Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!gatech!hubcap!ncrcae!ncr-sd!serene!pnet12!gbell
From: gbell@pnet12.cts.com (Greg Bell)
Newsgroups: sci.electronics
Subject: Using "Free" telephone power
Message-ID: <636@serene.UUCP>
Date: 28 Apr 89 08:16:09 GMT
Sender: root@serene.UUCP
Organization: People-Net [pnet12], Del Mar, CA
Lines: 24


This is in response to the person who was talking about lighting an LED from
the 50 VDC phone line voltage as an indicator for hook status:

That's the easy project!  But, how 'bout making a circuit that will light an
LED when the line voltage DROPS (ie. the phone goes off hook).  I tried it
once, and got it to work moderately well.   I used a combination of voltage
dividers to keep a transistor off if the voltage is 50 or so.  Worked alright
until the phone rang... the circuit answered the phone!  Got a little
annoying.

I'm still amazed at how phones run entirely off the phone line power.  They
have lit dials, and audio amplifiers... and I had trouble lighting an LED!
(trouble means the phone company equipment gets loaded down enough that it
shuts off your line for a while!).  

Can anybody offer insite on the secrets of running audio amps and chips off
the meagre phone line power?  How 'bout the ammount of current the phone line
can source?

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