What would you say if I told you that I could make a Joule Thief without a transistor or other electronic parts, just a single mechanical part? Would you believe me?
I have seem this method used many times in old radios and old equipment made before transistors were born. It’s not magic! You may have even experienced it before, accidently.
I knew a guy who liked to play jokes on (former) friends. He had a small device that fit in his palm, and held a small battery. When he shook hands with you, it would give you a mild, harmlessness shock, and your reaction would be immediate! Yow!
Well instead of a palm shocker, we will use a small, low voltage DC relay. Preferably it’s for a voltage less than 5 volts DC, but if not available, then use a 5 volt relay. The contacts should close at less than 5 volts, perhaps 4 or even less volts.
We connect the C or armature terminal of this relay to one of the coil terminals. The other coil terminal is connected to the negative end of two AA cells in series. Two white or blue LEDs are connected in series, and the cathode lead with the flat spot is connected to the battery negative. note that the LEDs do not light because they require 6 volts, but the battery is only 3 volts. The other lead of the LED is connected to the coil and C terminal. The positive of the battery is connected to the NC terminal of the relay.
When that positive lead is connected to the NC or normally closed contact, the current goes to the C terminal and to the coil. This energizes the coil, which disconnects the NC terminal. With no more current flowing, the magnetic field in the coil collapses, and the voltage rises higher and higher, until it gets to 6 volts, where the series LEDs start to conduct. This causes the LEDs to light, and the coil discharges its stored energy into the LEDs. After this, the C and NC contacts close and the process starts over again.
We have just made a mechanical Joule Thief! We also have made a buzzer, depending on how loud the relay is. I should warn that this may put out so much energy to the LEDs that they might get too hot and burn out.
By adjusting the wire size and turns on the relay coil, it can be made to operate on just about any voltage. If the voltage is very low, a winding with additional turns may have to be added to step up the voltage to the LED or LEDs.