DEVICE AND METHOD OF DETECTING VOLTAGE CHANGE OF TELEPHONE SUBSCRIBER LINE

摘要

PURPOSE: To precisely detect the voltage change of a subscriber line to which plural telephone sets are connected in spite of an interference signal existing on the subscriber line, to securely recognize the off-hooking of the other telephone set connected to the same subscriber line and to prevent the invasion of privacy. CONSTITUTION: Nodes N1, N2 and N3 are provided. The node N2 is provided with a voltage divider 10 between the nodes N1 and N2, a voltage supply means 20 supplying voltage corresponding to subscriber line voltage, a low pass filter constituted of a resistance 30 and a capacitor 32, a comparator 16 generating an output pulse in response to the drop of subscriber line voltage and a comparator 18 generating the output pulse in response to the rise of subscriber line voltage.