Simple Intercom From a Pair of Old Corded Phones




Introduction: Simple Intercom From a Pair of Old Corded Phones

My name is Jason Poel Smith I am a Community Manager here at Instructables. In my free time, I am...

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
Phone Cord
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

Wire Strippers
A Sharp Knife
Soldering Iron
Screw Driver

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:

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. 

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113 Discussions

Hi there folks - used to work for the phone company Downunder - modern electronic phones can operate on a ring voltage as low as 25 or 30 volts AC around the house - it's only a short cable run. The higher voltages (between 90 and 120 volts AC) were employed in order to drive an electromechanical bell set, and assumed that many phones may have been up to several miles from a telephone exchange (sorry - 'central office').

Here's a link to one of my projects from years ago on another site: and another one here on Instructables, which uses the 4 way cordless phone system, with no cabling costs:


2 replies

Thanks. I checked it out and they've completely redone that website and have deleted all of the old projects from years ago. I've got another project for a wired phone intercom so go to my home page here MK484 and have a look at that one...

Can you show how to make it for multiple intercoms? Can you put them in a string of intercoms i dont know if that makes sence or not but can you show me?

1 reply

How difficult would this be to convert to a multi person system. say 5 users. Do you just pigtail all the wires together?

2 replies

Connect in a big series loop, but you may need to add the power circuit again after 2nd phone. Each phone in the loop will use up power as the current passes through. By the time the current reaches the 4th or 5th phone, the voltage will still be the same, but there will not be enough amperage left on the line. Even in a 2 phone setup you can notice a difference from one phone to the next.

I am not sure. You could probably just connect them in series in a big loop. Try connecting the red wire to the green wire of the previous one until they are all connected.

Hello, I just attempted to do this project with two old 2554 phones. I have tried 3 different power supplies ranging from 5-8 volts DC with 200ma to 1 amp of current and none seem to be able to power the phones. Then I finally tried my 12 volt battery and it supplies enough power, but only 1 phone will transmit. I even replaced the handset cord to rule out a possible bad cord and eliminated the possibility of broken power wire. Any ideas as to why my circuit isn't working correctly? Thanks

3 replies

It's because the wiring is all in series. Once the signal passes through the first phone, there isn't enough amperage left on the line for the second unit to use. You can add an additional circuit on the return line to boost signal up again, or use a power source with more amperage.

I tested both out prior to attempting to do the project. Both phones worked on the phone line. I'll figure it out sometime. I only did the project because I thought it'd be something fun to mess with.

Looking at the circuit, do you even need the base? All the phone circuit you show (speaker & carbon mic) is in the handset, and it doesn't have any phone-supplied dialing or ringing. Anyone tried this connecting two handsets together?

4 replies

Yes! As an electrician we use this very thing (handsets only) to confirm long wire pulls using the actual wire pulled to talk over and confirm good wire. This works great where radios aren't allowed or don't work well.

Hey guys! Jumping in years after this, but if you did happen to use only the headsets, how would you go about connecting the jack onto this. I mean regarding the wiring. The headsets I have are using rj9 plugs with four wired wires in it (I guess for the mics and the speakers). How should I connect them onto this circuit then?

Thanks in advance!

We connected everything in series, including the battery. So the rj9's would be fine and make things clean. A small box like shown in one of the newer comments would work good. just do the wiring connections inside said box to loop it all in series. We used 9 volt batteries. 2 batteries in series gave 18 volts and simply made it much louder. Too loud really but in a loud industrial environment it helped. No resister needed for handsets only in series. Unplug the rj9's when not used to preserve battery or integrate some kind of on off button, perhaps in the handset itself.

Hey iphblue! Thanks so much for replying to this question!!!

May I ask you how you would go about connecting the 4 wires that go from the rj9 to the headset? I understand that there are two for the speaker and two for the microphone, but I only have two ends to wire. You say they should go in series one with the other?

Thanks a lot!!

​I just want to clarify the use of the resistor in this circuit. I see a lot reasons why but the actual reason is this; All wired telephones are made to work of 48-52Vdc on the audio and 90Vac on the ringer circuit. 12Vdc will not harm it. When you talk on the phones, small ac (audio) signals are moving between the two phones. If you should connect the battery without the resistor, the internal resistance of the battery will "short"out the small ac signal. The resistor just "removes" the low internal impedance of the battery from the ac circuit.