The Joule Thief (or just another white LED torch).


Update: 5/3/2014.

You can order professionally made circuit boards via this link:

https://oshpark.com/shared_projects/Pv0rRfN6

Thank you very much to Brian Compton for making this board available to a wider audience.

Brian and I did spot and eliminate a little problem. I believe it was probably caused by me using an old version of CadSoft's Eagle, and then (later) a newer version. Therefore I have checked the Eagle project files in the current version (6.4), and re-made the zip file (linked at the bottom of the page).

I think the circuit boards made by oshpark look very smart and professional, Brian has kindly shared some images and there is a discussion about the project here.

That completes the update, the original text follows:

As seen on the MakeZine blog!

First some notes:

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.


circuit schematic diagram

Parts list and assembly notes.

NameDescription/valueCommentsPart numberSupplier
R1, R31k ohmThe smallest wattage you can buy  
R22k2 ohmThe smallest wattage you can buy  
C1470pFCeramic disc4057713Farnell
C2100uF / 10VVoltage rating not critical1918126Farnell
D1White LEDThe brightest you can buy1139www.ultraleds.co.uk
L1470uH choke/inductorSee below  
JP12 pin jumperOptional for on/off switch  
T1, T2BC548B NPN-type transistorBC549C, BC550C may work933910Farnell
EnclosureCASE, ABS 80x35x17 BLACKSuggested3536439Farnell

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.


image of assembled Joule Thief image of assembled Joule Thief

PCB mask, PDF format.

PCB mask, SVG format.

Eagle files for the circuit and board, zip compressed.