The output frequency from IC pin 3 is determined by the voltage input to pin 6. A standard frequency counter can be used to measure voltages directly over a limited range from 0 to 5 V. In this circuit, the 555 is wired as an astable multivibrator. Resistor R2 oetermines the output frequency when the input to the circuit (the voltage measured by the voltage probes) is zero. R4 is a scaling resistor that adjusts the output frequency so that a change in the input voltage of 1 V will result in a change in the output frequency of 10 Hz.
Frequency-counter - schematic

That will happen when the combined resistance of R3 and R4 is 1.2 MO. To calibrate short the voltage probes together, adjust R2 until the reading on the frequency counter changes to 00 Hz. Then, use the voltage probes to measure an accurate 5-V source and adjust R4 until the frequency counter reads 50 Hz.

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