Using the LM3909 this circuit was found in the January 1992 issue of Radio Electronics magazine. Resistor R1 should be chosen according to the table given next to the diagram.
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....