Intercom system

Abstract

A communication system includes a master control with optional inside, door or patio remote units, door chime, phone coupler, or programming such as radio, phonograph, tape player or the like. The inside remote units and the master control are connected to common lines including an audio line, control line, and talk-listen line. The remote units signal the master control on the control line by one of five voltage levels selected by depressing intercom talk, door talk, phone talk, or end-call push button switches. Tri-state signals on the talk-listen line from the master control operation of speaker circuits and microphone circuits in the units wherein the units other than the calling unit are automatically operated for hands-free answering during a timer period after release of an intercom talk switch. Phone ringing signals are broadcast, and depressing the phone talk switch causes the audio line to be coupled to the telephone line permitting the unit with the phone switch depressed to talk to the telephone caller whose response is broadcast on the indoor units during the timing period after the release of the switch. A chime circuit generates electronic chime signals with both rise and fall times.