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 valid for 106 series SCRs and can be modified to operate on the 110V power grid....
Found in an October 1992 Radio Electronics magazine article, this circuit generates a low-power tone in...
This circuit was obtained in a Popular Electronics of the 90s. The circuit is compatible with...