Perpetual Pendulum

Perpetual Pendulum


A pendulum, once started, will always swing in less and less of an arc until it comes to rest. A ‘Perpetual Pendulum’ appears to swing forever, thus seeming to defy the laws of physics.

The trick for a perpetual pendulum is to give the pendulum a little ‘push’ occasionally. This is much like the push you give a child on a swing. Usually, you apply just enough force on each swing to keep the child from slowing and stopping.

There are a number of these devices on the Net. Typically, they use a magnet or a piece of metal and a coil of wire. The coil of wire, if magnetized with exact timing, becomes an electromagnet that will ‘pull’ or ‘push’ the pendulum to maintain a consistant swing for a long time. Here a few YouTube videos of these devices:

These are well made, clever devices, but what I notice is that the mechanics and electronics are all quite visible, leaving nothing to the imagination. My goal is to make a similar device which has no visible components except for the pendulum. By hiding the electronic and electromagnetic components, I hope to intrigue those without electronics and electromagnetics knowledge, and have them wonder "How does that thing work??"

The Microprocessor

Since I teach microprocessor classes, I have plenty of microprocessors to choose from. I have 8051 development boards (, SSE8680 PIC boards, Board of Education boards ( and Arduino Duemilenova and Mega boards ( I will try to write this page in a generic way so that you can easily adapt the principles here to any programming language or microprocessor. I will use the Arduino Duemilenove for this project.

The Software

Since the Arduino class of chips uses the C programming language, C will be my choice.

The Hardware

There are several different hardware configurations that will work. The one that I like uses a magnetic sensor to decide when the pendulum at a particular point. At that time, or a few milliseconds later, a magnetic coil is energized, ‘pulling’ on the pendulum. This pull, with the proper timing and duration, will be just enough to keep the pendulum swinging indefinitely.

  • Magnet: I have several sizes of magnet and will select the smallest one that will do the job.
  • Electromagnet: I have a roll of wire that will let wind my own electromagnet coil and I also have a pre-made electromagnetic coil.
  • Magnetic Sensor: On the net, I have seen a number of devices. I don’t like magnetic reed switches because they are mechanical and may make noise. I have two different types of Hall-Effect magnetic sensors, so will try them both
  • Coil Switch or Relay: Since I want silent operation, a solid state relay seems to be the best choice.