Hi: Here's a kit I just built that's a pendulum that's a metal ball powered by a small solar cell. Haven't done any measurements yet, but the period is about a second. http://www.prc68.com/I/GeekStuff.html#Zendulum -- Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html