This circuit came out in an English magazine from 1977, but it can be easily assembled today, as the components used are common. The circuit provides a signal whose frequency depends on the capacitance displayed by the transducer. The signal is rectangular, and the circuit can be powered by voltages from 5 to 12 V. In the formulation section for 555 the reader will find how to calculate Ra and Rb as a function of C, for the desired frequency.
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