DIY
CIRCUITS
“SOLAR JOULE” BRACELET
Solar-cell links are cleverly boosted to
drive an LED jewel. By Edwin Wise
My wife fell in love with Alice Planas and Hatti Lim’s
glowing bracelet project from CRAFT magazne
(Volume 06, page 123, “Solar Jewelry”), so of course
we had to make one. I built the circuit and she did
the fabric. It came out nicely, but I felt it could be
improved, so I decided to combine the solar jewelry
idea with a Joule Thief circuit that would make the
LED glow brighter. The result is the Solar Joule!
Joule Thief Theory
Photography by Edwin Wise
The Joule Thief is a small circuit that converts low
voltages, like from dead batteries, into an oscillating
voltage with peaks high enough to be useful. There
are many versions online (see makezine.com/19/
diycircuits_solarjoule).
The heart of the circuit is a pair of inductor coils
wound together into a transformer or choke. When
current runs into one coil, it’s resisted as it builds up
a magnetic field. This field pushes current through
the other coil, going in the opposite direction. In the
Joule Thief, one coil provides the kick of voltage to
light up the LED, and the other generates feedback
that drives a transistor into oscillations.
Here’s how the feedback works (Figure A, left side,
following page). When you first connect power to
the circuit, the transistor is off, there is no magnetic
field in the choke, and there’s not enough power
to turn on the LED. Some power leaks through the
resistor into the transistor’s base, turning it on a little
bit. This lets a bit of current run backward through
inductor coil 1-2 of the choke, creating an expanding
magnetic field. As a result, current is run through
inductor 4-3, which turns the transistor on even
more. This positive feedback loop continues until the
transistor is completely activated.
Once the transistor is done opening, the current
117 Make:
