These circuits were found in Exar documentation from the 90s. The circuits use both a small speaker and a piezoelectric transducer. The integrated circuit is not common today.
This circuit is from 1977 documentation, but can be assembled with BC548 for Q1, Q2 and Q4, BC558...
Found in American documentation from the 90s, this circuit uses components that are still common...
This high impedance input amplifier is suitable for the use with piezoelectric sources such as...