Line circuit for a key telephone system

Claim 1

A line control circuit, for a key telephone system including a telephone line and a telephone set, comprising: a timing network including a single capacitor and a plurality of resistors for providing a plurality of time-delay intervals; a trigger circuit responsive to delayed trigger signals from said timing network; a line relay responsive to ringing signals and direct current signals on the telephone line, for actuating contacts to apply a supply voltage to the timing network so as to charge the single capacitor through at least one of said resistors and provide a delayed trigger signal after a time-delay interval; a first control relay responsive to an off-hook condition of the telephone set including contacts for disabling the output of the trigger circuit; a second control relay responsive to the trigger circuit for indicating a ringing state of said telephone set and including connections to the resistors to rapidly charge said single capacitor so as to substantially decrease said time-delay interval and thereby sustain a continuous ringing condition in said telephone set; a third control relay responsive to actuation of the first relay for indicating a busy state of said telephone set and to alter connections to the resistors to discharge the single capacitor in the timing network, said third control relay including selfholding contacts for holding itself actuated via the trigger circuit; a hold key for releasing the first control relay during said off-hook condition and actuating the second and third control relays to indicate a hold state of said telephone set; and including means wherein the single capacitor is discharged during the idle and off-hook states of the telephone set, thereby substantially preventing false indication of conditions in said set immediately following a power disruption thereto.