Path: 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: 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 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 Brought to you by Super Global Mega Corp .com