We found this circuit in a Popular Electronics from the 90s. It operates at a frequency of 15 kHz. Components can be difficult to obtain, such as the integrated circuit.
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...