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.
We found this circuit in the December 1998 issue of Electronics Now magazine. RF transistors such...
This circuit was found in an American documentation from the 80's. The sensor is a special...
This circuit automatically controls the degree of illumination. As the circuit is from 2001, it...