Introduction: Simple Intercom From a Pair of Old Corded Phones

Picture of Simple Intercom From a Pair of Old Corded Phones



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

Picture of Materials

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

Tools
Wire Strippers
A Sharp Knife
Soldering Iron
Screw Driver

Step 2: How the Intercom Circuit Works

Picture of 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

Picture of 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.

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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. 

Comments

hans.schou made it! (author)2017-04-23

That was easy. I just put a clamp on the two connectors and then 6 volt from a PSU on the others to make a loop. I didn't use the 300 ohm resistor but it was fine anyway - just draw 0.04 Amp. The grey box from Lübke can give 48 volt AC and can make the black phone really noisy. I have to work more on that.

hsnopi (author)2015-11-16

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

JenniferK169 (author)hsnopi2017-02-25

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.

benk40 (author)2015-12-04

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

JenniferK169 (author)benk402017-02-25

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 don't know. One of the phones might be broken.

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.

bradw (author)2014-02-07

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?

iphlue (author)bradw2014-02-08

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.

JavierS114 (author)iphlue2017-02-22

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!

iphlue (author)JavierS1142017-02-23

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.

JavierS114 (author)iphlue2017-02-23

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!!

GeorgeN11 (author)2016-12-05

​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.

ConnieB82 (author)2016-11-28

https://youtu.be/pE5WwZikTD0

Ringer!!!

ConnieB82 (author)ConnieB822016-11-28

your ringer voltage can be run on top of battery voltage. That is how it is done on a phone line.

Oliwia. (author)2016-03-30

Hi, do you think a public payphone type of phone would be suitable for this? Or does this only work only on the ordinary kind?

ConnieB82 (author)Oliwia.2016-11-28

yes. The voodoo that made it a pay phone was at the central office. First job in high school was at a gas station. Pay phone out front and a phone in the office. The office phone had no dial.

I don't know how public pay phones are wired up. But it is worth a try.

DASA44 (author)2016-06-27

what is the use of resistor in the circuit ?

The resistor helps to lower the voltage from the 9V supplied by the battery to the about 6V that the phone circuit needs.

also (as I am just learning a lot of this terminoligy), is this using a NPN or a PNP?

which part are you asking about?

So if I had a AC to DC power source that was already outputting at 6V I wouldn't need to do anything but splice the source in much like you did the battery right?

That should work if it is a regulated output power supply.

EnzoP4 (author)2016-05-01

hi is it possible to connect 4-5 phones ?? How?

Is it possible to ring the phone 2 using a number or switch?

Is it possible not to use a resistor?

Yes. Connect all the phones in series.
It is possible but adding a ringing function makes it a lot more complicated
If your power supply is 6 volts such as 4 AA batteries, then you would not need a resistor.

Please tel me how to add a ringing function in this..

AaronP91 (author)SandipM62016-09-11

From my memory, the ring is about 90volt ac at I don't know what frequency. Make sure that you have a current limiting power supply(set to a very small current) so you don't melt something. An inverter and a few solid state voltage increasers... I don't know what they are called... Or the inverter with transformers should work from a battery.

DougL2 (author)SandipM62016-07-23

the phone only uses 2 wires so there's at least 2 more wires in the phone line to use. Why not make a tiny piezo electric buzzer run off the other 2 lines? In the old days, everyones phone would ring so people were assigned ring codes to indicate their home/phone was being called. Like Morse code with long and short rings.

Adding a ring function would greatly increase the complexity of the system. And to be honest, I am not really sure how to do that without damaging the phone.

ben-bradley made it! (author)2016-06-24

Designed the box to have two RJ-45 jacks so that any old wall phone can be plugged in =)

ayandasgupta (author)2016-06-06

I have made it with 220 Ohm resistor and connected one cordless and one corded phone. Working good.

NancyS115 (author)2016-02-21

What type of 300 and 600 ohm resistor do I buy? How many watts, volts, etc? I looked online and there's many different types. A link to Amazon would be great.

I used a 1/4 watt. The specific type doesn't matter.

HansF3 (author)2015-12-08

Hello. I successfully hooked up two old rotary phones together by using a 9v battery between them. Now, will using 9v with no resistors harm the phones in any way either in short time use or long term use? I can use 6v if that is better.

Another question: Inside the phone wire there are three wires; red, blue and white. I found out that white is (-) and red is (+). I am unsure what the blue one does.

Do you know if there is any signal output when phone 1 is lifted off? I wanted to hook it up so that a 12v buzzer sounds(next to phone 2) when the handle of phone 1 is lifted off. Maybe the third wire gives power when the phone is lifted off?

6 volts would be best. There can be as many as 4 wires in the cord. Just worry about the red one. The switch that is on the base of the phone just connects the signal wire when the phone is picked up so that you only hear someone when the phone is off the hook. But you could rewire it to use one of the extra wires to activate a buzzer.

PratyushC (author)2015-12-03

Hi, I am looking for any readily available cordless phones(pair) for intercom connectivity without phone line ...can you please suggest any model available online.

Sorry. I have no suggestions.

Thanks

egull2 (author)2015-11-26

I built this to do an off grid intercom between buildings about 200 ft apart at my property. This part is really simple - the trick is to get it to ring. I successfully rigged a buzzer system, but now I'm trying to refine it to make the phones actually ring. I also modified the system to run of a micro solar system with a car battery rather than a 9 volt battery (Overkill, but I plan to add more stuff to the system). When I get it done, I'll post my own instructable.

Cool idea.

keshavsai (author)2015-10-17

Hi am Keshav see brother ur given just 2 telephone intercom connection yes that is really good but i want go hot line connection how its please give me suggestion

Thank you

I am not sure I understand what you are asking about. Could you please clarify.

Thomas LeeC (author)2015-09-06

Hello! Can I ask something? Can you put a schematic on how you put a buzzer / led to both telephones?

You can just wire them up in series like this
https://cdn.instructables.com/F2D/MLM1/HEBNHKGW/F2DMLM1HEBNHKGW.LARGE.jpg

GamerDudecnr made it! (author)2015-08-08

I made the project and I also added a on/off switch. I also changed the project box to an altoids tin lined with duck tape on the interior

FarmerKJS (author)2015-06-11

I need to run a line about 150ft long will this work?

Can I power it off a 12v battery?

What does the resistor do? Will I need it?

Thanks, Kaden

Yes it will work. Yes you can use a 12 volt battery. All you to do is use a slightly larger resistor. The resistor drops the voltage down to about 6 volts (the voltage that the parts are designed to work with). If you are using a 12 volt battery try using a resistor that is close to 600 ohms.

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Bio: My name is Jason Poel Smith I am a Community Manager here at Instructables. In my free time, I am an Inventor, Maker, Hacker, Tinker ... More »
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