This circuit was found in a Popular Electronics from the 90's. The output signal can be applied to a headphone or speaker. The microphone can be a piezoelectric tweeter capsule or other type.
This circuit was found in a telecommunications publication of 1978. Its purpose is to obtain...
This setting appears with variations at various points on this site. This is from a Motorola...
This circuit is from a 1967 documentation. The circuit uses germanium transistors and has an...