Missing pulse detector will receive a source of repetitive pulse in normal condition. The output of this detector circuit will be inactive for normal condition (output pin 3 of  555  IC will be high, turning off the LED indication). If now the input pulse is missing, delayed, or stopped, then the output of this detector circuit will be active, giving an indication that the condition is abnormal by turning on the LED.  The period of no-pulse condition can be adjusted by changing 0,1uF capacitor. This circuit assume that active condition is low, and inactive is high. This assumption applied for both input and output. When the input is actively switched between high and low, the capacitor will be alternately charged toward +10V and discharged to ground. In normal condition, fast discharging will make the capacitor voltage never reach the triggering level of 555 IC. If the input pulse disappears of delayed, which means the input is tied to high voltage for certain period, then the charging of capacitor will reach the triggering level of the IC (2/3 Vcc) and activate the indicator. Here if the schematic diagram of the pulse detector circuit:

 

555 Missing Pulse Detector circuit schematic