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Missing Pulse Detector

Figure 1.  Missing Pulse Detector Circuit Diagram

This circuit is a missing pulse detector, i.e., it can be used to detect missing pulses in an incoming pulse train. The main component of this circuit is the 555 timer IC.  In this circuit, it is configured as a monostable multivibrator, i.e., a circuit which will output a single pulse every time it is 'triggered'.

Every time a pulse arrives at pin 2 of the 555 timer in this circuit, the 555 is triggered to output a single pulse at pin 3. The width of this output pulse is defined by the values of R2 and C1.  R2  and C1 must be chosen such that the output pulse width is slightly greater than the time between each incoming pulse.  If the arrival of the pulses at pin 2 is continuous, the output will never complete a single pulse.  This is because the 555 timer will always be retriggered and C1 will always be discharged through Q1 every time a new input pulse arrives.  As such, the output of pin 3 will always be 'high'.

However, a missing pulse will allow pin 3 to output a complete pulse.  This means that it will change its state from 'high' to 'low' after the set pulse width has been attained. Thus, a change in state of the output of this circuit signifies that it has detected a missing pulse in the pulse train arriving at the input.