output:
Tape the LDR to the screen of a computer running Max/msp or PD.
So now you can control the relais (i.e. anything) from your computer.
input:
Or use the relais to control one of the keys of a PS/2-, USB- or ADB-keyboard,
to measure light (or temperature when you replace the LDR by an NTC).
(click the image to enlarge)