<p>The Joule Thief is a delightfully simple circuit that can light an
LED that requires 2V or more from a battery that is depleted to 1V or
less. There are three components in addition to the battery and LED: an
NPN transistor, a resistor and a double wound inductor. The key thing
about the inductor is that the same core shares two windings but in
opposite directions.</p>
<p>The explanation of how it works is simple enough, as long as you
already understand how inductors and transistors work. In other words,
it isn't simple at all! In short, the double wound inductor and
transistor conspire to generate a transient high voltage spike and so
turn LED on and off so rapidly that the human eye cannot perceive
it.</p>
<p>The idea of such a circuit is approaching its centenary but the name
itself is only around 20 years old. I recommend you check out <a
href="http://bigclive.com/joule.htm">Big Clive's web site</a> as it was
he who came up with the name "Joule Thief". I also recommend <a
href="https://www.youtube.com/watch?v=K53beWYdIpc">this video</a> in
which he constructs a circuit using some ninja-level soldering
skills.</p>
