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 circuit was obtained in the Motorola Linear Interface ICs of 1993. The current is determined by the...
This circuit was obtained from a Linear Technology manual from the 1990s. The components may have...
This configuration can be found in several articles on this site. The one shown in the diagram is from...