The Joule Thief (or just another white LED torch).
As seen on the MakeZine blog!
First some notes:
- This is the first project where I've used the "toner transfer" technique to make a printed circuit board. There are plenty of other web-pages which give excellent descriptions of how to do it, so I won't replicate that.
- This circuit was published in the June 2002 issue of "Elektor Electronics" and the circuit was designed by B. Kainka.
- This is the first time I've used Eagle to design the PCB layout.
- I wanted to make this circuit as small as possible, so I've not followed some of the circuit-maker's conventions. There's a surface-mount component on the same side as the other components, only the battery holder is on the "correct" side.
Here's the schematic, it works as a voltage converter to draw just 20mA from a 1.5V cell to light that white LED. The term 'Joule Thief' is used because the circuit will work from a cell which would otherwise be considered spent. I found that if the batteries in my Gamecube Wavebird controller stopped working, the same "spent" batteries will quite happily run this torch for a couple of weeks. Continuously.
Parts list and assembly notes.
| Name | Description/value | Comments | Part number | Supplier |
|---|---|---|---|---|
| R1, R3 | 1k ohm | The smallest wattage you can buy | ||
| R2 | 2k2 ohm | The smallest wattage you can buy | ||
| C1 | 470pF | Ceramic disc | 4057713 | Farnell |
| C2 | 100uF / 10V | Voltage rating not critical | 1918126 | Farnell |
| D1 | White LED | The brightest you can buy | 1139 | www.ultraleds.co.uk |
| L1 | 470uH choke/inductor | See below | ||
| JP1 | 2 pin jumper | Optional for on/off switch | ||
| T1, T2 | BC548B NPN-type transistor | BC549C, BC550C may work | 933910 | Farnell |
| Enclosure | CASE, ABS 80x35x17 BLACK | Suggested | 3536439 | Farnell |
Sourcing the inductor was one of the trickier parts of this project. If I'd had a greater choice of suppliers, rather than just RS Components or Farnell it might have been easier. As I recall I got the inductor from RS Components, as it was the only way to get the correct value. If you have some varnished wire and a iron core of your own, then you could wind your own.
Sourcing the PCB-mount single AA holder wasn't easy either, that may have been from Farnell also. I had to design a battery holder part within Eagle because I couldn't find one pre-defined.
Given the PCB mask below, you could make your own Joule Thief readily. As mentioned the jumper JP1 is optional, so you could edit the mask before printing to short that out. I suggest you measure your battery holder's pins as well.
The circuit will also work from a 1.5V button cell, and if you find a cheap source of battery holders for these cells then please tell me.
Images and downloads.
