the way to the bottom.
Step 6: Step 6
Because the magnet wire has protective enamel around it, it is necessary to remove it in order to have connectivity with the circuit. Using a sharp object such as a hobby knife or blade, gently “shave” off the enamel. Observe the copper is shinier when the enamel is removed. Do this to all four ends of the magnet wire.
KIDS ASK AN ADULT FOR HELP DURING THIS STEP.
Step 7: Step 7
After stripping the enamel off the wire, continue twisting the wire previously twisted till the wires are connected, however, just before the end, cut one of the ends shorter and wrap the rest of it around the longer end. The purpose of twisting the magnet wire is because it’s too thick to fit in the holes on the bread board.
Step 8: Step 8
Connect the twisted end to the same positive bus as was the battery. For the other two ends, put them in different holes apart from each other. To make things a little easier, these instructions connected one of the ends into F5 and the other into F11—it does not matter which one.
Step 9: Step 9
Take the 1 kOhm resistor and connect one end to I5 and the other to F12. No need to worry about polarity.
Step 10: Step 10
Take the transistor and orient the component where the **flat side faces to the right.
Put the top lead into G11, the middle lead in G12, and the bottom in G13.
Step 11: Step 11
Take one of the extra wires, preferably a black one, and insert it behind the bottom transistor lead—hole F13.
Step 12: Step 12
Take one of the LEDs and insert the anode into hole J11. The anode is the longer lead on the LED. Connect the cathode, or shorter lead, into J13
Step 13: Step 13 - Last One!
Lastly, take any AA battery and place it in the battery holder. If all went well the LED should turn on
Step 14: Optional Steps
1. Take the LED out
2. Place a wire where the LED anode was, or J11, and connect the other end to the opposite positive bus on the bread board
3. Take another wire and place it where the cathode was, or J13, and connect the other end to the opposite negative bus on the bread board.
4. Place the other LEDs on the opposite side of the breadboard where you connected the wires with the anodes, or longer leads, going into the positive bus, and the other end going into the negative bus.
*It is necessary to count how many times you wind the torroid to keep the frequency as harmonious as possible. Uneven or overlapped windings could cause the Joule Thief circuit to not work.
**The transistor has three leads. One is the Emitter, the middle is the Base, and the third lead is the Collector. Its acronym is EBC. An easy way to remember the acronym is “Eat Big Cookies.”
Now that the Joule Thief circuit has been built, it can light many LEDs with a double A battery; before it was impossible to light just one.