When no one is on the phone, voltage is at ~50VDC. When someone picks up
the phone, voltage drops to ~8VDC (someone check that number?) So a 20V
Zener would let the current through when not in use, and stop when it's
picked up. Hook that up to an Opto-isolator or something simliar, invert
it, and you have a phone in use indicator. As for the DTMF, maybe someone
else can help...

-Greg


At 07:22 AM 02/6/99 , you wrote:
>Does anybody know, how to build a circuit to detect how much time my
>brothers  used the phone and which number they dialed. I was thinking to use
>DTMF with 8870 and a pic to calculate the time, but don4t know how to
>trigger the start of time calculation, is there any tone for that??? (start
>and stop of the conversation)
>Anything would help thanks
>Javier

-----------------------
Greg Cormier, age 17
Canadian living abroad in Kathmandu, Nepal
Digital electronics, programming, computer interfacing, microcontrollers
Local time : GMT + 5h30m
gcormier@wlink.com.np
ICQ # : 565465