http://www.merl.com/reports/docs/TR2003-35.pdf has instructions for doing this with microcontrollers.
http://www.instructables.com/id/E6PN27M4CPEP286H58/ uses LEDs as light sensors too. I like that one a lot. 
http://www.merl.com/reports/docs/TR2003-35.pdf has instructions for doing this with microcontrollers.
http://www.instructables.com/id/E6PN27M4CPEP286H58/ uses LEDs as light sensors too. I like that one a lot.