I have a negative-recovery monostable circuit that might do the trick, it uses a 555 and two transistors, when a signal is recieved the 555 will turn on as long as there are pulses the 555 will stay on (high) in the absence of pulses the 555 will turn off after a short delay. You would need to invert the output for you application.
I can put in a postable format if you are interested.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.