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Telephone Party intercom circuit

2016-02-04 02:10  
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The circuit to make two phones work as an intercom is extremely simple. You can see in the picture below, all you really need is a little bit of spare phone cable (or a spare junction box), a resistor, and a power source. A single 680 ohm resistor should let you use the 12V directly from your car or even a standard 9V battery if you’re wanting to make this portable.We called it “the party line”, and tried very hard not to breathe or make noise while we listened to the neighbor’s phone calls.A large number of intercom stations can be tied together. All units are connected in parallel, and the entire system is buzzed by only one signaling circuit. Each unit is powered individually from 1.5-V cells for redundancy. For greater signal volume, 3-V sources can be used for the supplies without changing any other parts of the system. The carbon microphone of a standard telephone handset at each station feeds into a common-base amplifier, and a tandem high-gain common-emitter stage drives the intercom line. All phone earpieces are in parallel across the line.The signaling circuit, also connected across the line, is a simple oscillator that drives all the earpieces.The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line. If another telephone is required at some distance, a parallel line is taken for connecting the other telephone.
 

Party intercom