>     I am making a project which will ring me up if a certain condition is
>met.  My problem is, how do you tell (electronically) when someone picks up
>the phone at the other end after placing a call?
>Thanks for any info,
>Brian.

I once constructed an alarm system that interfaced to the phone line
via a relay and isolation transformer if it triggerred.

A simple high gain amplifier was used in conjunction with a
comparator to 'sense' the signals on the phone line. Any time a
signal was present on the line the comparator output would go high.

If this high was continuous I would assume that Dial Tone was present
and I could begin dialling a number.

Busy Tone, Ring Tone, Number Unobtainable, etc., all have set bursts
of tone which can also be detected in this manner. I have never tried
it but I would think that if the signal from the comparator was
erratic, then you could assume 'talking' on the other end.

Note that in Australia there are some stringent laws that prohibit
any such devices to be connected to phone lines unless approved by
the appropriate authority. With good reason too. Safety to equipment
and more importantly - personnel.

Regards
Tony


Just when I thought I knew it all,
I learned that I didn't.