This circuit was found in documentation from the 80s. It is based on unijunction transistors not very common today being synchronized by the 60 Hz power grid.
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...