The circuit shown in the figure is quite interesting in control and alarm applications. This circuit detects when one or more pulses are missing in a constant pulse that is applied to its input. The time constant R C must be calculated so that it is slightly larger than that given by the frequency of the input signal. Thus, in the absence of a pulse at the input, there is time for the circuit to time the pulse and return to zero, thus presenting a negative transition at its output, as shown in the graph of the same figure. Among the applications of this circuit we have a protection link in which a rectangular signal of fixed frequency is sent through a wire. If this link is interrupted (wire cut), the detector placed on the other end of the wire indicates the lack of pulses and generates a trip signal. Another application would be in a data monitoring system in which the absence of a synchronism pulse to capture this data can easily be detected by triggering a warning system. Note that the 5 V power supply makes this circuit compatible with TTL technology and makes it a shield for microcontrollers.
