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BUILD THE MYSTERY
SOLAR POWERED
PENDULUM
By David Williams
Powered using the energy of the
sun, this solar pendulum does not
require batteries or an electrical
outlet to operate. So, how does the
solar energy make the pendulum
swing with no visible connections?
It will have your family and friends guessing until
you reveal the secrets of the electromagnetic
coil and circuit hidden in the base.
This device is a wonderful demonstration of clean,
green solar power and is based on BEAM technology. It
was inspired by the Magbot pendulum project in Dave
Hrynkiw’s book titled, Junkbots, Bugbots & Bots On
Wheels. When the solar cell is exposed to the sun or any
bright light source, the pendulum will start to swing —
almost magically. Each time the pendulum passes the
bottom of its swing, a hidden coil in the base gives the
pendulum a little nudge, using stored electricity
generated by the solar cell.
The invisible nudges keep the pendulum in
perpetual motion as long as there is enough light. The
solar pendulum is both fun to watch and is a learning
tool, all in one. It can also be a great school science
project, demonstrating both solar energy and
electromagnetic principles.
How the Circuit Works
32 August 2012
Figure 1 shows the simple schematic for this project.
As light strikes the solar cell, it generates a DC voltage
that begins to charge up the two electrolytic capacitors,
C1 and C2. If the pendulum is not already swinging, the
circuit acts in a mode that tries to “kick-start” the
pendulum into motion. During the kick start mode, both
electrolytic capacitors continue to charge until the
voltage on C2 reaches approximately 0.8 volts. At that
point, the NPN transistor Q2 turns on which, in turn,
causes the PNP transistor Q1 to turn on. When Q1 turns
on, it dumps
all the power
stored in C1
into the coil. The current flowing through the coil briefly
creates a magnetic field around it, as C1 quickly
discharges. Then with no more current flow, both
transistors turn off and the magnetic field collapses,
generating a negative voltage spike which causes the
LED (light emitting diode) to flash.
