intercom with less wires

[Mickey]

Hello guys;

I am thinking of designing and building an intercom device which will have about 50 lines available (50 apartmants in a building), which could be called from one place - and that calling is what bothers me most. I was thinking if it's possible to use just one wire (and GND) for calling. For example, every apartmant could have a circuit which responds on specific binary or DTMF code, but I have no idea how to send the code over one wire, with a push of only one button on the "calling board" which is placed on the entry door of the building.The calling board should have one push-button-switch for each apartmant.

So- I need some design ideas from you guys...HELP!

[audioguru]

Hi Mickey,
You are planning to make 50 signal decoders when only one is needed.
1) Connect the dialer to a single decoder.
2) The decoder selects which intercom station to call.
3) Reset the system at the end of the call.

You are probably going to need cheap two twisted pair cable to each intercom station. One pair for transmit and the other for receive. The station's power can be between the two pairs.
Each station will need a microphone preamp and a speaker amp.
The main equipment can switch the lines using Cmos transmission gates.

[Mickey]

I think you did not understand me -

Your solution tells me to lead one twisted-pair cable to each phone. If I get it right, we're talking of 50x2wires = 100wires through the building walls.

My idea was that all the intercom stations, phones, are in parrallel connection to same bus which has for example 2 wires (data & gnd). A code would be sent through the data line to all phones, but only one should respond.

[audioguru]

Each cable would have two pairs (a four-wires cable). Only one cable to each apartment. If the building has multiple floors then install the cables up the shaft where the water pipes and telephone wires go between floors. Then along the hall where the water pipes and telephone wires go to each apartment.

If you install a common buss then the cabling is nearly the same because each apartment needs a connection to the cable. If the common buss gets damaged then the entire system or a big part of it will fail and the problem might be hard to find.