System for and method of indicating intruder presence on telephone

Abstract

A compact telephone intruder alarm system is operative for indicating to a current on-line user of a primary extension telephone which is already interconnected with the telephone line of the subsequent interconnection and disconnection of another auxiliary extension telephone with the same telephone line. Electrical OFF- and ON-hook signals are respectively generated each time an extension telephone is interconnected or disconnected with the telephone line, and each such hook signal is successively detected and electrically amplified before being conducted to a light-emitting diode. The first detected OFF-hook signal indicates the start of the interconnection of the auxiliary extension telephone, and the second detected ON-hook signal indicates the duration and the end of the intruder presence on the telephone line. All electrical components are of miniature size and are mounted within the handset to simplify and to make installation less discernible. The light-emitting diode is readily viewable during conversation because it is mounted directly on the handset and instantly conveys the intrusion to the user.