Telephone Switching > General PAX & Intercom Systems

Can two 500's be connected as a Manual Intercom System?

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Can one convert say 2 500 type phones to local battery with ringer boxes and use as a intercom system?

Yes, sort of.

See the second circuit here.

Yes it can be done,
The circuit over is one example. If you use voltage of high enough, e.g. 18V (or more) you may bridge each telephone with a beeper in series with a zener-diode of a little higher voltage than half of the battery voltage.  When one telephone goes off hook the voltage drop is reduced, and the beeper will sound in the other end.

If you really want the high end local battery speech circuit, you have to get a low resistance/high impedance coil in series with e.g. 3V battery connected in parallel with the transmitter.  To get a hookswitch for turning on/off the battery power, you may use one of the hook-switch contacts for that and leave one of the lines in circuit all the time.
Then you have a circuit equal to the field telephone  TA43/pt.


I am using the diagram metioned in the second post for my model railroad. I will be using a seperate circuit for the call lights. They consist of flashing LEDs run by momentary contact pushbuttons. Note about the circuit diagram, the phone on the right has to be on that end by itself. The ones on the left may be all on the same pair of wires. I will power my phones with a 12volt jump pack, no resistor needed. If you need mode details, I will try to get a rough sketch for you.  Jim


Would that be a model railroad layout that's so large it becomes necessary to talk over a phone circuit to manage the operation of the whole thing when it's all running?

I've seen pictures of layouts like that: museum-scale setups that manage to duplicate entire vast swaths of countryside and cityscape and everything in between.

For those who have never seen anything like this: envision building a duplicate of reality on a scale of 1/87 (think "Matchbox cars" though they aren't exact scale), and doing it so well that from photos you can't tell it's not real. 

I have high respect for people who can build stuff on a tiny scale like that, that's photographically realistic, whether it follows reality 100% or whether it depicts an entirely fictional world.   


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