Found in Maxim's documentation from the 90's, this circuit uses an integrated from the time providing a 12 V output from 10 to 12 V inputs.
This two-channel wired intercom was found in the April 1990 issue of Radio Electronics magazine....
This sensor triggers an oscillator emitting sound when a magnet is approached by a reed switch....
This circuit is based on the voltage drop of the diodes in conduction, in the order of 1.6 to 1.7...