Wearable Joule Thief

So what is a Joule Thief? Well, it's a circuit that can run an LED from a 1.5V battery. At first this may not seem such a big deal, but LEDs typically require between 1.8 and 3.8 volts to work (approx depending on their colour) so normally they would not light up using a single AA cell. It gets its name from the fact that it can light an LED even when the battery is nearly dead (less than 0.5 volts) and so it is effectively 'stealing' every last bit of energy (or Joule) from the battery! I take no credit at all for the circuit presented, this is just my interpretation of it. There is already a wealth of information on the internet about its history and operation so I won't repeat it again here, though I have put some links at the end of the page for reference


Veroboard layout

The Joule Thief circuit has interested me for some time but I've never really had a reason to build one. Recently though, I was on a beach at night trying to photograph the Milky Way with a new camera that I hadn't yet learned to 'feel' my way around. It was pitch Black and I had to keep getting a torch out to see what I was doing which was difficult to hold at the same time as operating the camera and I remember thinking what I really need is a small hands-free light, not too powerful, that could be worn around my neck. I put it to the back of my mind until Christmas came around and we were carol singing. It was dark and I thought if only I had a small hands-free light it would be so much easier to see the words on the carol sheet. In that moment I realised I now had a good excuse to build a Joule Thief

The Joule Thief in action!

I did deviate away from the original design slightly by winding the transformer linearly rather than bifilar style, but that was just my choice. I also changed the value of the base feed resistor from 1K to 4K7 to reduce current consumption and get a bit more battery life. The LED brightness drops slightly but in fact, for its intended purpose, I found that with a 1K resistor it was a bit too bright anyway. A switched double AA battery box was used for the enclosure with a hole drilled in each side of the lid allowing a shoelace to be threaded through to act as a neck strap. The circuit consumes about 30mA with a fresh battery but as the battery dies the current drops dramatically, which I think is the secret to its magic. I get 2 days of full brightness from the LED before it becomes too dim to be of any use, but it does stay illuminated at low level for a further 4 days!

Details of the transformer

Toroidal Core: 10mm OD x 6mm ID x 5mm Ht (green ones seem to work best!)
Number of turns: 26 turns tapped at 8 turns (wind 8 turns, make a tapping point, then carry on winding for a further 18 turns)
Type of Wire: 0.4mm enamelled copper (27SWG or 26AWG) or 0.45mm if measured with vernier calipers (due to the enamel)

There's plenty of scope for experimentation. Core sizes of 8mm to 11mm can also be used and 25 turns tapped at 9 turns works OK as well

Useful Links

Wikipedia article

Big Clive's YouTube video (He coined the name 'Joule Thief')

Julian Ilett's YouTube video

The Joule Thief as an art form?

Lots of info about winding the inductor