The circuit presented here can beused for connecting two telephones in parallel and also as a2-line intercom.Usually a single telephone is connectedto a telephone line. If another telephoneis required at some distance, a parallelline is taken for connecting the othertelephone. In this simple parallel line operation,the main problem is loss of privacybesides interference from the otherphone. This problem is obviated in thecircuit presented here.Under normal condition, two telephones(telephone 1 and 2) can be usedas intercom while telephone 3 is connectedto the lines from exchange. Inchangeover mode, exchange line is disconnectedfrom telephone 3 and gets connectedto telephone 2.For operation in intercom mode, onehas to just lift the handset of phone 1and then press switch S1. As a result,buzzer PZ2 sounds. Simultaneously, theside tone is heard in the speaker of handsetof phone 1. The person at phone 2could then lift the handset and start conversation.Similar procedure is to be followedfor initiation of the conversationfrom phone 2 using switch S2. In thismode of operation, a 3-pole, 2-way slideswitchS3 is to be used as shown in thefigure.In the changeover mode of operation,switch S3 is used to changeover the telephoneline for use by telephone 2. Theswitch is normally in the intercom modeand telephone 3 is connected to the exchangeline. Before changing over the exchangeline to telephone 2, the person attelephone 1 may inform the person at telephone2 (in the intercom mode) that he isgoing to changeover the line for use byhim (the person at telephone 2). As soonas changeover switch S3 is flipped to theother position, 12V supply is cut off andtelephones 1 and 3 do not get any voltageor ring via the ring-tone-sensing unit.Once switch S3 is flipped over for useof exchange line by the person at telephone2, and the same (switch S3) is notflipped back to normal positionafter a telephonecall is over, the next telephonecall via exchangelines will go to telephone2 only and the ring-tonesensingcircuit will stillwork. This enables theperson at phone 3 to knowthat a call has gonethrough. If the handset oftelephone 3 is lifted, it isfound to be dead. To maketelephone 3 again active,switch S3 should bechanged over to its normalposition.

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