Building an electromagnetically-assisted pendulum

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By Matt Mets
Posted 2010/06/01 @ 12:19 pm
Category Science

For an upcoming clock project, Stephen Hobley is experimenting with an electromagnetic pendulum. The goal is to keep a pendulum swinging at a constant speed, in order to use it as a timing reference. By adding a permanent magnet to the bottom of a pendulum, and placing a coil of wire at the bottom of the pendulum’s path, he is able to use the coil both to sense the motion of the pendulum, and to give it an electromagnetic boost every cycle to keep it swinging.

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12 Responses to Building an electromagnetically-assisted pendulum

Jack on June 1st, 2010 at 10:17 pm said:

In 2007 I made such a “Grandfather’s Clock”. It seems to move without any source of input, but it keeps very accurate time (better than a minute a month), using a basic 1 second pendulum. I designed it completely from scratch, and this prototype has controls to set how often the pendulum’s magnet gets a push, when it begins, how long the duration is, etc. It also displays seconds with an led section and can reset seconds with a push-button to synchronize with standard time. The handmade oak case looks like a grandfather’s clock (about 6.5 feet tall) and only on closer examination can one see the difference. I’d like to send a photo, but I don’t know how to post it in this comments section.

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- Matt Mets on June 2nd, 2010 at 12:02 pm said:
  
  Sounds like an awesome project! I’m not sure that you can embed a photo in the comments, but if you post it on a photo sharing site (Flickr, Picassa Web Albums, etc), you can add a link. Even better, if you have a video of it running we’d love to see it!
  
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CameronSS on June 2nd, 2010 at 12:17 am said:

Well, not by me, but that’s because I’ve been too lazy to come up with a good magnet coil. The “The BEAM Magbot Pendulum” is chapter 11 of Mark Tilden’s “Junkbots, Bugbots, and Bots on Wheels.” It doesn’t even use a 555, much less an Arduino…just two transistors powered by a calculator solar cell.

http://junkbots.solarbotics.com/

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Silverman on June 2nd, 2010 at 6:26 am said:

This is similar to how the amazing Bulova Accutron watch works. The timebase in the Accutron is a tiny tuning fork instead of a pendulum. The tuning fork is kept in motion by a pair of coils and a transistor or two. Everything else about the watch is mechanical. I want to build a clock that uses an ordinary tuning fork as the timebase.

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Accomplished on June 2nd, 2010 at 11:59 am said:

The simple solar powered version CameronSS mentioned is a great circuit. Easy to build and a nice quick weekend project. You can check out my own attempt in brass and wood here:

http://www.accomplished.org/2010/06/02/solar-pendulum/

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- CameronSS on June 2nd, 2010 at 12:06 pm said:
  
  Accoplished: did you pull that cute little coil out of something, or wind it yourself? Goodness, I’m lazy…
  
  That’s quite a nice build, really. The gears over the magnet are a nice touch.
  
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  Accomplished on June 2nd, 2010 at 12:13 pm said:
  
  I’m in the UK and bought mine from a seller on ebay, but they’re actually the ones that Solarbotics sell here if you fancy picking some up for yourself: http://www.solarbotics.com/products/cmh/
  
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  CameronSS on June 2nd, 2010 at 12:52 pm said:
  
  With shipping, Solarbotics would cost $10 for a little wire loop. I think I’ll just find some magnet wire and make a spindle for the lathe.
  
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  Matt Mets on June 2nd, 2010 at 1:04 pm said:
  
  It’s the price of convenience!
- Matt Mets on June 2nd, 2010 at 12:31 pm said:
  
  That’s beautiful, thanks for sharing!
  
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Steve Hobley on June 2nd, 2010 at 1:13 pm said:

Thanks for all the comments.
There is a reason for the uController, and this will become apparent later on.

But you are quite right, just to get it working you don’t need a uController, or a timer.*

The first circuit I built was the NPN/PNP design – by Rod Elliot:
http://sound.westhost.com/clocks/free-pendulum.html

*The Arduino is purely for blog cred:
http://hackaday.com/2010/05/08/xkcd-takes-a-swipe-at-the-arduino/

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tagno25 on June 2nd, 2010 at 3:06 pm said:

There was a project guide in the September 2009 issue of Nuts and Volts. The article is write by Tony Gasparovic and is on page 36. At the time you could see a video of it running on Youtube.

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