skimask, I have built a version of a solar tracker about 2 or 3 years ago using Green Led's as light sensors with great success. Here is a link to a site I found about 2 years ago with some circuits for an interface. I have since gone over to using the TSL-230 light to frequency sensors as they have a wider light to dark ratio. I hope this helps.....
http://www.redrok.com/electron.htm#chaceshadow

Dave Purola
N8NTA