At work we have some old pulse-dialing (rotary) phones. We want to connect two of them to make a simple intercom. Whatever the number is composed (even one single digit) the other should ring and of course we need to speak/hear the voice.
How should I connect them? I read the Wikipedia article but it does not say much about how to use them.
There is a similar question here, but I don't understand why he needs an external buzzer: don't phones already have this built in?
For basic communication, a power supply, somewhere between 24 and 48 VDC, with 600 Ω series resistor, polarity unimportant, will power the phones. As soon as both are off-hook, they'll draw a few mA from the supply and one can talk and listen. Note that off hook, the voltage across each phone should drop to ~12 VDC: I.E., 24 V dropped across two phones, and ~24 V across the 600 Ω resistor.
To make the phone ring, 100 VAC, 20 Hz (other frequencies may be used for party lines, to selectively ring one phone) is inserted in series with the ~48 VDC, and the AC should switch off when the phone is off-hook (low impedance).
See "State of the Art Networking" for POTS basics.
You can buy something called a ring down generator to do exactly what you want. Pick up either phone, and the device sends ring voltage to the other. Pick up the other phone, and the device switches to bias current for communication.
https://vikingelectronics.com/products/dle-200b/
https://www.graybar.com/two-way-phone-line-simulator-32vdc-talk-battery/p/95017169?pcode=95017169
https://vikingelectronics.com/find-a-distributor/
To be clear, the 200b does everything. With a single DC power input, it creates the 40 V bias voltage, the 90 Vac ring voltage, off-hook detection in both directions, ringing in both directions, etc. None of these functions is particularly difficult, although the ring voltage can be a challenge, but they add up to lotsa parts.
You can connect two Plain Old Telephone Service (POTS) phones together using just wires and a battery, and they can communicate.
However, ringing in POTS phones is achieved by sending high voltage pulses from the central office to the phone. One POTS phone usually can't cause another POTS phone to ring directly. It either requires circuitry in between the two phones, or a source of high voltage at the calling phone. Back when the dinosaurs roamed the earth, phones had a hand crank which was connected to a small generator which could generate that ring. That same mechanism was used in portable military phones at least through World War II.
So, if you want to ring a POTS phone, you need a source of high voltage, and a mechanism to control it (even if that mechanism is just a hand crank and a generator).
Also note that the button that is depressed when one hangs up the phone causes the same electrical effect as the rotary switch. You could actually dial a number without the rotary switch by drumming your thumbs on the hang up buttons. (It was quite easy to call the operator by this method, but it is quite difficult to dial a long number without making a mistake.) So, the circuit that decides to send a ring pulses to the other phone needs to distinguish between a "real" hang up, and a dialing pulse.
When I was in college in the UK in the 1970's - there were no cellphones - no Internet - and the college didn't provide phones in people's rooms! VERY inconvenient.
Two friends and I wanted phones to call each other's rooms. We found three old-school phones (mechanical bells - no electronics) connected a ground wire to the college radiators - and a single thin wire dropped out of the bedroom windows - around the building to all three phones. We needed to disguise the wire because this would have been frowned upon by the college.
I applied (IIRC) somewhere between 12 and 24 volts DC between power and ground in my room - and we could talk to each other...but no ringers.
What we found was that using the rotary dial did cause both of the other two phones to "jingle" weakly when you dialled a big number - 8, 9 or zero.
Of course then we didn't know who was being called - so, simple solution - we said that one phone was 888 and another 188 and the third 818. It was easy to tell from the pattern of jingles - whether it was you or someone else who was being called. 888=ring-ring-ring. 188=click-ring-ring. 818=ring-click-ring.
It was kinda neat that all of the phones jingled together because these numbers were "people-numbers" not "phone numbers"! If you were hanging out in one of your buddie's rooms - and the third person phoned you - you could pick up.
Very, very, primitive...but for a simple intercom...simple.
I just bought a house - built in 1910 and it has little cupboards where the phones were hidden. The wiring is still in place - but it's a large, rambling house and having an intercom would be great - with old school candlestick phones...so my college trick MIGHT be the way to go.
