Here's a great, simple Make project that teaches you how to make an intercom from a pair of old, corded phones, a 9V battery and a resistor. I loved walkie-talkies and intercoms when I was a kid — the idea of setting up your own house-wide wireline intercom is super-cool, and the project is dead-simple.
At its most basic level, a telephone network is just two microphones, two speakers and a power source. In this project we are reducing the phone to these basic elements. The handset of the phone contains the speaker, the microphone and any necessary processing circuitry. All we need to add is the power source.
A regular corded telephone doesn't require much electricity to operate. It just needs about 9 volts and less than 30mA. It normally gets this from the phone line itself. This is why many phones can still work even during a blackout. However in this project, we are using a single 9 volt battery to power our phones.
The battery is wired in series with a 300 ohm resistor and connected to either the red wire or the green wire in a phone cord. The phone cord is then plugged into both phones. The battery is able to supply enough electricity to power the speaker and microphone circuits of both phones. This allows you to use them to talk back and forth.
Simple Intercom From a Pair of Old Corded Phones [Jason Poel Smith/Make]