A retriggerable monostable multivibrator and a D flip-flop can form a simple, yet reliable frequency comparator that compares an input frequency with a predetermined reference. To determine whether an input frequency (f) falls between two known frequencies, f1 and f2, two one-shot/flip-flip combinations are used, as shown.
Here both the one-shot and the flip-flop ICs are wired for positive-edge triggering. Each input pulse causes the monostable output to go high for the period of its preset timing interval. The flip-flop is triggered simultaneously, but its output is determined by the state of its D input at the time of trigger threshold.
If the period of the input frequency is shorter than the preset timing of the monostable, a constant high level will be present at the D input, forcing the flip-flop’s Q output to remain high. If the input frequency period becomes greater than that of the monostable, the flip-flop’s Q output will go low.
VR1, VR2, and C1, C2 determine the value of the time period of f1 and f2. Some typical values for measuring a range of input frequencies is given in Table 1.
One way to measure the frequency is to increase f2 by decreasing the value of R2 until LED3 goes ‘off’ and LED4 goes ‘on’. Then R1 is decrease so that LED1 goes ‘on’ and LED2 goes ‘off’. Now both LED1 and LED4 glow and the value of R1 and R2 are measured. Frequencies f1 and f2 are calculated by the formula
f1 = 1/(1.1R1C1) and f2 = 1/(1.1R2C2)
and the input frequency ‘f’ falls in between f1 and f2. If
f1 f2 It can be further noted that the values of the resistors and the capacitors can be taken according to one’s application.
PARTS LIST
Resistors
R1, R2 = 10 Ω
VR1, VR2 = 4.7 KΩ
Capacitors
C1, C2 = 10 µF/10V
Semiconductors
IC1 = 74123
IC2 = 7474
LED1 – LED4 = different color LED
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