Carrier subscriber telephone off-hook detector

Details Carrier subscriber telephone off-hook detector Carrier subscriber telephone off-hook detector

H - Electricity

04

M

379/57

H04M 3/22 (2006.01) H04Q 3/00 (2006.01)

Patent

CA 1190681

CARRIER SUBSCRIBER TELEPHONE OFF-HOOK DETECTOR by TOM L. BLACKBURN ABSTRACT OF DISCLOSURE A resistor and diode means are electrically connected in series between nodes A and B on the base electrodes of first and second transistors of a differential amplifier. These nodes A & B are also connected to a source of temperature-compensated load-sensitive refer- ence voltage and to a tap point, respectively, that senses current in the loop of a carrier-serviced subscriber's telephone set. The base and emitter of a control transistor are also connected to associated nodes A and B. Conduction of the diode means and control transistor prebiases the latter and reduces the reference voltage during an on- hook condition. When the handset goes off-hook, the node B voltage rises faster than that on node A for decreasing the conduction rate of the control transistor to indicate an off-hook condition, and eventual- ly cutting off the control transistor prior to the diode means. This operation of the diode means extends the loop length over which the circuit will detect an off-hook condition. With the diode means cut off, the second transistors collector voltage biases an FET in the loop for varying the resistance thereof and making the loop current a pre- scribed value for loop lengths that are less than a limiting value. - 19 - -19-

424580

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