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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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kjsrocks24 days ago

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

DIY Hacks and How Tos (author)  kjsrocks24 days ago
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.

thanks for the clairifictaion!

You could set up a buzzer system by reusing the same 2 wires. Since the phones are out of the circuit when hung-up, you could reverse the polarity of the power supply & use two diodes to "steer" the power to a set of buzzers.

This is what I mean. Expand image to see it all.

intercom buzers.png

I dont quite get what you are saying. could you give a bit more detail

shizumadrive3 months ago

I want to set something up like this but with rotary phones. Preferably candlestick phones. Problem is I dont have the phones yet to figure it out. Something so you click the holder twice it rings phone 2. Three times for phone three ect. Someday Ill figure it out.

bsnapp4 months ago
And you can get all of the parts you need, at you local Radiosh......Damn
DIY Hacks and How Tos (author)  bsnapp4 months ago
HA HA HA!
Good point. I should update that.

Every plan I've seen for this requires the 300 ohm resistor. Why? What does it do? Wouldn't it just drop the voltage and nothing else?

Yes. It drops the voltage to the appropriate level. The phone actually needs about 6 volts to run. The only reason that a 9V battery is used is because it is a little more convenient to work with. If you used a 6 volt battery pack you could probably leave off the resistor and be fine.
b1nman5 months ago

Would it possible to do this with two fax machines? I am curious as most fax machines have a separate mains connection for the mechanical parts to operate, the printer and scanning components for instance. I am wondering if the analog phone components run in parallel to this circuit and they are isolated or they use the same source.

DIY Hacks and How Tos (author)  b1nman5 months ago
Maybe. I don't really know how a fax machine's internal circuits work.
b1nman5 months ago

Would it possible to do this with two fax machines? I am curious as most fax machines have a separate mains connection for the mechanical parts to operate, the printer and scanning components for instance. I am wondering if the analog phone components run in parallel to this circuit and they are isolated or they use the same source.

Silence1 year ago

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

That would significantly increase its usefulness.

Like veeguy said, much more practical to utilize whats already there, the other 2 unused lines in the phone cord. Theres a million and one ways to get it to ring (oscillate) the phones onboard transducer, if your not savy with discrete components, use a MCU like and arduino or something.
veeguy Silence1 year 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".

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

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

Just curious.... Why would this be useful? :)

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?).

A 5 cent diode will fix that issue :)

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.

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.

Skruffles1 year ago
I've measured the average voltage of a phone line to be about 50v.. Surely a 12v supply would be adequate?

The -48 volts is an on-hook voltage. The actual voltage, thanks to the resistance in telephones, drops to around -8 to -9 volts. The 12 volt supply would be more than adequate.

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 veeguy1 year 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.

You can use different power supplies but you may need to change the resistor value
SIRJAMES099 months ago

This is a cool idea!! :)

TY for sharing Sir.

2 Q:

1) Because of my lack of knowledge of electronics, would it be possible to add a volume control Potentiometer to one or both phones without having to increase the voltage?

2) Is there a regulator made that has a dial type id adjustment to it?

Again, I ask because I am uneducated in electronics...

I know what a regulator does, and I know what a resistor is/does....but when it comes to building things that contain electricity and/or electronics, that is where I get lost.

DIY Hacks and How Tos (author)  SIRJAMES099 months ago

Yes adding a simple resistor or potentiometer will let you adjust the volume. No there aren't voltage regulators with built-in dials.

SIR:
TY for your time & effort in answering my Qs...'
Much appreciated.

James
mk48411 months 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

dbuckley61 year 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.
jfgorm011 year ago

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

cbelitz1 year ago

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

DIY Hacks and How Tos (author)  cbelitz1 year 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.
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