1978-10-25
1980-05-27
Moffitt, James W.
H04M 176
Patent
active
042052041
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.
REFERENCES:
patent: 3504127 (1970-03-01), Slana
patent: 3508009 (1970-04-01), Henry et al.
patent: 3626101 (1971-12-01), Fitzsimons et al.
patent: 3627952 (1971-12-01), Person
patent: 3757052 (1973-09-01), Tothill
patent: 3783198 (1974-01-01), Couch
patent: 3828139 (1974-08-01), Chambers, Jr.
patent: 3872258 (1975-03-01), Chambers, Jr.
patent: 3889066 (1975-06-01), Simokat et al.
patent: 4024351 (1977-05-01), Ingle
patent: 4127747 (1978-11-01), Levi
Moffitt James W.
Myers Randall P.
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