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blinkinlights 04-12-2011 08:27 PM

2-way phone line intercom
i want to set up a 2-way intercom system for my house...

from what i understand i need to add a separate power source to the phone line. the line needs 30mA to work, i was planing on using a 9v battery or just wiring a 12v car jack to it then plugging it in through an adapter to a wall socket. now to do this i think i need i resistor that is ether 300ohms for 9v or 400ohms for 12v

can anybody tell me if im planing this right any info or advice would help...
i could post a circuit diagram if it would help

gregzoll 04-12-2011 09:19 PM, or go to local radioshack. Nowdays, people just use their cellphones to communicate between people, either by texting, or talking, no matter where they are in the house, or on the property.

Leah Frances 04-12-2011 09:24 PM


Originally Posted by gregzoll (Post 628309), or go to local radioshack. Nowdays, people just use their cellphones to communicate between people, either by texting, or talking, no matter where they are in the house, or on the property.

:yes: DH and I no longer holler at each other in the house (in a honey-can-you-get-me-something-while-you're-in-the-kitchen way, not in a mean way), we just pick up our cell phones. :yes:

blinkinlights 04-12-2011 10:27 PM

its to connect a phone from the house entrance to another phone at the top of the stairs sort of like a cheap intercom system

nap 04-12-2011 10:36 PM

Current flow is a result of the circuit construction and is based in Ohm's law. A circuit draws whatever power it needs as long as you have the power available for it.

You do not want to put any power on an active phone line. It already has its own and varies between 15-48 VDC and ~100 volts AC (ringer)

yes, post a circuit drawing if you have it.

blinkinlights 04-12-2011 11:01 PM

the current is flowing through the phone jack not the power jack

nap 04-12-2011 11:48 PM


Originally Posted by blinkinlights (Post 628371)
the current is flowing through the phone jack not the power jack

power jack? are you in the US or some other country?

Are you speaking of an active phone line?

If in the US and it is an active phone line, there are the voltages I already spoke of on the line. You cannot impose more voltage on the line and expect the phone to work.

I can only accept that the resistance rating for each of the voltages is correct to result in the proper applied voltage to the system. Without knowing the resistance imposed by the phone sets, I cannot calculate anything.

so, not sure what you are looking for. Based on your drawing, you need to insert the proper resistance for whatever voltage you are providing. If using 12 volts, the resistor has to be at least a 1/2 watt rated resistor. The other two voltages would require at least a 1/4 watt rated resistor.

vsheetz 04-13-2011 02:35 AM

My cordless phones have intercom capability - 1 of 5 handsets is in my detached workshop and I can easily intercom to any of the 4 handsets in the house.

blinkinlights 04-13-2011 06:19 AM

there is no external power source at all the battery is soupposed to provide the power source and it is connected in series through phone cord modification the phone cord just goes from one phone to another but has the battery as a power source

gregzoll 04-13-2011 07:21 AM

If you really want to go with this, look at either building a home PBX box, which would give you the feature you want & more, or you will have to purchase two line phonesets, and a special box made for creating in sense a Intercom, that when you pick up one handset on line 2, it would ring the other. That equipment can be found at places like and others.

Cheapest route, is to just go to Radioshack and get a intercom set, or if both of you have cellphones, just use them. We do all the time, when the wife is in the house, and I am in the backyard or garage.

AllanJ 04-13-2011 09:40 AM

Run separate wires. You do not use the wires hooked up to the telephone company lines for a set of private intercom units.

Some regular phone installations use just two wires although the cable has four wires. Here you can use the other two wires for your intercom.

You would not need the resistor unless the instructions called for it.

Ohms or kilohms are mentioned in the instructions because the handsets have some resistance. Generally, handsets with higher resistance work better over a longer distance using thinner wires connecting them.

DangerMouse 04-13-2011 10:11 AM

We just use our old baby monitor for this! :laughing: 20 years old and still working fine!

vsheets has a good idea as well, we had those at one time too, though rarely used the intercom feature.


Wildie 04-13-2011 06:54 PM

I have set up phones for intercom use, on many occasions. I always used a 6 volt lantern battery because it had press clips for the wires.
For standard telephones, such as those provided by the telco, a resistor isn't necessary. You can safely use up to 48 volts on these phones.

The problem is how to signal back and forth. I used to set up a return call buzzer system for this purpose. A return call buzzer system requires 3 conductors.

Just a note to say, the wires from a working telephone line using service from the telco cannot be used for this.

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