Step 1: Materials
Two Corded Telephones
9 Volt Battery
300 ohm Resistor (270 ohm or 330 ohm can also work)
Heat Shrink Tubing
9 Volt Battery Connector
Small Plastic Project Housing
A Sharp Knife
Step 2: How the Intercom Circuit Works
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.
Step 3: Separate the Internal Wires of the Phone Cord
To access the wire, I used a sharp knife to cut down the center of the cord for several inches. Then I cut off the insulation from one side to reveal the red wire. I cut the red wire at the middle and stripped the insulation off of the wire at each cut end. This is where we will connect the battery and the resistor.
Step 4: Connect All the Components to Test It.
Connect one terminal of the battery to one of the red wires and connect the other terminal of the battery to the resistor. Then connect the free end of the resistor to the other red wire. Plug the phone cord into both phones and you are ready to test it.
Speak into one of the phones and have someone listening to the other. This works best if they are in a different room. You should also be able to hear them speaking. If their voice is too faint, then you should try using a smaller value of resistor.
Step 5: Solder the Battery Connector and the Resistor to the Phone Cord
Step 6: Mount the Parts in a Plastic Project Housing
Now just carefully fit all the parts into the housing and close it up. Plug the phone cord into both phones and your intercom is done.
Step 7: Using the Intercom
The only problem with this design is that there is no way to make the phones ring. One potential way to deal with this is to use a phone cord with four wires in it such as a “4 conductor line cord” and use the two extra wires to set up some kind of buzzer or LED indicator.
Step 8: Optional: Power the Intercom with an AC Power Adapter Instead of Batteries
If you would like to make this project a more permanent intercom system, you can replace the battery with an AC power adapter. I did a whole other project on how to replace batteries with an AC power adapter using a simple voltage regulator circuit. You can check it out here: http://www.instructables.com/id/Convert-Battery-Powered-Electronics-to-Run-on-AC/
If you try this method, I recommend setting the voltage regulator to 6 volts initially. Then gradually increase the voltage until you reach the appropriate volume in each receiver. Do not exceed 12 volts. If you go beyond this, there is a chance that you might damage the circuits.