This circuit was found in 80's telecom documentation. Its purpose is to control a microphone in the process of transmitting voice and Morse signals.
This circuit was found in a 1980s documentation. The indicated integrated circuits are uncommon...
This circuit is from Popular Electronics from March 1991. It can still be assembled with the...
This circuit is from the 1990 Motorola Linear Databook. The circuit can be implemented with other...