There are several methods of detection, much depends on your control system. The least intrusive involves using photo-cels and ambient light. The most difficult part is the logic to display aspects appropriate to the situation.
If links are permitted, look at **broken link removed** That will drop into my website at the correct page. If you think the equipment there will solve your problem, email me. The fundamental circuits are not proprietary, and I'm always ready to assist a novice.
Some documentation is posted there, also a PDF with installation procedures for my boards. The instructions will contain much insight into signal systems as systems.
Artificer (bhudson@hudsontelcom.com)