Found elsewhere in this section, but from other sources, this circuit is from Motorola documentation. The circuit uses a triac of the time and a tripping transformer. The circuit is for 110 V network.
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...