Discriminating network for telephone switching signals

Claim 1

In a telephone system wherein a central office is linked with a subscriber station by a line including a first and a second signal wire, said first signal wire being connected at the central office to a source of d-c voltage, said second signal wire being grounded at the central office, said subscriber station being provided with a hook switch for interconnecting said first and second signal wires to form a closed loop traversed by direct current from said source, said subscriber station being further provided with a key for selectively grounding said second signal wire whereby current passes only through said first signal wire upon closure of said hook switch, the combination therewith of a discriminating network comprising: a first light-emitting diode in series with said first signal wire; a second light-emitting diode in series with said second signal wire, said light-emitting diodes being cascaded in said loop for traverse by the same current from said source upon closure of said hook switch with said key unoperated; a first photon detector juxtaposed with said first light-emitting diode; a second photon detector juxtaposed with said second light-emitting diode; first and second amplifier means respectively connected to said first and second photon detectors for generating an ouput voltage indicative of current flow in the corresponding signal wire; and a logic circuit connected to both said amplifier means for distinguishing between current flow in both signal wires and current flow in said first signal wire only.