This circuit is from an ancient American documentation. The integrated circuit is 741 and the power supply must be symmetrical. The passing band is centered at 1 kHz.
This basic Schmitt trigger was found in an American publication from 1968. The circuit uses...
This circuit is from an American 1964 documentation. Transistors of the time are used, but modern...
This simple circuit uses small speakers for both listening and speaking (microphone). The feeding is...