Logic Rail has what you are looking for:
http://www.logicrailtech.com/
The 'Signal Animator' will make a signal go red when the detector photocell is covered, and change it to yellow after an adjustable delay. It will turn green after another delay.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
I know a couple of manufacturers used to make a "signal simulator" -- it was essentially a detection system tied into a basic timer. A train would pass, the signal would kick over to red, wait for 30 seconds, then kick to yellow, wait for 30 seconds, then go back to green.
Does anyone know if those are still made? I'm usually operating solo on my layout so I don't need a complex signalling system that actually works, but thought it would be a nice detail to have what looks like a signalling system.
This would be for HO scale, if it matters.
Thanks
Todd
Blog: www.smallrails.blogspot.com. Pictures, videos and tutorials.