Voltage-to-frequency converter (VFC) circuit is shown in the schematic diagram below. The circuit employs 555 IC as the core of its function.
This circuit uses +5V and -5V supply voltages, and the input is limited to 0-2V. You can adjust the zero point by grounding the input and adjust the 4k7 pot to obtain the lowest frequency oscillation. [Circuit’s schematic diagram source: lh-electric.4t.com]