Wearable Joule Thief

What is a Joule Thief? It is a circuit that can run a light emitting diode (LED) from a 1.5V battery. LEDs typically require between 1.8 and 3.8 volts to work (approx depending on their colour) so ordinarily 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 here and this is just my interpretation of the layout. There's a lot of information already on the internet about its history and construction so there is no need for me to repeat it again here (see links at the end)


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 pitch Black beach at Dungeness photographing the Milky Way with a new camera that I had not yet learned to 'feel' my way around. 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 in the courtyard of Ightham Mote. 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 reason to build a Joule Thief

Internal view

Notes: The ferrite core and the wire for the windings were salvaged from an old mains adapter. The number of turns that could be neatly wound onto the core side by side (bifilar style) was two lots of 19, which seemed to work fine. A double AA battery box with switch 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

My circuit consumed about 40mA when a fresh battery was first installed, though the current drawn reduced dramatically when the battery voltage began to drop. I got 2 days of full brightness from the LED before it became too dim to be of any use, but it stayed illuminated for a further 4 days!

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