BY TON Y GASPAROVIC
Here’s a really fun project that will get
a lot of attention. It’s an electronic
pendulum that operates on the principle
of magnetism. You’ve probably seen similar
devices at office supply or novelty stores.
I’ve owned a few of these over the years
and was always amazed by them.
Unfortunately, they never worked very well
or would stop running after a few days.
I searched all over the Internet looking for
a better model, but I couldn’t find one so
I decided to make my own. After many
months of testing and experimentation,
I have come up with a stable and robust
circuit that is easy to build and will run
for months on two AA batteries.
■ Pendulum in action.
Theory of Operation
The timing of this circuit is controlled by the magnet.
As the magnet crosses over the top of the electromagnet
(the coil), it creates a small EMF voltage spike. This voltage
spike is detected and amplified by transistor Q1. The
output of Q1 causes Q2 to switch on, which turns on the
electromagnet. The force generated by the electromagnet
gives the magnet a push at the precise time it passes over
the electromagnet, which keeps it swinging. With each
pass of the magnet, the electromagnet pushes the magnet
higher. When the magnet swings beyond the edge of
the electromagnet, the EMF voltage drops to zero. The
electromagnet turns off and the magnet continues to
swing through the pendulum arc undetected, until it
passes over the electromagnet again.
It’s best to make the base from wood. When
constructing the base, don’t use any steel nails or screws.
This will cause the magnet to be attracted to the metal
36 September 2009
and will hinder the smooth swing of the pendulum.
Use brass or stainless steel hardware. Keep in mind the
base should be at least as long as the pendulum arm.
This will help keep it stable. The weight of the pendulum
arm should be light so it will swing with as little resistance
I’ve had success having the pendulum arm swing from
a loose-rolling skateboard bearing, or you can have the
magnet swing from thread. If you’re going to use thread,
I recommend using upholstery thread because it is very
strong. You will also need to mount the magnet in such a
way to keep the pole of the magnet in line with the coil.
You don’t want the magnet to wobble, bounce, or tilt. The
magnet that I am using has a 1/4 inch hole in it. You can
glue a two inch piece of 1/4 inch dowel in the hole of the
magnet and then drill a small hole in the other end of the
dowel for attaching the thread.
Building the Circuit
The circuit is very simple to build. I used a
general-purpose RadioShack circuit board. There is one