After a days research and a day of looking for the parts i have finished the telephone intercom. It has 2 phones and both can talk together without interference or noise. Haven't found a way to make them ring, still looking into that Smile . Haven't tested it with cordless phones but it should work. If you cant find a 300Ohm resistor 330 would work just as well but not any lower that 300. Also i don't think you need the resistor if your cable is huge, 50m+ needs no resistor.

Here is the diagram i made. The red and green wire are 2 of the 4 wires found in any phone cable NOT VOIP

It would be good to use a high quality power supply as simple phone-chargers may have noise and noise could damage the phones. If you can't get a power supply a large electrolytic capacitor connected + to + and - to - would do the job ok. (4700μf or larger)

Btw: Is there a way to upload/host the images on
Looks fairly simple and straight to the point.

And no, sadly. You'll have to continue to use photobucket, or other similar resources. is good...

The second circuit will generate the AC waveform needed to make the telephone ring.
Wow, I had a much more complicated schematic for this. If I can find it, i'll post it.
pcb_master wrote:

The second circuit will generate the AC waveform needed to make the telephone ring.

With that design you must have audio through a audio jack, not through another phone.
This is quite nifty. What inspired you to build it?
elfprince13 wrote:
This is quite nifty. What inspired you to build it?

The idea of having to pay the phone company when i am just calling my friend at the 3rd floor next door. So i threw a phone cable from my balcony to his and he hooked it up to his phone. Since the phone only uses 2 of the 4 wires in the phone like i hooked up a 2 way buzzer circuit on the outer 2 wires so we can sort of ring each others phone.
Shock Im gonna have to see a picture of that. just leaving the wire hanging like that could raise a couple issues.
Wind and birds are problems but hey, i would rather deal with that than have to pay a fat phone bill since we can spend 4+ hours talking while playing minecraft or other games.
Tom wrote:
If you cant find a 300Ohm capacitor 330 would work just as well but not any lower that 300.

Do you mean resistor, Tom?

Anyway, looks like a nifty project, I might try it if I can find some old telephones. Kitchen to computer desk intercom++
Yes I do mean resistors
Why do you need a resistor? From my understanding, the resistor in a typical telecom office is to match the voltage of the line to that needed by the phone. The voltage at the telecom office is high (usually about 50 volts), so it can reach many miles away without having the voltage at your phone be too low. However, the phones themselves are 12 volt devices, so think of the cirtcuit as a voltage divider. The resistance of the telecom office resistor + resistance of long wires + resistance of the phone you are calling = resistance of R1. Resistance of your phone = R2. Current = 50v / (R1+R2). Voltage at your phone = current*R2. The proper voltage at your phone should be about 12 volts.

So if you have a battery that is already a 12 volt battery (or less, such as a 9volt battery), you shouldn't need ANY resistor at all in series with it (it's already at the proper voltage, without constructing a voltage divider by putting a resistor in series with the phone). If my assessment is incorrect, please state WHY it is incorrect. I plan to build this circuit using a 9v battery, and without any resistor at all (the fewer components the better), if it won't damage the phone. Of course, I don't want to burn out my phone with too much voltage and/or current, so I am holding off on doing this experiment, until I receive a reply to this post.
Yes, it's more convenient to do this with a 9V. This video has the simplest circuit I could find.
Actually from further reading, in typical scenarios with a normal POTS system with central office, etc, the voltage present across each phone (depending on a couple factors, like how far away it is from the central phone office, and how much resistance the phone itself has) will be between about 6 and 12 volts. This means that the voltage across a pair of connected phones will be between 12 and 24 volts (the rest of the voltage is dropped by the power-resistor in the central office, and the resistance of the phone lines). I also read, that a telephone typically has a resistance between 200 and 300 ohms. The largest possible current would therefore be for a 200ohm device when the voltage across it is 12 volts, which is 60mA. The smallest possible current would be when the voltage is only 6 volts, and the resistance of the phone is 300ohms, which is 20mA. So as long as the current flowing through the phone is between 20 and 60mA, then there should be no problem with it. Below 20mA and the phone may not work. Above 60mA and it may be damaged.

And I think to account for unexpected line conditions, telephones themselves are required to be manufactured with components that will handle levels of current and voltage significantly larger than typically encountered in a properly operating phone system, and even to be able to handle reverse polarity without damage (though depending on the phone it may not operate with wrong polarity, but will avoid being damaged).
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