CAT5 Intercom

Put your broken telephones or unused land line instruments at home to good use by making your own intercom! This workshop will show you how you can do this at home!

It has been over a century and a half since Alexander Graham Bell whispered the words ‘Mary had a little lamb’ and changed the way mankind communicated. Modern life is unimaginable without a telephonic device—be it a sophisticated cell phone or an archaic pulse dial telephone still in use in certain parts of India. If you have gone fully digital, don’t throw out your land line instruments in a hurry. This workshop will help you make a functional small distance intercom device. A straightforward way to build a simple intercom at home is to use two old land line telephone handsets, run a wire between them and apply some power. An improvised circuit can include a buzzer at both ends, so that the person at the other end can be alerted to pick up the phone. First of all, you need to decide which rooms you will install the intercom. Then you need to collect all the materials listed on this page before getting down to work. There are different ways to power the circuit. The first is to simply use a 9V battery. However, the downside to this option is that battery will drain out in no time leaving the intercom connection dead. Alternatively, you can power the circuit using any good pure 12 V DC power adapter. Any laptop’s idle or unused power adapter is the best choice. Don’t worry if the voltage level isn’t exactly 9 V—anything with an output voltage below 24 V will do the job. Note that the intercom needs to be powered at one end of the circuit only. The scenario for which we designed and built this intercom included two rooms having computers connected over a network CAT5 cross-cable. Since a network cable consists of four pairs of wires, of which only two pairs are used by the network adapter, the other two unused pairs were perfect for our purpose. We only require four wires for the intercom to function, so if you don’t already have a network cable connecting computers between floors or rooms, you can use any ordinary telephone cable with four cores or two pair wires. Construction If you plan on using an existing network cable, (like in our workshop), note that pins one, two, three, and six (from the RJ45 connector) are used by the network adapter while the rest are just not needed. Check which wires are used in your cable. In our case it turned out to be the orange and orange-white, green and green-white. Using a wire cutter, we need to cut and separate out the remaining four wires. We shall now be using these four wires for our workshop. Now simply cut the sleeve of the CAT5 cable carefully and dig out the unused wires, then cut them carefully. The other wires can continue to be connected to the RJ45 connector, and be used for the existing network. Now that the wires are ready to be used for the workshop, twist the ends so that you have pointed tips. Refer to the circuit diagram above and connect all the elements—the switches, buzzers and RJ11 jacks. Finally, hook up the telephone’s cord to the RJ11 jack and power up the circuit with the battery. Now, your intercom device is all ready to be used.

TEST IT: Press any one switch to check if the buzzer beeps at both ends to confirm whether the ringer is functioning. Next, pick up the handsets at both ends, start a conversation with the person at the other end of the line to confirm the working of the intercom circuit. And Voila! The intercom is ready. If the intercom doesn’t seem to be working properly, refer to the circuit diagram and find the fault to ensure that the connections of the circuit are correct. This short workshop described a method to install an intercom device in your home. To improve on this, you can design your own complicated array of switches in a more efficient manner by connecting more wires and handsets to make a larger intercom.


The Author likes to thank 'Deepti Krishnan' for her help in this workshop.