Abstract: A circuit monitors the period of separation between impulses and provides an output only when the period of separation between successive impulses is substantially the same as a predeterimed period. The impulses are applied to a monostable circuit which produces pulses of fixed duration in response. A switching circuit controlled by the pulses switches a binary counter to be driven by a low frequency oscillator output between pulses and by a high frequency oscillator output during pulses, the counter being reset to zero at the trailing edge of each pulse. A comparator determines whether, before reset, the counter reaches a count corresponding to the predetermined period and provides an output pulse to an AND circuit input when the count is reached. The AND circuit has another input connected to the monostable circuit output and thus provides an output pulse when the comparator output is coincident with a corresponding pulse from the monostable circuit.

Abstract: A total security time-delay circuit useful in railway communications, for example, provides a series of output pulses after a specific minimum interval in response to a direct voltage applied to an input of the circuit, the minimum interval being maintained even in the event of circuit component malfunctions. A pulse generator connected to the circuit input provides a series of pulses to control a first contact breaker which switches the primary winding of a transformer to partially discharge a capacitor which is series-connected to the primary winding. The capacitor charges through a resistive network of a transformer secondary winding circuit which includes a second contact breaker having a control terminal connected to the transformer secondary winding and operable to provide output pulses when the capacitor discharge reaches a sufficient intensity.