Instructables
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. 
 
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Step 1: Materials

Picture of Materials
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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
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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.
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mk4841 month ago

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:

http://electronics-lab.com/projects/telephone/004/... and another one here on Instructables, which uses the 4 way cordless phone system, with no cabling costs:

http://www.instructables.com/id/Cordless-Phones-Us...

mk484

dbuckley64 months ago
Could Lectric Wizard tell us exactly how to do his mod, as the question of a buzzer has come up so often? Thanks for idea, will be doing it for kids treehouse.
jfgorm016 months ago

Mind = blown. I'm going to wire the whole house and figure out a ringer.

cbelitz6 months ago

How much range can you get with this set-up (how long can the wires be)?

DIY Hacks and How Tos (author)  cbelitz6 months ago
As long a you want. A phone cord doesn't have a lot of resistance. So your main limitation will probably be the amount of money that you want to spend on the cord.
arikii6 months ago

On a Plain Old Telephone Service interface, the ringing signal is created by superimposing ringing voltage [90
volts AC at 20 Hz in the USA] atop the -48VDC already on the line.

The ringing signal sent to a customer's telephone is 90 volts AC at a frequency of 20 hertz in North America.

Surely this data can be used to create a ringing system...

bradw6 months ago

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 bradw6 months ago
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.

"I've been wanting to install an intercom system since I bought the house."

Then, put in a LAND LINE and buy a five-handset Panasonic remote handset phone system with answering machine. Phones located in five places make hearing a ring no problem and the handsets can intercom one another -even while taking an 'outside' call.

as far as the door bells are concerned, you can buy remote doorbells with Front and Back ringer tones that can be placed all over the house (receiver units) and all 'ring' the same tone when either front or back doorbell button is pressed.

"Why not just add a switch on each end (normally open) inline with the power,"

SPDT switches (require three conductors) would allow either end to activate the circuit. LEDs to indicate 'on' condition required.

Look at teh switch in the handsets see if it is a DPDT so that a RINGER circuit could be employed.

"Why not just add a switch on each end (normally open) inline with the power,"

SPDT switches (require three conductors) would allow either end to activate the circuit. LEDs to indicate 'on' condition required.

Look at teh switch in the handsets see if it is a DPDT so that a RINGER circuit could be employed.

lmcclane6 months ago
Why not just add a switch on each end (normally open) inline with the power, leave the phones off the hook, and break out the receivers to an external speaker? This way the circuit is always open when not in use, which reduces power drain. Not only that, but you won't have to concern yourself with a ringer and would just be immediately heard in much the same way that a regular intercome -actually- works. Would also make for some entertaining pranks on unsuspecting victims. ;-)
Rouverius6 months ago

zappenfusen, I think there are three voltages that are in play here. There is the low voltage required to talk on the phone (about 6V DC). That voltage goes up to about about 50V DC when you hangup the phone. Then, there is the ringer signal which is something like 90V AC at 20Hz.

zappenfusen6 months ago

Anyone know how the original phone system connected the phones with internal ringers operational by picking up sending phone to ring other phones on line?

royharper6 months ago

Well this is ONE way to beat the NSA wiretapping of your communications!

Picturerazzi6 months ago
Just curious.... Why would this be useful? :)

I have a two floor house with a very dispersed layout (lots of halls), a garage and a barn where I have a workshop on one floor, practice area on another and intend to make the third floor into useful space at some time...

My house also has three doors that get used a lot and have no door bells; people either call our cell phones or think we are snubbing them when we don't hear the knock - we can't hear the kitchen door (next to the driveway) from the living room and we can't hear any of the doors when we are upstairs in the bedroom.

We have to yell to be heard between the floors and even then we tend to have to walk to the middle of the house where the stairs are and ask the other person to repeat themselves..

I've been wanting to install an intercom system since I bought the house. With a trip to Goodwill, this one would cost me only about $25.

A lot of houses have phone wires throughout the house, with phone boxes in most rooms. These boxes are all, theoretically, connected. One option would be to inject current into the line and then have readily accessible intercoms for each room.

I had this same thought, but you have to remember to disconnect the house from the outside telephone system or you run the risk of sending a charge back to the central office and damaging their equipment or injuring a line worker (very small chance, but why risk it?).

Intercom house to shop, playhouse to kitchen, access gate to house, etc.

