At 07:15 PM 2/6/99 -0500, Karl A. Uscroft wrote:
>Old circuit i've seen involves one rectifier (B250C1500) with the output
>across a 22k resistor and a high efficiency diode.  With a line voltage of
>~40V coming from the phone lines when not busy and the LED will light.  The
>rectifier means that it doesn't matter which way round you have it, also
>polarity protection is of no consequence.  When in use the LED will dim.
>Using a zener as mentioned in another e-mail will stop it lighting when the
>phone is in use.  This could easily be altered for use with a op-to isolator
>for input into a PIC for timing.

This sort of thing is exactly what they DONT want connected. A device like
this can cause your phone to never be seen as hanging up. You can only draw
a few uA from the lines in the on-hook condition.

The only guaranteed method to check if a phone is off hook, is to look for
a series current >=20mA.
That's what the phone company does. Your phone has to draw at least 20mA,
they have to supply at least 20mA.

Voltage detect can work as I've mentioned, but it's tricky to make it work
on all phones and all lines.

>PS.. In most countries it is not allowed to connect anything other than
>approved equipment across the phone lines.  Hence talk to the phone company to
>see if you can get the Okay.  They check using line resistance or something
>like that.

You won't get it. If you ask them, you will cause youself trouble.

When designing phone circuits, we use line simulators for most of the work,
then PBX lines that are essentially a high quality line simulator, then
after everything checks out, we do a final test on telco lines to make sure
there isn't anything we've missed.

For the hobbyist, you can make a simple line simulator fairly cheaply, or
work on your home line, but  if your device wigs out the automated testing
routines and gets you disconnected, grin and bear it.

Another connect issue is that everything you attach to the line need to
have some serious AC/DC isolation. (it's been a while since I looked at
that number 1600 or 2200V IIRC) Otherwise you risk blowing up your circuit
(and anything attached to it) or putting dangerous voltages back up the
line.  You do NOT want the liability suit when your project shocks a
lineman and causes him to injure himself.  Optoisolators and certified
transformers are what you need.