Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!nrl-cmf!ukma!rutgers!gatech!hubcap!ncrcae!ncr-sd!serene!pnet12!gbell
From: gbell@pnet12.cts.com (Greg Bell)
Newsgroups: sci.electronics
Subject: Re: Telephone Interfacing (On/Off Hook Detection)
Message-ID: <365@serene.UUCP>
Date: 29 Jan 89 22:16:04 GMT
Sender: root@serene.UUCP
Organization: People-Net [pnet12], Del Mar, CA
Lines: 24


 
     I'm sure you're going to get a hundred responses to that message, but
maybe I can give you some info that nobody else will...
 
     The phone line sits at about 50vdc when all phones are on hook.
When a phone goes off hook, this voltage drops to about 10 vdc.

  
         See, you really don't need any precise specs for this since the
voltage variation is so large.   I have a project in front of me that happens
to include an on/off hook detection circuit.  I did it with a simple voltage
divider (a 100K and a 4.7K resistor) a zener diode to clamp the ring voltage,
and a transistor (with a pull-up resistor at the collector).   When the line
is at 50vdc, the transistor is on, and pulls the output to ground.  When the
line is at 8vdc (about), the transistor turns off and the output goes high due
to the pullup resistor.
 
         If you need more help, or a text diagram would help, let me know.

    Greg Bell_________________________________________________________
      Hardware hacker          |
      Electronics hobbyist     | UUCP:  uunet!serene!pnet12!gbell
      EE major at UC San Diego |