This circuit was found in 1980s American documentation. The transducer is a piezoelectric capsule and the power supply voltage can be between 3 and 18 V.
We found this circuit in a Hands-On Electroncs from the 90s. The circuit uses an LED indicator. Diodes...
This circuit was obtained in a 1973 national publication, translated from an English publication of the...
We found this circuit in a 1983 English magazine, but it can be easily assembled with the BC548...