Abstract: A subscriber loop extender for a telephone system in which a variable shunt conductance (41) is placed across the loop in response to a loop current indicating an off hook condition (32). An optical coupler (39) using a photo-resistor (41) or a photo-transistor as the variable conductance and a light source (40) responsive to loop current raises the value of the shunt conductance until sufficient current flows through the shunt to activate a central office relay (25). An SCR (48), the gate (47) of which is sensitive to the shunt current, extinguishes the light source thus removing the shunt from the loop. Loop current must fall to a level below the relay holding current to extinguish the SCR, thus providing hysteresis to the circuit. False ring trip is avoided by removing the shunt during each positive ringing pulse before its value increases so as to draw current to trip the ring. A second SCR (69) is provided to defeat slow decay of the photo-resistor conductance during dial pulses.

Abstract: An electrical apparatus for use with telephone equipment to extend the effective length of an electrical circuit such as a subscriber loop or the like. The disclosed apparatus includes a variable resistance in shunt across the loop between the central office and a subscriber station, and also includes a sensor which senses the level of current in the loop and varies the shunt resistance in response thereto. When installed with the central office telephone switching equipment, the disclosed apparatus supplements the sensed current level in the loop, thereby increasing the distance from which subscriber station stations may be connected to the central office or enabling the use of telephone loops having greater internal resistance.