This circuit uses a 555 timer chip to detect a missing pulse. The input is a square wave. When you click the logic input (the "L") to bring it high, then the input will stop oscillating and stay high. The 555 will detect this and bring the output low.
The 555 is wired as a monostable. But whenever the input goes low, it turns on a transistor which quickly drains the capacitor. When the input goes high, the capacitor recharges, but doesn't quite make it to the threshold voltage before the input goes low again. If the input stays high, then the capacitor is allowed to charge fully, and the 555 timing interval ends, bringing the output low. -- Credits: Mr. Paul Falstad.
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