an intercom for four users

[nhlanhla]

Design an intercom system that could enable communication between four clients within 10metres distance between them.
Must confy with the following specifications
*Any two of the four users could communicate with each other without disturbing other users
*if any two are using the line all four units must shoe engaged indicator by means of an LED
*each unit must have a ringer to indicate incoming call, but only the called party,d unit should ring and lastly not more than four cables are allowed between any two units!

 

[wa1kij]

There is no requirement in the rules for two conversations to go on simultaneously. Thus two wires could be used as a common audio bus. Individual units could be signaled by some code unique to that unit. DTMF, for example.

 

[wa1kij]

The second pair of wires could be used for signaling. For instance, unit 2 wants to call unit 3, so it generates a repeating series of 3 positive pulses on that pair. When unit 3 answers, it puts a continuous voltage on the line, which causes the sending unit to stop sending pulses, and also lights the "busy" light on the other units.