Frequency operated switch

Abstract

A switching circuit which is actuated when the frequency of an input signal falls below a specified value, includes an input circuit for providing a rectangular pulse train consisting of pulses having durations indicative of the frequency of the input signal. The rectangular wave train is used to actuate a pair of one-shot multivibrators adjusted to produce output pulses having durations such that the pulses from the first one-shot correspond to an upper reference frequency slightly below that of a nominal input frequency, and the pulses from the second one-shot correspond to a lower reference frequency equal to the minimum acceptable input frequency. The output signals from the one-shot multivibrators are applied to clock terminals of first and second edge-triggered flip-flops, whereas the D input terminals of the same flip-flops are actuated by the rectangular pulse train. Each edge-triggered flip-flop operates so that the first positive-going clock pulse transfers the instantaneous value of its D input signal to its output terminal. The transferred signals from the two flip-flops are applied to clear and preset terminals, respectively, of a latching flip-flop which produces a disabling signal when the frequency of the input signal falls below the minimum acceptable value and retains the disabling signal until the input frequency is increased to a level at least as high as that represented by the upper reference frequency.