The figure below shows a linear voltage-to-frequency converter using 555. Use an op amp and a 555 chip, as well as a small number of R, C components. A linear V/F conversion circuit can be constructed. The op amp constitutes a differential integration circuit, and the trigger level of 555 is Udd/3. The input voltage is within Udd/2. 
Linear voltage to frequency converter circuit
Linear voltage to frequency converter circuit
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