Intercom system through door panel - has multiplexed duplex audio channels as well as spy glass

摘要

Set into the panel of a door, e.g. a front door, is a rectangular case which has two main case parts with flanges. The two case parts are inserted in an aperture from each side of the door and fastened together using self-tapping screws. At the top of the unit is a 'spy-glass' which allows the occupier to see who is outside the door. When the occupier presses a push-button on the inside, a 9 volt battery is connected to amplifiers and a multiplexer which allow time divided audio communications via microphones and loudspeakers. A variant of the unit has an indirect spy-glass via mirrors for extra protection for the occupier.