Dear sir,
Your project will be a simple 3 modules :
-Timer (that provide ,3 sec, 750 ms...etc)
-Checking logic (for the heart beat)
-Alarm functionality
As I understood from your clarification
that the coming signal is On/Off
i.e (High for a period , then low ) ->no need for ADC
I will provide :-
Simulation through ISIS proteus
Source code with high modualrity , winAVR make file
I also made the days(5) for clarification & totally understand your requirements.
Let's start