HOOK-OFF DETECTION SYSTEM FOR TERMINAL EQUIPMENT HAVING AUTOMATIC DIALING FUNCTION AND EXTERNALLY MOUNTED TELEPHONE SET

PURPOSE:To attain detection of hook-off with smaller number of components by using a common current inversion detection function for automatic dialing and manual dialing so as to detect hook-off of an externally mounted telephone set. CONSTITUTION:The hook-off of an externally mounted telephone set of a terminal equipment controlling the detection of the hook-off of an externally mounted telephone set with an output of a ringer detection circuit 10 is detected in the manual dial mode with an output of a common current inversion detection circuit 11 and in the automatic dialing mode with an output of the ringer detection circuit 10, the manual dialing switching identification state of changeover circuits 6, 8, a detection output of the common current inversion detection circuit 11, or the output of the ringer detection circuit 10, the automatic dialing switching identification state of the changeover circuits 6, 8 and the detection output of a single direction current detection circuit 13. Thus, the hook-off detection circuit has only to detect a current in a single direction from a DC power supply 7 to an externally mounted telephone set, then the constitution of the single direction current detection circuit 13 is simplified.