•  
  •  

Two line intercom plus a telephone changeover switch

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. In this simple parallel line operation, the main problem is loss of privacy besides interference from the other phone. This problem is obviated in the circuit presented here.
Under normal condition, two telephones (telephone 1 and 2) can be used as intercom while telephone 3 is connected to the lines from exchange. In changeover mode, exchange line is disconnected from telephone 3 and gets connected to telephone 2.
For operation in intercom mode, one has to just lift the handset of phone 1 and then press switch S1. As a result, buzzer PZ2 sounds. Simultaneously, the side tone is heard in the speaker of handset of phone 1. The person at phone 2 could then lift the handset and start conversation. Similar procedure is to be followed for initiation of the conversation from phone 2 using switch S2. In this mode of operation, a 3-pole, 2-way slide-switch S3 is to be used as shown in the figure.
In the changeover mode of operation, switch S3 is used to changeover the telephone line for use by telephone 2. The switch is normally in the intercom mode and telephone 3 is connected to the exchange line. Before changing over the exchange line to telephone 2, the person at telephone 1 may inform the person at telephone 2 (in the intercom mode) that he is going to changeover the line for use by him (the person at telephone 2). As soon as changeover switch S3 is flipped to the other position, 12V supply is cut off and telephones 1 and 3 do not get any voltage or ring via the ring-tone-sensing unit.
Once switch S3 is flipped over for use of exchange line by the person at telephone 2, and the same (switch S3) is not flipped back to normal position after a telephone call is over, the next telephone call via exchange lines will go to telephone 2 only and the ring-tone-sensing circuit will still work. This enables the person at phone 3 to know that a call has gone through. If the handset of telephone 3 is lifted, it is found to be dead. To make telephone 3 again active, switch S3 should be changed over to its normal position.

If there are any problems please contact webmaster@electronic-circuits-diagrams.com

Please share & like


Vintage Notifier Fire Alarm Control sales binder 1950's picture

Vintage Notifier Fire Alarm Control sales binder 1950's

$40.00



vintage Unbranded Multimeter Untested As Is  picture

vintage Unbranded Multimeter Untested As Is

$4.99



Vintage RCA SENIOR VOLTOHMYST METER Model WV-98C w/ PROBE, Powers Up picture

Vintage RCA SENIOR VOLTOHMYST METER Model WV-98C w/ PROBE, Powers Up

$65.00



Vintage Brown & Sharpe Combination Square No. 4 USA Providence RI (Pls Read) picture

Vintage Brown & Sharpe Combination Square No. 4 USA Providence RI (Pls Read)

$19.95



People standing near large building -- vintage letterpress printing block picture

People standing near large building -- vintage letterpress printing block

$5.00



Caterpillar Performance Handbook Edition 32 Vintage 2001 picture

Caterpillar Performance Handbook Edition 32 Vintage 2001

$24.99



Vintage DYMO 1720 Label Maker picture

Vintage DYMO 1720 Label Maker

$18.99



Vintage Micronta Range Doubler Multi Tester 22-204C (Unit Only / No Cords) picture

Vintage Micronta Range Doubler Multi Tester 22-204C (Unit Only / No Cords)

$12.97



Vintage Jacobs Multi-Craft Drill Chuck  DC4G61 Made In USA picture

Vintage Jacobs Multi-Craft Drill Chuck DC4G61 Made In USA

$20.00