Intercommunication and front door opening system in flats - has four-function contacts to switch loudspeakers in different ways to lines

摘要

The intercommunication and front door opening system, for blocks of flats etc., has control contacts (11-13) in the terminals (4) in the flats to separate the flat loudspeakers (9) from the lines (2, 3) to connect them over a ballast resistor (10) to the lines, to connect them directly to these lines or to short circuit them. The central station has a coupler (17, 18) that a.c. connects one (3) of the lines to the amplifier (24) and d.c. connects the same line to a resistance network that controls various transistors. During 'speak' the control contacts connect the loudspeaker via the ballast resistor to the lines.