I want to design a circuit wherein a permanent beam of light is present in an equipment. When the user uses the equipment the beam of light is cut momentarily and another device is switched on. How should I go about designing such a circuit?
Thanks for your help

 

This schematic is a good start.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=11366&d=1249853393

 

I just made that a few days ago and edit it today to include a counter.