Found in American documentation from the 1980s, this circuit can be designed with equivalent components. The microphone can be of the piezoelectric type.
We found this circuit in the September 1992 issue of Radio Electronics magazine. It can be...
This circuit adds an amplifying transistor that can supply loads of up to 250 mA. The circuit is...
This circuit was found in a 1964 Electronic Design. The transistor may be a current equivalent of...