On Sun, Aug 16, 2009 at 5:12 PM, Marcel Birthelmer wrote: > Could you somehow detect an audio signal (some sort of mechanical > click or something produced by the wheel) using a dsPIC or something > like that? Or maybe even use an ultrasound transducer and receiver to > pick up the spokes moving past? Overengineering FTW! You should look at the Polar power measuring device. It is very clever. It seems the top chain vibrates like a guitar string. If you know the length of the top run of chain, and the mass of the chain, you can determine tension from the vibration frequency. So Polar have rigged a pickup (similar to a bass guitar pickup) and they measure the frequency of this vibration. Then the chain speed is measured with a special rear derailer pulley. Then chain speed * chain tension gives power transmitted. I met the inventor at the Interbike trade show. Pretty cool. Although in practice it is quite a bit messier than the strain gage based systems. Regards, Mark markrages@gmail -- Mark Rages, Engineer Midwest Telecine LLC markrages@midwesttelecine.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist