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Looking around, I find a variety of phone line interfaces. Some are
for simple ring detection, others are for analogue signals. I'd like to
know if the circuit attached will do what I hope it will, and also
what components (protection, isolation, steering etc) may need to
be added

For compliance, because there's a mains-powered PIC-based
circuit, I believe a transformer would be mandatory. Most of the
projects I found on the web use resistors or optos, often with
zeners/caps/diodes. They are generally powered by line voltage.
Many have a few hundred pF at various positions across the line.
There seems to be no concensus about that

The rectified HVAC ring voltage is detected by the PIC, which
then turns on the relay for the LV analogue voltage. I figure this is
the equivalent of the hook switch. The incoming signal is buffered,
the outgoing is driven

Should I wait for a ring pause (ie no AC) before engaging the relay,
to protect the analogue side from ring voltage, or does the exchange
stop that voltage as soon as the relay is engaged ?

Any other gotchas to be aware of ?

TIA

=================================
For a successful technology, reality must take precedence over
public relations, for nature cannot be fooled

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