What does D2 do?Below is the LTspice simulation of AK's basic circuit with a few modifications:
I added D2 to reset the circuit after the time-out.
I eliminated C3 and R4, and replaced them with R4 in series with D1 to provide about a half second delay in the reset at the output to give the half second output pulse.
Thus C1 does double-duty.
R3 and R5 prevent excess current into the IC inputs from the capacitors if power is suddenly removed.
View attachment 251098