This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
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....