We found this circuit in a documentation for radio amateurs from the 70s. The transistors can be the BC548. With it, small speakers of 4 or 8 ohms can be used as microphones.
This circuit, from a 1988 documentation, makes successive actuation of the shower to control the...
This circuit was found in a 90s Popular Electronics. The circuit indicates if a transmitter is working,...
This circuit was found in a Motorola documentation from 1975. Modern equivalent transistors such...