This is a simple yet very useful circuit that can be used in so many systems to detect a missing pulse from a train of continuous negative pulses. 

LM311 is a comparator that compares two voltage signals, an input signal and a reference signal. The input signal is placed on the terminal "Neg_Pulse", pin 2 of U1, while the reference is placed on the inverting pin of the comparator, pin 3 of U1. Since pin 2 of U1 has very high impedance then any amount of low negative voltage, from the negative pulse, will help charge C1 between the +5VDC and the negative peak of the pulse. The charge and discharge time of this capacitor charge depends on the cap value and the setting or RV2. This determines the time constant via C1 and RV2. RV1 sets the reference voltage to be compared with the input negative pulse magnitude. The output "Output_Detect" at pin 7 of U1 follows this logic:

Assuming the reference voltage Vref is at pin 3 of U1 and the Output is at pin 7 of U1, then

1. Output goes LOW if Input < Vref.
2. Output goes HIGH if Input > Vref.

By varying RV1 and RV2 you can observe when the output goes high and low. Now apply the train of negative pulses on the Neg_Pulse input and observe the output. Adjust RV1 until the output goes HIGH. If you can, slightly decrease the frequency of the input signal and re-adjust RV1 setting until the output goes LOW. Re-apply the original input frequency and observe the output going HIGH again. You may need to repeat this process until you get RV1 setup properly. The output also should go LOW if you disconnect the input negative pulses. This not only depends on the input signal frequency but also on the magnitude of the input signal. Adjust both RV1 and RV2 until you observe the best performance of the circuit that fits your need, in term of input signal frequency (RV2) and input magnitude (RV1). These two parameters do interact however.

From now on you can connect the output to any decision making circuit such as alarm or relay circuit, or to a micro controller circuit for further processing.

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