Also, be careful about ringig voltage. Apart from anything else, your circuit
will probably see this as lots of on/off hook transitions. I used a similar
circuit to what Russell described with a transistor and a large electrolytic
to smooth the ring and it worked fairly well.

Also, there are a lot of telephone circuits out there. Spend some time
searching and you'll find lots of onn/off hook indicator examples.

Simon.
---

On Thu, Aug 01, 2002 at 09:31:00PM +1200, Russell McMahon wrote:
> >   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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