Here is the project of a missing pulse detector using 555 IC. The 555 timer IC is wired here as a monostable multivibrator. The circuit will detect a long period pulse between a train of pulses.
frequency of pulses. The timing of the circuit can be increase or decrease by increasing or decreasing the value of R2 and C1. For setting the circuit to work properly you have to set the time delay of the circuit little long as compare to the frequency of the pulses. The operating voltage of the circuit is 9 to 12V DC.