74HC121 PDF

This frequency comparator circuit see Fig. If it is higher, then the output of the circuit is high, otherwise it is low. The IC DD1 74HC is a monostable multivibrator with reset, it produces a short positive pulse when a negative edge appears at A input. The IC DD2 is a D - flip-flop, it used as a memory cell, a rising edge on its clock input captures a value at D input. If a half-period of input signal is less then the width of positive pulse produced by monostable multivibrator, then the signal at D input is high, so the output of D - flip-flop is high see Fig.

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This frequency comparator circuit see Fig. If it is higher, then the output of the circuit is high, otherwise it is low. The IC DD1 74HC is a monostable multivibrator with reset, it produces a short positive pulse when a negative edge appears at A input.

The IC DD2 is a D - flip-flop, it used as a memory cell, a rising edge on its clock input captures a value at D input. If a half-period of input signal is less then the width of positive pulse produced by monostable multivibrator, then the signal at D input is high, so the output of D - flip-flop is high see Fig.

For an input frequency F the pulse width should be about 0. In this circuit the monostable multivibrator is based on gates DD1. Because the pulse is negative, the inverted output of D - flip-flop DD2 is used for output. The pulse width calculation is almost the same as in the previous circuit with 74 series, except the coefficient is 0. In both circuits a potentiometer can be used, connected in series with resistor R1, it will allow to adjust the pulse width, so the circuit can be tuned for desirable frequency.

LTSpice models of this frequency comparator circuit can be downloaded here. It is necessary because logic gates in program LTSpice are almost ideal, but real gates comprises protection diodes.

Without this diodes especially without diode D2 the capacitor C1 couldn't be discharged fast enough. See also how this circuit working in Falstad Circuit Simulator on Youtube. Frequency comparator circuit based on digital IC's This frequency comparator circuit see Fig.

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