But the only place with the simplest circuit has me confused. http://electronics.howstuffworks.com/telephone4.htm

It says I need a 300 ohm resistor, but I don't know why. Phones are simply appliances that use the DC voltage on the phone line as their power source, and since they are appliances (loads) they have their own resistance (and the required resistance is usually just achieved by having 2 phones in series, I think). Right?

Further more it shows a battery's positive terminal connected to the red of one phone, but the battery's negative terminal connected to the red of the other phone. From my understanding of other sources I've read that talk about Tip and Ring, the Ring is the negative connector and has a red wire, while the Tip is the positive connector and has a green wire. Because of this, I'd think the correct wiring is the red wire of phone-1 should go to the negative terminal of the battery, the red wire of phone-2 should go to the green wire of phone-1, and the green wire of phone-2 should go to the positive terminal of the battery. If you don't do it this way, one of the phones will have proper polarity, and the other will have reverse polarity. This may not be a problem if you are lucky enough to get your hands on an antique telephone that simply had a moving-coil speaker, and a carbon granual microphone, but in modern phones the reverse polarity will fry many circuits. Even the microphone in a modern phone is an "electret" mic, with its own builtin semiconductor pre-amp, and the buttons on many phones are illuminated with LEDs. These semiconductor devices will be fried with reverse polarity. Many modern phones have all kinds of circuits that depend on the DC voltage of the phone line for power. If I follow the diagram on HowItWorks, one phone will work, but the other will go up in smoke (literally).

Now maybe I'm wrong about this, and if so, please explain why.

 

But the only place with the simplest circuit has me confused. http://electronics.howstuffworks.com/telephone4.htm

It says I need a 300 ohm resistor, but I don't know why. Phones are simply appliances that use the DC voltage on the phone line as their power source, and since they are appliances (loads) they have their own resistance (and the required resistance is usually just achieved by having 2 phones in series, I think). Right?

First: This is a very crude setup.

The 300 Ω resistor may be for limiting the current thru' the circuit as an added protection, though really, it is unnecessary for the 9 volts supply.

Further more it shows a battery's positive terminal connected to the red of one phone, but the battery's negative terminal connected to the red of the other phone. From my understanding of other sources I've read that talk about Tip and Ring, the Ring is the negative connector and has a red wire, while the Tip is the positive connector and has a green wire.

There is no Tip and Ring on a Telephone instrument. There are just 2 wires.


Because of this, I'd think the correct wiring is the red wire of phone-1 should go to the negative terminal of the battery, the red wire of phone-2 should go to the green wire of phone-1, and the green wire of phone-2 should go to the positive terminal of the battery. If you don't do it this way, one of the phones will have proper polarity, and the other will have reverse polarity. This may not be a problem if you are lucky enough to get your hands on an antique telephone that simply had a moving-coil speaker, and a carbon granual microphone, but in modern phones the reverse polarity will fry many circuits. Even the microphone in a modern phone is an "electret" mic, with its own builtin semiconductor pre-amp, and the buttons on many phones are illuminated with LEDs. These semiconductor devices will be fried with reverse polarity. Many modern phones have all kinds of circuits that depend on the DC voltage of the phone line for power. If I follow the diagram on HowItWorks, one phone will work, but the other will go up in smoke (literally).

Now maybe I'm wrong about this, and if so, please explain why.

Your argument about Polarity is correct. But, in all electronic instruments, the first unit at the telephone terminal is a Bridge Rectifier to take care of the Polarity. So No Smoke; No Fire. And the Exchange feed is 48 Volts, Not 9

Ramesh

 

Exchange feed is 48V until the line is in use when it drops to around 6V

 

For modern telephones which have polarity steering built-in; use simply a 12 V battery in series to only one of the interconnecting wires, any polarity, no resistors added :

------------------------------(+)12V(-)----------------------------------Tel 2
Tel 1
----------------------------------------------------------------------------

This will provide the intercom function, not telephone operation with dialing, dialing tone nor ringing.

 

you will not get the phones to ring unless you use a 20hz 90V ac source (in the US anyway).

 

It's very difficult to build a "private exchange" just to run two phones.

A much easier option would be to buy a couple of cheap household intercoms, and pull out the guts and install it in the telephone.

 

Your argument about Polarity is correct. But, in all electronic instruments, the first unit at the telephone terminal is a Bridge Rectifier to take care of the Polarity. So No Smoke; No Fire. And the Exchange feed is 48 Volts, Not 9

Ramesh

It's 48 for onhook. The moment you pick up a phone, so it goes offhook, it drops to about 9volts. Since a phone in its onhook state it is pretty much useless, I really only care about simulating its off hook state with 9volts.

As for the bridge rectifier, do standard computer "56k" modems also have a bridge rectifier in them? I hope so, because I eventually want to be able to connect 2 computers together via their modem and be able to send data between them. My use as an "intercom" with 2 telephones would just be to test out my setup.

Also, do modems require the presence of an actual dialtone audio signal to work? Do they require the presence of a ringing signal to work too? What about the presence of an onhook 48v DC, is that also required for the proper working of a computer modem? Please let me know. Thanks in advance?

 

...I eventually want to be able to connect 2 computers together via their modem and be able to send data between them.

Really? That's been obsolete for what, over 25 years?

 

It's 48 for onhook. The moment you pick up a phone, so it goes offhook, it drops to about 9volts. Since a phone in its onhook state it is pretty much useless, I really only care about simulating its off hook state with 9volts.

As for the bridge rectifier, do standard computer "56k" modems also have a bridge rectifier in them?
No, they do not because they don't require to use the Exchange supply to function. They are powered externally.

I hope so, because I eventually want to be able to connect 2 computers together via their modem and be able to send data between them. My use as an "intercom" with 2 telephones would just be to test out my setup.

Also, do modems require the presence of an actual dialtone audio signal to work?
No, they don't. You can disable Dialtone sense by an AT command.

Do they require the presence of a ringing signal to work too?
No.

What about the presence of an onhook 48v DC, is that also required for the proper working of a computer modem?
No.

Please let me know. Thanks in advance?

..........

Ramesh

 

Check out Hack A Day's post on doing this:

http://hackaday.com/2012/06/08/using-old-phones-as-an-intercom-in-your-vw-bus-or-anywhere-else/