# Creating your own phone network?

Discussion in 'The Projects Forum' started by Mathematics!, Apr 7, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
http://communication.howstuffworks.com/telephone4.htm

And me and my 3 next door neighbor's are curious to set up our own phone network.

We all have spare phones , crimping and stripping tools ...etc
And we are wondering if we can make our own network.

Forget the ringing for a second. Can we just use a 9 volt battery and hook all
the greens together and reds together.

Then hook the green bunch to positive and red to negative of the battery.
Or must I use a larger battery for more phone's then two (I don't think so because all of these are hooked in parrell )
But the more phones I add in parrell the less current for each phone while the voltage will stay the same.

But what about the 300 ohm resistor? I don't quite understand why they use it and what I should use.

I would like to know what voltage and resistors I need for 100 or 1000 ,...etc phone's their should be some formula for it ?

2. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Does anybody know why the 300 ohm ?

3. ### Mike2545 Senior Member

Mar 26, 2009
116
3
Try this. You are on the right track

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4. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Yes but that is for 2 phones and a microphone...etc

How would I create a network of say 20 houses with a battery and resistors capacitors ,...if needed etc ?

I am looking for the easiest way other then a string and a paper cup thing.

I am still curious on why the 300ohm could I just not use it or by not using it will it damage the phone?

I don't really understand what line balancing actually does and if it is important. Is that what the 300ohm is in the link?

I am also curious on how long the wire can be before their is to much of a voltage drop to hear anything? ...etc

I know I can calculate voltage drop by the gage and length of the wire but I don't know if voltage drop is what screws up the analog voice?

5. ### Wendy Moderator

Mar 24, 2008
21,500
2,961
Because early phones used carbon mikes to pick up sounds, and the AC component rode on DC. Standards die hard, a lot of what we do harkens back the technology invented at the beginning of last century.

That DC turned out to have a lot of practical uses later. Pulse dialing, for example intermittently shorted the line in pulses that counted to the number dialed. Later it was used to power the electronics in touch tone phones. Because that is the standard, phones still require the DC carrier for their electronics and to carry the audio. It is built in, and not easily gotten rid of. Otherwise you could build an intercom, which is easy enough (I did this as a noob).

What you are asking about is a switch board, aka exchange. How else are you going to route calls to various users. How many people would you like to use the exchange at the same time? Think switches, lots of switches.

I'm interested myself, I had to give up an extra phone line due to financial reasons, and need something to test my BBS out. At the moment it is effectively down because I can't call in. What I need is a dial tone.

6. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
8
Try an internet search using the exact phrase "private branch exchange."

7. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I get the basic's of how the telephone switch network works.

But I don't want it this invovled I just want anybody that picks up the phone to hear the other person.

I guess you could say it is an intercom using 3 or 4 ..etc phones tied together.

All I am wondering is do I tie all the phones together in parrell or seiries?
And How much voltage will I need to power all the phones in my simple intercom network?

Can I just use a 9 volt with 4 or 5 phones.
I am looking for a formula to calculate given N phones how much voltage/current will I need to power the phones to hear everybody talk thru any of them? And if I need some sort of resistor how can I determine it by the number of phones N.

I would think that the more phones the the less resistance 300ohms?

I am not even sure if I need a 300 ohm resistor even?
I know if I hook everything in parrell all the phones get the same voltage but each phone will get less current.

So in the long run I want it just to have an intercom network with phones using the simplest things.

Later I will make a basic switch network by making a circuit that when a person picks up his phone a led comes on then the center station which is my house will pick up and ask who would you like to talk to. Then I will break my connection with him call the other guy plug his phone directly into the other guy's ...etc

I don't think this is to hard it is like a manual switch network like the first telphone network was based on.

But before I can do all this I need to know how much volt's for the phones...etc etc. I measured the voltage in the regular phone network (POTS) to be around 45 or 47 ish volts DC. I think when you pick up the phone it goes up to 90 volts.

This is why I am getting confused why the link say's 9 volt battery.
Because clearly the real phone network uses much higher.

Last edited: Apr 9, 2009
8. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
8
Parallel.

They will be in parallel, so you'll need the same voltage (48V, not 9) for a dozen as you would need for one or two. Multiply current requirement of a single phone by the total number of phones. http://www.allaboutcircuits.com/vol_1/chpt_5/index.html

48V is the specification. 90V is only for ringing.

9. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok, say I have a 48V DC voltage source and every phone is hook in parallel.
As I hook more phones in parallel each phone has the same voltage on it but the current is reduced.

I can measure the current of a regular phone on the POTS. But what I am not sure of is how if I exceed the current min then how am I suppose to bump up the current while keeping the same 48V?

Also I am wondering in the POTS system is their generator DC or AC and what is it's voltage/current readings. I would think DC since I measured the green and red wires using DC on my multimeter.

Also curious to know how to make the phone ring. Do they just bump the voltage up to 90V DC and the phone circuit rings at that voltage or is it they have to send some kind of plusing DC at a certain voltage like 90V to get it to ring?

I guess what I am asking is how is the ringing speaker tripped to go off in the phone circuit?

Forgetting cordless phone just the old cord ones we are talking about.

10. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,182
415
I know this Chinese-American lady who's a rather eccentric computer programmer (aren't they all?) She absolutely hates telephones. She wired up an antique hand-cranked telephone to a friend's matching unit in the same apartment building. The only time she'll take a call is if it comes from her "operator:" She loves freaking people out if they happen to need to borrow her phone for an outgoing call. She tells them to give the phone a few cranks...and lo and behold, her "operator" in the other apartment actually answers. (She then patches it to a modern phone line...all unseen to the "victim"). She gets the strangest looks!

I love it.

Eric