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
​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.
<p>your ringer voltage can be run on top of battery voltage. That is how it is done on a phone line. </p>
<p>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? </p>
<p>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. </p>
<p>I don't know how public pay phones are wired up. But it is worth a try.</p>
<p>what is the use of resistor in the circuit ?</p>
The resistor helps to lower the voltage from the 9V supplied by the battery to the about 6V that the phone circuit needs.
<p>also (as I am just learning a lot of this terminoligy), is this using a NPN or a PNP?</p>
<p>which part are you asking about?</p>
<p>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?</p>
<p>That should work if it is a regulated output power supply.</p>
<p>hi is it possible to connect 4-5 phones ?? How? </p><p>Is it possible to ring the phone 2 using a number or switch?</p><p>Is it possible not to use a resistor?</p>
Yes. Connect all the phones in series.<br />It is possible but adding a ringing function makes it a lot more complicated<br />If your power supply is 6 volts such as 4 AA batteries, then you would not need a resistor.
<p>Please tel me how to add a ringing function in this..</p>
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.<br>
<p>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.</p>
<p>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.</p>
Designed the box to have two RJ-45 jacks so that any old wall phone can be plugged in =)
<p>I have made it with 220 Ohm resistor and connected one cordless and one corded phone. Working good. </p>
<p>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.</p>
I used a 1/4 watt. The specific type doesn't matter.
<p>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.<br><br>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. <br><br>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? </p>
<p>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.</p>
<p>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</p>
I don't know. One of the phones might be broken.
<p>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. </p>
<p>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.</p>
Sorry. I have no suggestions.
<p>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.</p>
Cool idea.
<p>How difficult would this be to convert to a multi person system. say 5 users. Do you just pigtail all the wires together?</p>
<p>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.</p>
<p>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 </p><p>Thank you </p>
I am not sure I understand what you are asking about. Could you please clarify.
<p>Hello! Can I ask something? Can you put a schematic on how you put a buzzer / led to both telephones?</p>
You can just wire them up in series like this<br />http://cdn.instructables.com/F2D/MLM1/HEBNHKGW/F2DMLM1HEBNHKGW.LARGE.jpg
<p>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</p>
<p>I need to run a line about 150ft long will this work?</p><p>Can I power it off a 12v battery?</p><p>What does the resistor do? Will I need it?</p><p>Thanks, Kaden</p>
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.
<p>thanks for the clairifictaion!</p>
<p>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 &amp; use two diodes to &quot;steer&quot; the power to a set of buzzers.</p>
<p>This is what I mean. Expand image to see it all.</p>
<p>I dont quite get what you are saying. could you give a bit more detail</p>
<p>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.</p>
And you can get all of the parts you need, at you local Radiosh......Damn
HA HA HA!<br>Good point. I should update that.
<p>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?</p>

About This Instructable




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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