One way to do that -- design an up-counter that increments continuously, but resets to zero whenever an input pulse occurs. If the count reaches your desired timeout value, then stop counting and trigger the alarm.
One way to do that -- design an up-counter that increments continuously, but resets to zero whenever a pulse is detected. If the count reaches your desired timeout value, then stop counting and trigger the alarm.
Not all monostable can be re-triggered ..
One example of those which can is the 74123 ..
The time can be set by RC and every incomming pulse that comes within this time will re-trigger the circuit ..
If there is no pulse the output will jump from "1" to "0" or from "0" to "1" depending on which output you choose ..
Should't be difficult to build ..