I was wondering if it would be possible to hook 2 POTS phones togeather witout the telephone network. What voltage is required to run a phone and what voltage is required to make it ring? I think it is somewhere around
18vac and 55vac to ring but I was a kid playing with a voltmeter when I took those voltages so I'm not sure what the exact voltages were. I guess I could just find out with my voltmeter but I was wondering if there was anything else needed other than approx 18vac and a wire between the two.
You can use two telephones to talk, but you will have to shout to the other end to get the person to pick up the phone. It is 90VAC at 20 Hz for ringing, and 24-48 volts DC to power the electronics in the phone. Some cheap personal radio service walkie-talkies would be better unless your house is made from shielding wall and floor and ceiling materials. Foil-covered sheet rock is normally used only for Kitchen walls, so you should be aboe to use the psesonal radios anywhere. In addition to
24-48Vdc to run the telephones, you have to have wires, etc that are a pain.
If you do go the telephone route, some phones are polarity sensitive to operate the touchtone dialing signals, but you don't neet them so it is not a big deal. I would take any power supply 24 - 48 VDC and put the power supply in series with the two telelpohones that are also connected in series, Then bypass the power supply with a 10 uF or larger capacitor of 50VDC or higher rating. This makes the power supply a low impedance to the talking frequencies.
I designed telco central office circuits for Bell Labs/AT&T 40+ years ago and did what you are proposing doing for a home novelty. But, the radios are far easier, have internal signalling, don't tie you down to one location, can be used for car caravanning, skiing trips, and all sorts of other things and are not very $$. Available at sportuing goods and Radio Shack stores.
Well I have an out building with cat 5 running to it for a network. I'm using only 4 of the 8 wires and I figured that I'd use 2 for a telephone line and 2 for an intercom since I have a 2 line phone. I have radios but keeping batteries in them is a pain and this phone has speakerphone and a headset.
I can't imagine any easy way to produce a 90v 20hz signal. Hmm...
Just for the heck of it, in case you're a younger fellow, a mag set is a self contained single telephone set. Doing remote radio broadcasts for instance we'd rent two pairs from the local telco. One audio quality for the actual broadcast, and one DC pair. Just a pair of wires from the remote site back to the station control room. To each end we'd connect a "mag set". Crank the handle on the set, and ac would be generated that "rang" the other end. :)
However, enough old man tales of the olden days, how about an alternative suggestion, if I may?
Take a look at the new'ish two set cordless phones. Cheap now. Plug the first one into the house line, use one set in the house. Plug the second set base into ac in your outbuilding. You then have a telephone set to use there, and an intercom between the outbuilding and house. Transfer calls back and forth between handsets, do three way calls. They're great! (I use a pair at the lake, one in the cottage, the other in the boathouse)
The only downside I can think of is that a power failure disables them.
I don't know if I'd run 90vac down the same cable as my network, but it actually 'should' work.
In my case, I took a couple of 'power line' intercoms and modified them to use the phone line instead. I see them all the time at thrift stores for less than $5 apiece. Disconect the FR output to the AC cord, and solder on a phone plug (or jack, if you want to get fancy). Route that signal (RF) down a pair on your Cat 5.
Michael, I have some power supplies from the old 1A2 Key telephone systems that supply the DC talk voltage, and also have a subcycle 20 Hz ring generator if you want one. I'm not far from you, just north of Belleview on Hwy 441.
If I understand correctly, you suggest connecting a 48V power supply across the series combination of the two phones. I believe that in addition to the power supply, you probably need a current limiting resistor. The phone system must get shorted quite often and they wouldn't want the wires to melt when this happens. I think the phones might rely on the current being limited. I think that the phone loads the line so that it has an off-hook voltage of about 6V or so, on a normal phone line, but you could check this and you could even measure the off-hook current in a phone too. Chris
Chris - You are correct. There is impedance built into the central office supply system to limit the current to about 100 ma as I remember. I did this work 40 yhears ago when I first joined Bell LAbs, been doing EMC work since 1970 or so. You can put a 1000 ohm resistor in series with the two phones and the power supply, and then bypass the resistor with a 10 uf electrolytic polarized correctly so there are no audio losses across the resistor.
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