Path: utzoo!telecom-request Date: Fri, 21 Jun 1991 18:30:01 GMT From: Nelson Bolyard Newsgroups: comp.dcom.telecom Subject: Re: Modem vs. Line-Powered 'In Use' Light Message-ID: Organization: Silicon Graphics, Inc., Mountain View, CA Sender: Telecom@eecs.nwu.edu Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 478, Message 3 of 9 Lines: 69 gsteckel%Ecd@sun.com (geoff steckel) wrote that his line-powered phone "in use" light was keeping his modem from being able to receive at 9600 bps. He correctly stated that line-powered lights may be sufficiently non-linear to affect the signal. Here is a schematic for a phone line in-use indicator that won't affect your modem. This circuit is a modification of one published in the article "Phone Line Busy Indicator", by Robert M. Harkey, Modern Electronics, November 1988, page 49. In that article, Q1 was incorrectly shown as an NPN transistor. I built this using Radio Shack parts and the box is now sitting next to the phone in my kitchen. It has quite succesfully prevented the old problem of my spouse picking up the phone while in use by my modem. Unlike some of the other circuits I've seen posted, this one presents a balanced load to the phone line, and does not interfere with high speed modems (e.g. I have no trouble communnicating at 19200 bps with my Telebit T-2500 modem). This circuit draws less than 12 microamps from the phone line when on-hook, and even less when off hook. It should not cause any problems on your phone line (e.g. it won't cause any telco equipment to flag you line as leaking/needing repair). o-----------------o--------o | | | | V LED | | --- | | | | o > | /E > R5 | | / > | + B |< | ------- Green (+) o-----VVVVV----o----| Q1 o --- 3Vdc R1 | |\ | /C ------- | | \C B |/ --- > o----VVVVV---o--| Q2 | Phone line > R3 R4 |\ | > | >E | | o | | | | Red (-) o-----VVVVV----o--------------------------o--------o R2 R1 = R2 = 2.2 Megohms Q1 = 2N3906 PNP xistor (276-1604 pkg of 15) R3 = 330 K ohms Q2 = 2N3904 NPN xistor (276-2016) R4 = 33 K ohms Battery = 2 AAA cells. R5 = 100 ohms LED = garden variety red LED Miscellaneous hardware (Radio shack part numbers): 276-159A pkg of 2 printed circuit boards (great for little projects). I built this circuit and a "hold" circuit on ONE of these PCBs. 270-230 project box (3.25x2.125x1 inch) 270-398 Double AAA battery holder 23-555 pkg of 2 AAA alkaline batteries Construction note: when I first built this circuit, the light shone very dimly. The problem was that the Q1 transistor had a "beta" (Hfe) lower than its rating and was passing insufficient current to the base of Q2 to light the lamp. Replacing Q1 with another one from the Radio Shack package of 15 solved the problem. Disclaimer: I have no relationship with Radio Shack except as a (usually) satisfied customer. Nelson Bolyard nelson@sgi.COM {decwrl,sun}!sgi!whizzer!nelson Disclaimer: Views expressed herein do not represent the views of my employer.