Fig. D: Solar battery with the Schottky diode and
supercap soldered in. Fig. E: Solder the transistor
collector (C) and base (B) to pins 2 and 4 of the
transformer. Fig. F: Solder the resistor between pins
1 and 3 of the transformer. Fig. G: Solder the
capacitor between the transistor emitter (E) and pin
3 of the transformer. Fig. H: Solder the LED “jewel”
between the transistor’s collector and its emitter.
Start by bending the emitter pin on the NPN transistor 90° away from the flat side. Then bend the
collector and base pins so that you can solder them
to pins 2 and 4 of the choke, respectively (Figure E).
(Note where pins 1 and 4 go when you turn the
choke upside down.)
Position the resistor across the base pin, bend
its leads down the sides of the choke, and solder
it between pins 1 and 3. Trim the pin 3 lead, but leave
the pin 1 lead long to connect to the battery’s positive (+) terminal (Figure F). Any resistor from 1K to
3K should work. A larger one will be more efficient,
but I happened to have a 1K resistor on hand.
Flip the assembly over and solder the axial capacitor between pin 3 (or the resistor lead connected to
it) and the transistor’s emitter pin (Figure G). This
capacitor isn’t required for the circuit to work, but
it increases efficiency. An axial capacitor fits better
here than a regular disc-shaped cap would, so it
makes the circuit more compact.
Now let’s set the LED jewel. Bend out the LED’s
shorter cathode lead (–). Solder the anode lead
(+) to the choke’s pin 2 or transistor’s collector.
Solder the cathode to the transistor’s emitter, which
should be conveniently poking up. Trim the LED’s
anode but leave the cathode lead long (Figure H).
Solar Jewel + Joule Thief
Using wires or alligator clips, wire the Joule Thief
and the solar battery together, + to + and – to –.
The LED should glow! If you watch the LED’s positive lead with an oscilloscope, you should see it
pulsing up to the LED’s forward voltage at 300kHz
to 500kHz. Since that’s too fast for your eyes to
discern, it looks like a steady glow.
If it doesn’t light, use a voltmeter to confirm that
your solar battery has a charge. It doesn’t take
much; half a volt is plenty. But it may require many
minutes of bright sunshine to charge the cap the
first time. If you have voltage but no glow, confirm
that the diodes, capacitors, and the choke’s inductors are all oriented correctly.
I’m ignoring the final part of the project here:
a crafty mounting that you provide for the other
2 parts. That’s up to your imagination.
Edwin Wise ( simreal.com) is a software engineer with 25
years’ experience. He develops software during the day
and explores the edges of mad science at night.
for schematics and a link to the original Joule Thief