Article View: sci.electronics
Article #10380Re: Telephone "line in use" indicator"
From: Jon Sreekanth
Date: Sat, 23 Feb 1991 15:10
Date: Sat, 23 Feb 1991 15:10
36 lines
1783 bytes
1783 bytes
In article <2231@umriscc.isc.umr.edu> robf@mcs213f.cs.umr.edu (Rob Fugina) writes:
a telephone "line in use" indicator. Here's the circuit...
1M 1k / E (to +5)
>----- -----\/\/\----+---\/\/\-----|
| \ C
\ |
/ 220k \
from \ / 1k
phone bridge | \
line | |
| LED
| |
>------ --------------+---------------+
|
GND
To repeat myself just a little, the 1M resistor is too small; FCC
requires a minimum of 5M to qualify as a device with Ringer Equivalence
Number of 5.0. In practise, I'm told, a low resistance might be
interpreted by your local office as line trouble, and result in a
(unnecessary) service call.
Since you're using a power supply anyway, you might consider a
potential divider made up of 22M and 1M, with a cheap JFET opamp
(like the quad opamp LF347) as a comparator.
Regards,
/ Jon Sreekanth
Assabet Valley Microsystems Fax and PC products
346 Lincoln St #722, Marlboro, MA 01752 508-562-0722
jon_sree@world.std.com
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Path: arc1-ca.pugleaf.net!utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!world!jon_sree From: jon_sree@world.std.com (Jon Sreekanth) Newsgroups: sci.electronics Subject: Re: Telephone "line in use" indicator" Message-ID: <JON_SREE.91Feb23101003@world.std.com> Date: 23 Feb 91 15:10:03 GMT References: <154@datatec.DATATEC.COM> <2231@umriscc.isc.umr.edu> Sender: jon_sree@world.std.com (Jon Sreekanth) Organization: The World Lines: 36 In-Reply-To: robf@mcs213f.cs.umr.edu's message of 22 Feb 91 22:47:40 GMT