Kajimuro The Figure 1 shows the circuit diagram of the frequency meter, the circuit based on the digital IC This device uses the ammeter with full-scale deflection of 0. Also refer to Transistor Multivibrator Circuits. Do the same operation for all ranges. The ammeter PA1 indicates the voltage across the capacitor C1.
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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 depends on the values of capacitors C C5 and trimming potentiometers R 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? A quartz crystal oscillator circuit that can generates 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 Hz and connect the device to the reference signal source of 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. Reference oscillator circuit diagram This device uses the ammeter with full-scale deflection of 0. By the way, the ammeter PA1 can be replaced with a digital multimeter. Diodes VD VD4 protects the integrated circuit from overloading. Instead of , the SN or DM can be used. George Kuzev, "Radio Televizia Elektronika" 11,
Monostable Multivibrator 74121
IC 74121 PDF