Importance

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Abstract

A detector for On-Hook/Off-Hook state of a telephone line includes a capacitor connected to the telephone line in series with a resistance large enough to be allowed as load on an On-Hook telephone line. A low impedance voltage sensing means in series with a normally open switch is connected across the capacitor, and the switch is closed in response to a polling signal to determine if the line is in On-Hook state. The capacitor transforms a very low charging current lasting seconds into a large discharge current lasting only milliseconds. This makes it possible to use an inexpensive low impedance voltage sensing device, such as an opto-coupler in series with a zener diode. Such components provide high-voltage galvanic insulation at low cost, and a very compact apparatus can be made. The invention also provides for inexpensive ring detector circuits and for overload protection for the equipment connected to the telephone line.

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First Claim