# Frequency meter with 74121 monostable multivibrator

Digital integrated circuits can be used in the design of an analogue frequency meter with a frequency range to several MHz. The Figure 1 shows the circuit diagram of the frequency meter, the circuit based on the digital IC 74121. This IC is the monostable multivibrator that may generate a pulse of a different width (from 35 ns and wider) that depends on RC components. All this allows to create very simple frequency meter circuit with several frequency ranges:

• 10 - 100 Hz,
• 100 - 1000 Hz,
• 1 - 10 kHz,
• 10 - 100 kHz.

Fig. 1. Circuit diagram of the frequency meter
D1 - 74121; VD1-VD5 - 1N4148;
PA1 - an ammeter with full-scale deflection of 0.5 mA;
C1 - 15 μF; C2 - 0.39 μF;, C3 - 47 nF; C4 - 4.7 nF, C5 - 470 pF;
R1 - 12 K; R2 - 6.8 K; R3..R6 - 33 K.

The number of frequency ranges can be increased by using a rotary switch SA1 with more than four positions, and by adding some additional RC components. The device can measure sinusoidal wave and square wave input signals.

The pulse width produced by the monostable multivibrator 74121 depends on the values of capacitors C2..C5 and trimming potentiometers R3..R6. The pulse frequency depends on the input frequency.

The pulse voltage is proportional to the input frequency, through the diode VD5 this voltage charges the capacitor C1. The ammeter PA1 indicates the voltage across the capacitor C1.

How to calibrate this frequency meter? You'll need a source of stable frequency reference signal that can produces a set of frequencies 100 Hz, 1 kHz, 10 kHz and 100 kHz. A quartz crystal oscillator circuit that can generates 100 kHz signal with a 3-stage decade counter can be used as a reference frequency source (see the Figure 2). Turn the rotary switch S1 of the frequency meter to the range of 100 Hz and connect the device to the reference signal source of 100 Hz. Adjust the trimming potentiometer R3 to move the needle of ammeter PA1 to the maximum value on the scale. Do the same operation for all ranges.

Fig. 2. Reference oscillator circuit diagram

This device uses the ammeter with full-scale deflection of 0.5 mA. It can be replaced with the ammeter with full-scale deflection of 1 mA, but in this case it takes to replace the capacitor C1 with one of a value of 25 μF, and the resistor R2 must be replaced with a resistor of 3.6 K. By the way, the ammeter PA1 can be replaced with a digital multimeter.

Diodes VD2..VD4 protects the integrated circuit 74121 from overloading. Instead of 74121, the SN74121 or DM74121 can be used.

George Kuzev, "Radio Televizia Elektronika" 11, 1993.