This circuit will detect a missing pulse or abnormally long spacing between consecutive pulses in a train of pulses. The timer is connected in the monostable mode. The time delay should be set slightly longer than the timing of the input pulses. The timing interval of the monostable circuit is continuously retriggered by the input pulse train, 1-J. The pulse spacing is less than the timing interval, which prevents Vc from rising high enough to end the timing cycle. A longer pulse spacing, a missing pulse, or a terminated pulse train will permit the timing interval to be completed.
This will generate an output pulse, V0 as illustrated in Fig. 25-3b. The output remains high on pin 3 until a missing pulse is detected at which time the output decreases. The NE555 monostable circuit should be running slightly slower, lower in frequency, than the frequency to be analyzed. Also, the input cannot be more than twice this free-running frequency or it would retrigger before the timeout and the output would remain in the low state continuously. The circuit operates in the monostable mode at about 8kHz, so pulse trains of 8 to 16 kHz can be observed.