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 is from an old manual of operational amplifiers. The setting can be adapted to work...
The operational amplifiers used on this Motorola circuit from the 1970s are no longer common, but...
This circuit uses a feedback system that latches the relay when closed. The circuit uses an...