Found in an old English publication, this circuit applies to the 240 V network, but can be changed to lower voltages. Transistors are mostly general-purpose, admitting modern equivalents.
The transistors are 2N918, 2N3464 and other RF equivalents of the time for Q1. For Q2, 2N3399, 2N2894...
This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit...
This interesting intercom circuit was obtained from an Argentine publication from the 1970s....