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.
The basis of this circuit is a CA3130 operational amplifier. The transistor can be BC548, and the...
This circuit, found in a 73 magazine for radio amateurs, is used in radio localization. The...
This circuit is from an American magazine Popular Electronics of March 1991. The circuit produces both...