Telephone line state detector system - has two detectors and comparators coupled into line by high value resistors

摘要

A detector circuit is coupled into subscriber telephone lines and the detector provides an output which signals the completion of the subscribers call by detecting the reverse current flow in the line which occurs when the handset is replaced. The detector is based on the use of two comparators connected into the subscriber line by high value resisters. One input of each comparator is connected to separate reference potentials the remaining comparator inputs being interconnected. An output is generated when the voltage on the subscriber line moves outside the range defined by the two reference potentials, two signals resulting from the comparator action inhibiting two other circuit states until such an excursion occurs.