The left timer is in astable mode creating a pulse(blue square wave). When the output goes low it starts timer 2 at pin two and sends its output HIGH. The orange wave is timer 2’s capacitor charging. Before it can reach 2/3 rail voltage, timer 1 pulses again sending it another negative pulse resetting it which discharges the capacitor. Timer 2 only completes its cycle if timer 1 misses a pulse. The time delay of timer 2 should be set to slightly exceed the time between the incoming pulses. The pulse of timer 1 can be interrupted using the switch I placed at its output. A short time after the pulse is “broken” to timer 2, it’s cycle will finish indicated by the green LED and the green wave. These types of circuits can be used to detect intermittent firing of an automobile spark plug or to monitor the heartbeat of a patient. Any other uses you can think of?
