DIYbanter - View Single Post - Hooking 2 POTS phones togeather as intercom

wrote:
Michael Kennedy wrote:

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.

Thanks,
Mike

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.

H. R.(Bob) Hofmann

Hi...

Or see if he can't find a couple of the old mag sets

Ken