An intercom can be a useful tool or a fun toy. In this project, I am going to show you how to make a simple intercom using a pair of old corded telephones. This is an easy electronics project that is great for beginners and fun to do with children.
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
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.
Step 3: Separate the Internal Wires of the Phone Cord
Inside most phone cords there is a red wire and a green wire. If your cord has four wires (red, green, black and yellow), you only need to pay attention to the red and green wires. We need to connect our battery and resistor to either the red or the green wire. Because we only need to connect to one of the wires, the other wire can be left untouched.
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.
It is always a good idea to test a circuit before soldering it together. To do this we need to connect all the components. You can do this with alligator clips, plain jumper wires, or you can just twist the ends of the wires together.
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
If everything is working, then it's time to solder everything together. I highly recommend insulating each connection with heat shrink tubing. If you decide to do this, you need to slide the heat shrink tubing onto one of the two wires before soldering them together. Then after you are done soldering, slide the tubing over the exposed wires. Now heat the tubing with your soldering iron or a lighter to shrink the tubing so that it is tight around the wires. Do this for each connection.
Step 6: Mount the Parts in a Plastic Project Housing
No project is complete without a housing to keep all the parts neatly together. I used a 3" x 2" x 1" plastic project enclosure from RadioShack. We only need to make one modification to it. You need to cut a hole in each side for the phone cord. To do this I held the cord up to the side of the housing at the opening and marked the outline of the cord. Then I cut out the marked area with a sharp knife. Do this on both ends of the top and bottom piece of the enclosure.
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
When you speak into one phone the person at the other phone should hear you clearly. Now you just need to run a phone cord between the two locations where you want the intercom to be located. To turn the intercom off, simple hang up one of the phones.
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
With a typical 9 volt battery, you should get 20-30 hours of use out of the intercom. Keep in mind that the phones still use a small amount of electricity even when they are hung up. So the battery will not last forever.
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: https://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.