Wooden box for the base
Brass tubing, 5mm diameter, 40cm length
Neodymium magnet, disc-shaped, 25mm×3mm
LED, red, 5mm
Coil, 1mH Solarbotics part #CMH, solarbotics.com
Solar cells, 3V output ( 2) used in parallel, such
as Solarbotics #SCC2422 or salvaged cells from
Capacitors, electrolytic: 3,300μF ( 1), 1,000μF ( 1)
Transistors: 2N3904 type ( 1), 2N3906 type ( 1)
Resistors, 100kΩ ( 2)
Copper wire, thin enameled, several feet I used wire
that was 0.45mm diameter, but any will do.
Monofilament fishing line, a few feet
Cotton thread or string for binding the fishing line
Assorted brass bits for decoration
Cyanoacrylate (CA) glue (optional) aka super glue
3V solar cell
photodiodes ( 2)
LED (Yes, it’s
Soldering iron, solder, flux paste, scissors, sandpaper, hacksaw or jeweler’s saw, electric drill, 3mm
and 5mm drill bits, vise or drill clamp, thick gloves,
1. Assemble the circuit.
Build the circuit, following the schematic
(Figure A). This should be simple if you’ve had
any previous soldering experience, as there
are only a few components. I’ve gone for a
slightly shambolic approach to this circuit
(Figure B) as I knew it was going to be hidden
away, so feel free to be much neater than me.
It doesn’t need to be pretty to work, though,
so don’t feel intimidated if you’ve not had
much experience building electronic circuits.
To connect the LED, solar panels, and coil,
you may want to use crocodile clips rather
than soldering, so you can test the circuit
before assembling it. I made long extension
cords for these parts using twisted pairs of
enameled copper wire to allow me to run
them through the brass arm and then down
through the holes in the base box.
To test the circuit, you can use any 3V DC
power supply in place of the solar panels if
there’s no sunshine. You should be able to
tape a magnet to a piece of thread and, holding it above the coil, trigger the magnetic
pulses. This will cause your magnet to start
swinging and will also light the LED as it
passes back past the coil.
2. Construct the brass arm.
Cut 2 lengths of brass tubing: 30cm for the
vertical section of the arm and 10cm for the
horizontal section. Clamp the vertical tube in
a vise, and drill a 5mm hole 10mm from one
end. You’ll run the LED wires through both
tubes, via this hole. Be careful not to drill all
the way through the tube; if you’re using a
drill press, it’s helpful to set the depth stop
before drilling to prevent this happening.
Next, join the 2 sections of tubing at right
angles by brazing (soldering) them together.
An LED connected across the coil takes
advantage of the back EMF (
counter-electromotive force) generated when the
moving magnet passes by. This energy is
harvested and used to flash the LED with
every swing. The cycle continues as long
as the sun is shining.
With no batteries to replace, this pendulum
will run for years without maintenance.
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