Hi. At first I want to appologize for my English, it isn't my native language, so sorry for any grammatical errors.

So, here is problem that I'm struggling with for a while (2-3 months).
My friend goes to shoot various events and actions, he ussualy has 5-6 cameras with 3-4 cameramem, but there can be as much as 6 cameras and 8 people that need to comunicate with each other. Now, he has separate intercom system for communication and device for Tally lights. But he is looking for some system or device, that could combine those two, it would be great if the supply voltage could be supplied via the same line. I was doing little research and it turns out that theoreticaly it is possible.
I thought that it would work, if somehow I could do it like old telehpone system, it had supply and it carried audio signal throught 2 wires, for signaling I would need to mix some high frequency to audio, separate it and rectifie it to get "control voltage" which ccould turn on any of 2 tally leds.
So I need to design some kind of interface circuit and "telephone device", but I don't know how circuit should look. I looked for any schematics of any older pots exchange that could use any op amp or transistors in it's interface circuit, but I didn't find any, as if they were not freely accessible or published on the Internet.

Can someone help me with this?

P.S. to my knowledge, there is no such device manufactured, we are two amateur electronics enthusiasts who have access to parts from discarded electronic devices, so any such device should not be a problem to build

 

If you use a version of a telephone "hybrid" you can get full duplex audio and use DC on the line at the same time to control whatever you want. On the other hand, with cheap MCUs you could do this all digitally now and have complete flexibility.

 

I would like it to be analog, and yes I could use telephone hybrid to get duplex audio, but i don't know, how to designe hybrid cirucit, how should it look like on exchange/telephone side and how to get signal from and to the line

 

I would like it to be analog, and yes I could use telephone hybrid to get duplex audio, but i don't know, how to designe hybrid cirucit, how should it look like on exchange/telephone side and how to get signal from and to the line

Two telephones and a battery are sufficient for it to operate. The exchange is nothing but switching.

 

Here is rought sketch, block diagram, how i imagine this cirucit