This circuit uses an optical coupler called Raystor created by Raytheon in the 1960s. The circuit uses components from the era that can be changed. The current is low.
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...