... and *I* would be interested in reports
on any and all circuits and their longevity
in light of real-world experience and all
that mother nature (in the way of secondary
induced secondary lightning transients as
well as man-made transients in industrial
and office environments) literally THROWS
at them ...

Doesn't anybody think safety and or "isolatin
of the line" from possible human contact anymore?
That used to be a consideration (BUT with the
declining cost of technology I fear safety has
taken a back seat to features and bottom-line
cost).


RF Jim


----- Original Message -----
From: "Russell McMahon" <apptech@PARADISE.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, August 01, 2002 4:31 AM
Subject: [EE]: Telephone monitor


> >   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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