Make:

From the MAKE Flickr photo pool

Flickr member Pocket-sized breadboarded a solar/magnetic pendulum as seen in the BEAM book - JunkBots, Bugbots, and Bots on Wheels
The self-sufficiency of solar power can really up the fun-factor in these builds - Magnetic Pendulum on Flickr

This site explains the project a bit more, and highlighting it's strengths:

• Back EMF powers the led
  
• slight vibrations allow the system to self start
  
• solar powered means years of joy with no support needed (this is my favorite part of beam art - it's allows there for you)
  
• self timing
  
  • the magnet induces a slight current into the coil
    
  • this extra current coupled with the energy stored in the capacitors trips the power dump
    
  • the power dumped into the coil causes the magnet and coil to be attracted to each other
    
  • there is only 1 degree of freedom in the system so it starts to swing predictably
    
  • the orientation of the magnetic field pulls the pendulum towards dead center on the way in
    
  • this same field pushes the pendulum away from the coil once it passes dead center

From the pages of MAKE:

MAKE:06 p.54 - A Beginner's guide to BEAM - Subscribers, read this article now in the digital edition.