An absence of pulses could mean an important things, depending on your system, it might be a grip loss when your vehicle’s wheel is locked during braking and anti-lock action should be taken immediately, a loss of image integrity and your monitor should be blanked while trying to regain the synchronization signal, or it could be any other important things (depending on what it represents). The point is that a missing pulse detection circuit is sometimes needed to catch important event so an important action could be handled.
A missing pulse detector will receive a source of repetitive pulse and output nothing in normal condition, but when the input pulse is interrupted or delayed or disappear then an active condition should be triggered on its output. In the presented circuit, the output of this detector will be inactive for normal condition (output pin 3 of 555 IC will be high, turning off the LED indication), but 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.