Mostly I see this as something that children would use as a toy. But you could set it up just like a regular intercom and use it to communicate across a house or to a workshop or front gate. There are a lot of potential uses even if they all seem a little ridiculous.

House to garage/barn/greenhouse etc.

If you can ring the other phones it would be infinitely more useful.

BurgersBytes6 months ago

Even if you could make the phones ring with AC voltage, you'd still need a button to press as most phones don't have one already. Might just as well use the other two phone line wires to carry a low voltage to a buzzer on each end. The other 2 wires are normally not used in the phone itself. They are used with a transformer on special phones.

heathbar646 months ago

If the phone required 9 volts why do you need the resistor?

It's been a while since Electronic Theory for me, but as I recall the 300 Ohm resistor won't change the Voltage much. The resistor is intended as a current limiting resistor. Amps (I) = Voltage (E) / Resistance (R); 9 / 30 = 0.03 Amps == 30 milliamps. Without a resistor, even a small one, the draw on the battery could be more than the phones could handle.

Again, it's been a while, but I believe that's the theory.

Kinda... Yes the resistor does limit the current, but you have to take the whole system into account: Battery, resistor, and both phones. Using the thevenin resistance of the two phones, the voltage drop across the phones and resistor can be found using a simple voltage divider equation. So the statement that " 9 / 30 = 0.03 Amps" is not very accurate to this system. If it were only the battery and the resistor that statement would be true.

This should help anyone find the required resistor if they wanted to use a higher voltage source:

http://www.facstaff.bucknell.edu/mastascu/elessons...

https://learn.sparkfun.com/tutorials/voltage-divid...

DIY Hacks and How Tos (author)  heathbar646 months ago
The actual operating voltage of the phone is more like 6 volts. Its just that 9 volts makes a really convenient supply voltage. You can use 9V battery or a 9V power supply.
whobedo6 months ago

Can it be made to ring too, so you can alert your party to pick up?

Boomer16 months ago

A buddy actually hooked up a similar home made device on a jobsite. This was before cell phones were in everyone's pocket, and the only ones with radios on the job were the general contractor crew.

We were pulling a long run of 500 MCM, and we just hooked it up to the emergency lighting circuit, (that was not energized). It worked really well for telling the guys when to start pulling, etc.

Skruffles6 months ago
I've measured the average voltage of a phone line to be about 50v.. Surely a 12v supply would be adequate?
veeguy Skruffles6 months ago

For quick learning experience, hold the phone wires in your fingers and use your cell phone to call your corded number. It's el-cheapo shock therapy.

Boomer1 veeguy6 months ago

Yeah...that's a really good method...especially if you're not expecting it.

Hooking up a phone line, bare wire in my fingers...mother in-law calls...ZAP!

Or in her case...try, try, again.

50 to 90 V is when the phone is on the hook, it typically drops to under 20 V when the phone is off the hook.

DIY Hacks and How Tos (author)  Skruffles6 months ago
You can use different power supplies but you may need to change the resistor value
Silence6 months ago

Is there a way to get the other phone to ring ? like an extension number or something ?

That would significantly increase its usefulness.

veeguy Silence6 months ago

To get a corded phone to ring, you need to send aprox. 90 volts AC through the same line as the 9 volts DC is sent through. It might be more practical to use the other 2 wires ( yellow & black ) in a phone cord to ring a chime or buzzer on the other end. Put a momentary on pushbutton switch on each end to send a ring signal to the other end. Push the button to signal the other end to "pick up".

phckopper Silence6 months ago

http://www.epanorama.net/circuits/telephone_ringer.html

To make the phone ring you'll need to raise the line voltage to ~70VAC@20Hz

wrsexton6 months ago

Wouldn't this also work for a pair of cordless phones if you connected the bases to this modified cable?

DIY Hacks and How Tos (author)  wrsexton6 months ago
It might work with cordless phones if you had the bases plugged into the wall outlet.
And, if you had matching cordless phones, swap bases (receiver 1 & base 2/receiver 2 & base 1) so you can "page" the receiver at the opposite end of your connection. That solves the problem of getting the phones to ring and makes this project quite useful. I've got kids and a very large old house. I'd prefer to use something like this than have my kids tempted to run or yell through the house whenever they need something.
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