It is an improved version of the previous circuit with a voltage follower that reduces the load impedance. The circuit was found in American documentation from the 1980s.
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...