Quote:
Originally Posted by AndiiT
Hi, I tried experimenting to make a shared service emulator using relays in each station "line circuit" that were held via the loop after the "call exchange" button was depressed, this seemed satisfactory for outgoing calls until I began to look at the incoming ringing and drew up a couple of other designs which weren't very successful.
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It's good to know that you had a go at this. Apart from anything else, it helps to reassure me that wanting to do so isn't entirely crazy!
I find it interesting that at the practical level you hit the snag that I have been envisaging in thinking through what is needed - which is why I commented earlier that emulating incoming calls might be easier if it involved pressing "call exchange", as this would seem to enable incoming calls to be handled in a similar way to outgoing. On incoming, all I can envisage is that the ringing current itself will need to engage a relay which in turn connects the exchange line carrying the ringing current between the appropriate leg and earth and enables the making of a loop at the appropriate station to engage the connecting relay - which is where I start to come unstuck.
My thought experiment for outward calling goes as follows:
In the idle state, each leg of the subscriber line is connected via a relay coil to battery. The other end of the relay is connected to earth.
When a subscriber goes off-hook and depresses "call exchange" (CE), this connects that subscriber's leg to earth and actuates the above relay which first connects itself (via a relay powered by line current) to a supply to keep itself engaged once CE is released, and disconnects the subscriber line from the relay and connects it to the exchange line. Both these relays probably need to be delayed-release so that they don't release during either the above switching or (for the one powered by line current) during dialling.
Once the subscriber clears down, the line-powered relay then releases, in turn releasing the first relay and everything returns to the idle state.
In theory, would this work?
To be honest, I can't yet get my head round what is needed for handling incoming!