This is a very simple post for the electronic enthusiast on how to build your very own intercom connection at home. Before we discuss on how to make an intercom connection, I advise the reader to brush up the concepts related to the working and networking of the telephone lines.
The following links give the required details regarding the working of the telephones. Those who are already aware of the working can skip this section.
http://en.wikipedia.org/wiki/Telephone_network
http://communication.howstuffworks.com/telephone-image.htm

 MATERIAL REQUIRED:

Now lets build up our own intercom connection which is similar to the telephone network. We require:
1) 2 old telephone sets

2) One 9volts battery

3) One 300ohm resistor

4) telephone wire as per requirement

 CONNECTIONS:

The telephone wire may consist of 2 or 4 or 8 number of wires out of which we only require the centre two wires which are usually red and green in color. Strip the red wire and make the following connections as shown in the figure.

Here the red wire act as the live wire and the green wire as the ground wire. Insert the jack at the ports provided in both the phones.


APPLICATIONS:
Once your connections are complete, you have your very own intercom connection which you can install in your homes across various rooms. The drawback of this connection is that two people can talk at the 2 ends of the connection only when both of them have lifted the receiver. It doesn't have a ringing bell. But there is a solution to that too. just keep both the phones on speaker mode and decide a tone pattern for dialling. when u dial that tone the other person at the receiving end listens to it being dialled on his speaker of the phone. This acts as the pseudo ringtone .