Line card circuit for a key telephone system

Claim 2

A telephone line circuit for use with telephone systems including at least one key telephone subscriber''s set and a pair of line conductors to which ringing signals and line current are applied and which are adapted to be connected through said key telephone set by operation thereof, said telephone line circuit comprising: first means comprising a multi-winding first relay normally connected with said windings in series between said line conductors, said first relay initially operated in response to alternating current ringing signals applied to said line conductors and conducted over said conductors to said serially connected relay windings; second means including a second relay connected to said first relay, operated in response to said operated first relay, to enable audible and visual indicators at said key telephone subscriber''s set.

Claim 3

A telephone line circuit as claimed in claim 1 wherein: said second means further include at least a first transistor connected between said first relay and said second relay, said first transistor rendered conductive in response to operation of said first means and said second relay operated in response to said first transistor being rendered conductive.

Claim 4

A telephone line circuit as claimed in claim 1 wherein: in response to initial operation of said key telephone subscriber''s set said second means are rendered inoperative to disable said audible and visual indicators; and there is further included third means including a third relay, also operated in response to said initial operation of said key set, to render said first relay inoperative and to reenable said visual indicator in a second manner distinct from said original enablement. 4.A telephone line circuit as claimed in claim 3 wherein: there is further included a second transistor connected between said first relay and said second and third relays, and in response to further operation of said key telephone subscriber set, said third means rendered inoperative thereby rendering said first relay reoperable; said fist relay re-operated in response to line current applied to said line conductors; said second transistor rendered conductive in response to said re-operaTion of said first relay thereby re-operating said second means, said third means re-operated in response to said re-operation of said second means and said second transistor being rendered conductive, said re-operation of said second means and said third means in combination effective to connect a bridge circuit between said line conductors, shunting said key telephone subscriber''s set; said reenablement of said second means and said third means in combination further effective to disable said visual indicator operation in said second manner and reenable said visual indicator in a third manner distinct from said second manner and from said original enablement.