A quad comparator forms the basis of a frequency detector that is faster and less expensive than more complex versions designed around frequency-to-voltage converter chips. Positive feedback through a 5MO resistor allows the circuit to resolve changes as small as two percent; the output responds to those changes in about one cycle. When the input frequency is high, V, is pulled low; it`s never allowed to exceed 213 V. When the input frequency is lower than the limit, V, exceeds 213 V once each cycle, but 1-`3 is held below that limit.
Frequency-detecting-comparator - schematic

The trip frequency is defined by F = 1/(1.1R2C2). R2 can be adjusted to permit trimming of the trip point, but the value of R3 must remain larger than R2.

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