Using 555 to form an additional circuit for measuring voltage with a frequency meter

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As shown in the figure, this circuit consists of an unsteady multivibrator consisting of 555 and R1, RP1, and C1, and a decay resistor. As a voltage-to-frequency converter, it can measure a DC voltage of 0~5V.
Potentiometer RP1 determines that when the input voltage is 0V, the frequency meter reads 0Hz; RP2 is used to adjust the frequency meter reading to 50Hz when the input voltage is 5V, that is, the full scale adjustment. This additional circuit allows the measurement frequency to be replaced by the measured frequency.