Found in American documentation from the 1980s, this circuit can be designed with equivalent components. The microphone can be of the piezoelectric type.
This circuit is from an English publication from 1965. The transistors used are rare, however, they...
This low voltage circuit was found in the November 1960 Radio Electronics magazine announcing new...
We found this circuit in a 2003 publication. The indicated coil is for capturing the signals in an...