Device for adding intercom functions to a phone system

Claim 1

Apparatus for adding an intercom capability to a plurality of telephones connected in parallel to an incoming telephone company line, comprising: an input connection for communication with the "tip" and "ring" wires of the telephone company line; an output connection for communication with the "tip" and "ring" wires of lines leading to telephones having the intercom capability provided by the apparatus; computing means; a house to central relay responsive to the computing means being activated by a first keypad signal generated at any one of the telephones connected to the output connection, the house to central relay being disposed to disconnect the telephone company line when a first keypad signal is detected by the computing means on the lines connected to the telephones; a hold relay responsive to the computing means to simulate an off-hook telephone on the telephone company line when the house to central relay has disconnected the telephone company line while a call is in progress; a ringing voltage sequence generator responsive to the computing means being activated by detection of a second keypad signal from any one of the telephones connected to the output connection, the ringing voltage sequence generator generating a sequence of voltages corresponding to the second keypad signal to ring all the connected telephones with the sequence; an intercom power supply for enabling voice communications over the connected telephones when the house to central relay has disconnected the telephone company line; an incoming call signal generator responsive to the computing means being activated by means for detecting an incoming call, the incoming call signal generator causing a signal tone to be generated at the connected telephones when an incoming call is detected and the house to central relay has disconnected the telephone company line; off-hook detect means in communication with the computing means for detecting when a connected telephone is off-hook; the computing means being responsive to the off-hook detect means to activate the ringing voltage sequence generator to generate a sequence of ringing voltages when all connected telephones become on-hook after detection of a second signal and to terminate a ringing voltage sequence upon detection of an off-hook connected telephone; and the computing means being responsive to the off-hook detect means to deactivate the house to central relay and thereby reconnect the telephone company line after detection of an off-hook connected telephone during a ringing voltage sequence.