Devices used to connect between outside and inside buildings for incoming visitors.
Intercom main panel has a number of buttons equal to the number of housing units in the building by pressing the corresponding button for each unit, it sends an electrical signal through the cables that connects the main panel to each intercom device in each unit then it gives a bell.
In some of Intercom there is open the door option by sending an electrical signal from the internal Intercom to the external Intercom Which located at the entrance, then the Intercom sending an electrical signal to the knob by closed circuit. Several doors can be connected to the same internal intercom panel, and the door can also be opened from the inside through the internal exit button.