An NE555 timer IC connected as shown here can detect a missing pulse or abnormally long period between two consecutive pulses in a train of pulses,Such circuits can be used to detect the intermittent firing of the spark plug of an automobile or to monitor the heart beat of a sick patient.
The signal from the pick up transducer is shaped to form a negative going pulse and is applied to pin 2 of the IC which is connected as a mono stable.As long as the spacing between the pulse is less than the timing interval,the timing cycle is continuously reset by the input pulses and the capacitor is discharged via T1.A decrease in pulse frequency or a missing pulse permits completion of time interval which causes a change in the output level.
Source: http://www.circuitstoday.com/category/automotive-circuits/page/3
