Title: Common bell ringing apparatus for multiple button key telephone set

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Abstract: A common telephone bell ringing device has a plurality of input terminals adapted to be connected to the ringing sides of a plurality of incoming telephone lines, an output terminal adapted to be connected to a telephone bell, a common terminal for said output and input terminals, a plurality of photoresistors connected between said output terminal and said input terminals, respectively, each being essentially non-conductive when not exposed to radiant energy and conductive when radiant energy impinges thereon, and a plurality of radiant energy sources such as neon lamps connected between said common terminal and said input terminals respectively, each neon lamp being mounted in a light tight enclosure with a corresponding photoresistor so that ringing current on one of said input terminals energizes a corresponding neon lamp to reduce the resistance of a corresponding photoresistor and permit the ringing current to pass to the bell without feedback to any other input terminal.


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