Probably, this may be the smallest 1 Watt LED Torch working on a Watch battery. It is the “Micro Joule Thief circuit” that is made as compact as possible. It rests on a heat sink of the 1 watt LED and powered by a 1.5V Button Cell. We can make different types of Joule Thief circuits and this experimentation is to make it as small as possible. Try it.
You need the following components
1. A small Ring core
2. 1 Meter enameled copper wire 22-26 SWG
3. Transistor BC 547
4. 1K Resistor
5. 1 Watt White LED with Heat sink
6. 1.5V Button cell
Take a small Ring core to make the Toroid. You can procure it from an old CFL or SMPS or discarded Computer or TV board. Cut the copper wire equally. Mark the opposing ends of the wires. We have to tie them together later. Wind both wires around the core to make a small Toroid. Now it has four leads. Two marked leads of the opposing wires and unmarked ends. Join both the marked wire ends. These are the opposite ends of the two wires. See image below.
See the link to get more info of Joule Thief circuits and Toroid winding.
Now the Toroid has three leads. Connect one lead to the base of BC 547 through 1K resistor. Connect the other lead to its collector. Now connect the + of White LED to the Collector of Transistor and – to the Emitter. Finally connect the jointed leads of the Toroid to the Positive side of battery. The Negative of the battery should go to the emitter of Transistor. LED will light. You can also provide a Push switch in series with the battery positive.
Now apply some glue over the Toroid and fix the White LED over it. Apply glue on the lower side of Toroid and fix the battery. Your Button Torch is ready.
Even though it is not too bright as powered in battery, the light is sufficient to illuminate an area. This reduction in brightness is due to low current from the Joule Thief circuit. 1 Watt LED requires 100-300 mA current for maximum brightness. Here only 50-80 mA current is available from the Joule Thief even though the voltage is around 4V.