Line circuit for a key telephone system

Claim 2

A telephone line circuit for a key telephone system comprising, in combination, a pair of line conductors, ringing detector means bridged across said conductors, a transistor network responsive to said detector means for initiating the operation of local ringing means, means including a timing circuit capacitor for protecting said network against operation by spurious signals thereby eliminating the need for a ringing ground return path, said transistor network being isolated from said ringing detector means, a plurality of relays for performing logic and output functions, said network including an interconnected pair of output transistors for driving one of said relays, one of said pair having the emitter electrode thereof connected directly to the collector electrode of the other of said pair and means connecting the collector electrode of said one of said pair to said last named relay.

Claim 3

Apparatus in accordance with claim 1 including means for ensuring the availability of the combination of the BVCES of said other of said pair and the BVCEO of said one of said pair for driving said last named relay.

Claim 4

Apparatus in accordance with claim 2 wherein said ensuring means includes a junction point from which operating current iS applied to the base electrodes of said pair, means including a first diode connecting said point to the base of said one of said pair, means including second and third diodes connecting said point to the base electrode of the other of said pair, a third transistor having the collector-emitter path thereof in parallel with the base-emitter path of said other of said pair and means directly connecting the collector electrode of said third transistor to the base electrode of said other of said pair.

Claim 5

Apparatus in accordance with claim 3 including a local power source for biasing the transistor of said network and for operating said relays, and dual function means comprising a single transistor connected in common emitter configuration between said source and said network for operating as a current limiter and voltage stabilizer.

Claim 6

Apparatus in accordance with claim 4 wherein all the transistors of said network including said pair and said single transistor are included in a single integrated circuit.

Claim 7

A telephone line circuit for a key telephone system comprising, in combination, a pair of conductors connectable to a telephone line, a bridged ringing detector including a ringing-responsive relay connected across said pair of conductors, means including a transistor network, a timing circuit and a plurality of relays responsive within preselected parameters to the operation of said ringing-responsive relay for effecting operation of said plurality of relays thereby to initiate local signaling response, said bridged ringing detector being isolated from said means with the exception of contacts of said ringing-detector relay being included as a part of said timing circuit, said said network including first and second interconnected transistors arranged as a relay driving circuit for one of said plurality of relays, the emitter electrode of said first transistor being connected to the collector electrode of said second transistor, the collector electrode of said first transistor being connected to said last named relay and means for ensuring the availability of the combination of the BVCEO of said first transistor and the BVCES of said second transistor for driving said one relay.

Claim 8

A telephone line circuit for a key telephone system comprising, in combination, a pair of conductors connectable to a telephone line, a bridged ringing detector including a ringing-responsive relay connected across said pair of conductors, means including a transistor network, a timing circuit and a plurality of relays responsive within preselected parameters to the operation of said ringing-responsive relay for effecting operation of said plurality of relays thereby to initiate local signaling response, said bridged ringing detector being isolated from said means with the exception of contacts of said ringing-detector relay being included as a part of said timing circuit, said said network including first and second interconnected transistors arranged as a relay driving circuit for one of said plurality of relays, the emitter electrode of said first transistor being connected to the collector electrode of said second transistor, the collector electrode of said first transistor being connected to said last named relay and means for ensuring the availability of the combination of the BVCEO of said first transistor and the BVCES of said second transistor for driving said one relay.

Claim 9

Apparatus in accordance with claim 6 wherein said ensuring means comprises a third transistor having the collector-emitter path thereof connected across the base emitter path of said second transistor, said third transistor being in a saturated state when said one relay is to be operated.