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I'm looking for an in house pa system circuit that should use existing phone wiring. What I want to do is be able to switch from normal phone operation to pa mode and when I pick up a phone have all others ring, or be able to select which phone I want to ring. Thanks for any suggestions.

A DPDT switch will allow you to disconnect from Ma Bell and do what you want with your system. You will need an 80 VAC generator to ring the other phones. In order to select which phone to ring, you will need an addressing scheme. When you have all that working, it is not likley that it will still work on Ma Bell's system.

Ok, forget the addressing scheme for the moment, how would I make the other phones ring with an 80 VAC generator? I read somewhere that the 80 VAC must be applied only when the phone is on hook, if it is applied while off hook, phones will be damaged. Thanks

never heard of damaging a phone applying ringing when off hook, but your ear won't appreciate it.. the switchhook contacts open off the ringer when your off hook, so all you get is the ringing voltage on the R side of the pair, right in the ear.. you really need to look at a secondary circuit, such as the old blk/yel pair, or in the newer stuff, use the w/o o/w pair for signal and use a 10vac buzzer with a key system power supply.. or any 10vac transformer..

Don't know if this helps at all, but TI makes a couple chips that will generate that AC ring signal, and will automatically switch to the DC talk voltage when the phone goes off-hook.<p>UCC3751 Datasheet

only problem with that is if the phone is still on the network.. you can't send foreign voltages down the telephone companies wires.. they frown big time on that.. so, unless this is a completely isolated system, stay away from that one.. .. (have you read the e-mail I sent you re: putting in a small digital phone system?)

Yeah I meant in addition to the switch that pulls it off the network, sorry I didn't make that clear. Invloves extra wiring, but hey, that's cool