In the figure we have one of the many options found in this section for an intercom using ordinary phones not in use. This was obtained in a Popular Electronics from the 90s.
This 1995 circuit uses interruption sensors or NC connected in series in any quantity. With the...
This circuit transmits the audio signals from a telephone line to an FM receiver placed nearby. The...
This circuit is from an old manual of operational amplifiers. The setting can be adapted to work...