At 06:59 PM 3/19/96 -0600, you wrote:
>>    I've got a different question concerning phone interfacing.
>> How do you detect whether a phone is on or off hook without
>> inserting a loop detect relay yet keeping this detection circuitry
>> isolated from the rest of the PIC circuitry?
>
>[1] Use a very sensitive voltage sensor to detect whether there is more than
>    12 volts on the line.  If so, then nobody has their phone off-hook.  Note
>    that you need to draw only a few microamps when detecting the voltage, so
>    you may want to use a small transformer to power an op-amp which could
>    then detect the phone line voltage.  The output from this amp is then
>    either fed through an opto or back-fed through the transformer.
>
>[2] As above, but use a battery to power the phone-line side of things.
>
>[3] AC-couple yourself to the phone line and look for audio.
>

I just built a phone off-hook dectector and it works pretty well.  It might
not be what you want, but it is what I did.

I measured the voltage of the line with a phone on-hook and off-hook and got
about 50 V on hook, and about 5V off hook.  I built a 1/10 voltage divider
with high enough resistances so as to not draw too much current because the
volume on the reciever drops too much(can't remember the exact values but I
think I used 1K and 10K.)  This will then give me 5V when the phone is on
hook, and .5V off hook.  I then used a cmos inverter chip to drive a red and
green led to tell me if the phone is off or on.  I had to use a power supply
to power the chip, but it works very well.  I also use the power supply to
light some leds that I put under the numbers that turn on when my receiver
is picked up so I can see the numbers at night.  Be aware though, 1 green
led lights up a whole room at night!  Very annoying.

   Donald Frederick Wright Jr.

   School info: Lawrence Technological University
   E-mail: DW79208@LTU.EDU

   Work Info: AVL North America
   E-mail: DNWRIGHT@AVLNA.COM