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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
 Schematic
 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 |