This circuit was found in an English publication from 1967. The circuit is for listening on a phone. Transistors are types of germanium of the time.
This circuit was obtained in a 1995 Electronics Now. It switches off a relay after an interval that...
This circuit with a MOSFET is from an old American documentation. The capacitor should be leak...
We found this circuit in old documentation. Modern equivalent operational amplifiers can be used....