Intercoms serve a variety of purposes and are commonly found in schools and large companies. Their primary functions are to allow communication across the expanse of a large building with ease. Home intercom systems can be installed to enable instantaneous communication by linking one room to another. Although intercoms can be purchased commercially, they come with a price. However, you can build your own intercom system for your home for a low cost with existing corded telephones.
Use wire strippers to strip approximately 2 inches of insulation from the phone cable, beginning halfway down its length.
Cut 2 inches off the exposed red wire in the centre of the phone cable. Connect one end of the remaining red wire to one of the battery's jack terminals. Solder the connection for security.
Remove half an inch of insulation from both ends of the red 2-inch wire piece cut from the phone cable.
Join one end of the 2-inch red wire to the unoccupied battery jack's terminal, and solder the connection. Connect the other end of the 2-inch red wire to a 680-ohm resistor, and solder the connection.
Join the unoccupied end of the resistor to the free end of the red wire from the phone cable. Solder the connection for security.
Connect the two phones together by plugging in the two ends of the phone cable into their respective sockets on the phones themselves.
Insert the battery into the battery jack. Lift the phones off the receivers to turn on the intercom and speak into the phone.
You can also use corded phones that have a speaker option so you can leave the phones in speaker mode at all times.
Always use caution when working with electrical components. Wear gloves for protection against potential hazards.