The US thresholds are:
When on hook, greater than 32 volts (typically 48)
When off hook less than 30 volts
Current loop 20-30ma
So if you used a bridge rectifier to keep from worrying about line polarity
and connect a 30 volt zener in series with the line rectified line,
and a resistor of say 1meg to the other side of the line,
you could use a differential comparator with series resistors of at least
100K to the inverting and non-inverting inputs to read the voltage across
the 1M resistor. You probably should also include a pair of 12v zeners
reverse series as snubbers directly across the comparators input pins to
limit the differential voltage across the comparators inputs during ring
cycles.
Most such comparators have a small offset voltage so simply reverse the
inverting and non-inverting lines if it will not respond.

Alternately you could just pick up a device from Radio Shack that is
designed to monitor line-in-use. But where's the fun in that.

Lonnie - KF4HAZ

----- From: "Russell McMahon" <apptech@
> >   I am thinking of making a simple on/off hook
> > indicator. If the phone is "hung up", there are 42v on
> > the line. If it is off hook, there are about 4v on the
> > line. Would this work (in general)??
> >
> >   A bridge rectifier to a 12v reg. to a 5v reg to an
> > I/O line on a pic (or an LED, for that matter).
>
> May work but the telephone company will not be your friend.
> Any load placed across the phone line MUST have a very high effective
> resistance.
> Most regulators will draw substantial current in the above scenario. Also,
> the line will be unbalanced and this will cause hum due to noise being
> coupled into the line. The arrangement MAY also stop the line releasing
> after a call. There are also regulatory/permission issues about connecting
> things to phone
> liones.
>
> A simple idea to play with is to use a zener diode of say 10 to 20 volts
in
> series with a large resistor joined to a PIC pin with another resistor
> pulling the same PIC pin to ground. When the line voltage is greater than
> the zener voltage
> the PIC pin is pulled high. When lower than zener voltage the PIC pin is
> low. This is just the basic principle - have a play and see if you can
work
> out a practical circuit.
>
> Use of a simple differential amplifier is liable to more easily produce a
> more "real world" result.
>
>
>
>
>         Russell McMahon
>
>
>
>
> >
> >   My thinking is that if there is < 12v, the 12v
> > regulator wont power the 5v reg. and the LED (or IO
> > pin) will be 0v.
> >
> >   Will this work? As you can tell, I am fairly new to
> > this. I am reading everything I can get, though.
> >
> >   Also, Op-amps are used in quit a few circuits -
> > anyone know a link to a GOOD paper on theory and such?
> >
> > Thanks,
> >   Kevin

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