At Sunrise:Off-Grid I ended up mentioning this fascinating little circuit to many people, but hadn’t taken an example with me. The Joule Thief allows a single 1.5 volt battery to power one or more 3volt LEDs for many hours. The light won’t stop until the battery drops to approx 0.4 volts.
The construction is very simple indeed, and quite fun – you have to wind a little torroid with wire and a ferrite ring. You need one transistor, one resistor, an LED and your battery. You can also use a Rodin Starship coil in place of the torroid, allowing the whole thing to use wire with no ferrite ring. The image here shows a Joule Thief made this way. The picture shows a single AA (reading about 1 volt) lighting 36x 3 volt LEDs. I would have tried more but I ran out of LEDs! Interestingly, the whole lot were only drawing about 20 milliamps at 1 volt – less than the normal load of a single 3 volt LED!
Below is a diagram of the build of the Joule Thief. You will need the following parts:
- 3volt white LED
- 1kohm resistor
- Transistor 2n3904 or 2n2222
- Small ferrite ring
- 1.5v AA or AAA battery
- Some insulated wire
It is easy enough for a child to make and intriguing enough to keep an adult entertained and bewildered for hours…