>   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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