SolarPanels,
All you ever wanted to know about electronic trackers and sensors, http://www.redrok.com/electron.htm scroll down. I've built several versions and they all work but now I'm looking at the ones that interface with a microcontroller to prevent "hunting" and other problems with the simple ones.
Using LEDs as sensors really works better than I thought . The LEDcooker3 amazes everyone with its simplicity. 3parts.
Have fun.
TomJ