Intercom

Intercom system in the Pittock Mansion

An intercom is an electronic communications system within a building or group of buildings. Intercoms are generally composed of fixed microphone/speaker units which connect to a central control panel. A small home intercom might connect a few rooms in a house. Larger systems might connect all of the rooms in a school or hospital to a central office. Intercoms in larger buildings often function as public address systems, capable of broadcasting announcements.

In many schools, tones signaling the change of classes are sounded over the intercom, taking the place of the electromechanical bells used in older schools.

Basic Terms:

Master Station - These are units that can control the system, i.e., initiate a call with any of the stations .
Sub-stations - Units that are capable of only initating a call with a Master Station but not capable of initiating calls with any other stations(sometimes called slave units).
Door Stations - Like sub-stations, these units are only capable of initiating a call to a Master Station. They are typically weather-proof.
Power Supply - Used to feed power to all units.

An intercom may be connected to a door buzzer (see access control).

Wiring Intercoms

While every system is different, most systems have much in common. The following is based on wiring for the LEF system from Aiphone, which is a very popular brand in the Northeastern United States for residential use.

These must be connected to a 12v DC power supply:

+ Positive
- Negative

Other wires include:

E, or common wire
C, or unit identifier.

If one connects a wire color to the C screw terminal in a unit (called the 1 screw terminal in door stations), energizing that wire (by pressing the button associated with the screw terminal the wire is connected to) will initialize a call. In other words, if one attaches the pink wire to the LEF5C in the hallway, and attach the pink wire to screw terminal 4 in the LEF5C in the study, pressing button 4 in the study will energize the pink wire and the common (E) wire. This will initiate a call between the study and the hallway units, with the study unit monitoring the hallway unit.

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