I've posted a YouTube video that shows how to program an Arduino to “Magically” switch an LED on and off by shining a keychain light at it.
The video describes how the technique works for one LED as well as for a set of 8 LEDs, each of which can be switched on and off independently. There also a bonus effect shown at the end of the video. Complete source code is available for all 4 examples and can be downloaded page that’s linked in the description for the video. Enjoy!