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.
We found this multiple thermostat circuit in July 1980 Radio Electronics magazine. It uses...
This circuit is from a 1986 Revista Española de Electrónica. It consists of an amplifier with...
Suggested by a 1968 Philips documentation this Eccles Jordan rocker as it was called can operate...