On Mon, 19 Nov 2007 14:04:24 +1300 Jinx wrote: > Series DC-blocking cap (I see values 0.47u - 1u used) with a > relay pole to short that on connection, the impedance of the Tx > signalling off-hook to the exchange I thought a little more, and I have the impression that (from memory) you have the circuit mostly reversed. 1) Normally the line is connected directly to the bridge, and when a ring comes, the detection is done with an optocoupler connected between the bridge and the pic/computer. 2) When the ring is detected, the controller connects the transformer to the line with the realy, which signals to the central that the phone is off hook. The transformer must then transform only the audio signal, NOT the low frequency ring, which would require a large transformer. John -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist