JOULE THIEF AND SOLAR BATTERY SCHEMATICS
Fig. A: Dots indicate like-voltages on the choke.
Fig. B: Loops of solid wire wound around bolt, cut,
and bent. Fig. C: Photodiodes soldered in series,
connected by wire loops.
through 1-2 stops increasing, so the magnetic field
stops expanding and stops pushing the current
through 4-3. This causes the transistor to close a bit,
which reverses the feedback loop. Current reverses
through 4-3, closing the transistor more and blocking
current flow through 1-2. When the transistor shuts
off, the inductor’s magnetic field winds down and
unloads a blob of charge, which then runs through
the LED to light it. The current is quickly exhausted,
and we are once again at the starting state.
The capacitor between the resistor and the choke
provides a little “spring” to the feedback action,
buffering some of the voltage changes across
Build the Solar Battery
First I built the solar battery, which is a series of PIN
diodes bridged by a supercapacitor that stores the
energy they collect (Figure A, right side). The photodiodes also act as links in a chain bracelet, and
I connected them with loops of wire to provide some
spring and make it easy to sew them onto fabric.
To make the loops, wind solid wire around a
small bolt, cut it at every other turn, and re-bend
the ends of each loop into small solderable hooks
(Figure B). Then solder 10 (or more) diodes in
series, + to – (Figure C). The silver strip on each
diode’s face indicates the + side. If you want to go
crazy, make 2 strips of photodiodes and connect
them in parallel, side by side (+ to + and – to –).
Solder the Schottky diode to the + end of the
series, with its black stripe (the – end) pointing
away from the photodiodes. The Schottky diode
ensures that power flows into the capacitor when
the photodiodes are in bright light, but won’t flow
back out when they’re dark. Any diode will work
here, but a Schottky diode consumes less voltage
in the forward direction, saving more for the LED.
Solder in the supercapacitor, with its + side at the
signal diode and its – side all the way at the other end
of the PIN diodes. Finally, solder a wire to each end of
the capacitor (Figure D), to connect to the Joule Thief
later, and set the solar battery in the sun.
All values are approximate and not particularly critical
For the solar collector:
Wire, 22-gauge solid I used insulated wire and
stripped the insulation.
PIN photodiodes ( 10 or more) Mouser Electronics
part #782-BPW34, mouser.com
Supercapacitor, 0.22F Mouser part
Schottky signal diode Mouser #625-SB330-E3
Nice fabric or other bracelet material for mounting
For the Joule Thief circuit:
Common-mode choke, 51μH Mouser
NPN transistor Mouser #512-BC549
Resistor, 1kΩ to 3kΩ, 1W such as Mouser
Capacitor, 0.01μF axial Mouser #80-C114C103K5R
LED, any size or type such as Mouser
Small pliers Round-tipped are nice, for making loops.
Soldering iron and solder
Liquid flux (optional)
Third-hand vise (optional)
Voltmeter and/or oscilloscope (optional)
Build the Joule Thief
Many versions of this project involve winding your
own transformer, but I’m lazy, so I bought one. It’s
a surface-mount (thus, very small) common-mode
choke with a ferrite core. The white dot on top indicates pin 1, and the pins are numbered clockwise.
118 Make: Volume 19