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 adjustable oscillator has the traditional configuration already presented in other articles. The...
This circuit appears in other versions on this site. obtained from other publications. This is...
This circuit decodes a 1800 Hz tone used in communications. The circuit is from an American...