Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!optilink!jones
From: jones@optilink.UUCP (Marvin Jones)
Newsgroups: sci.electronics
Subject: Re: How Do I Make a Phone In-Use Light?
Summary: How about Low Voltage Detectors?
Message-ID: <3883@optilink.UUCP>
Date: 27 Jun 90 17:29:34 GMT
References: <2204@sauron.Columbia.NCR.COM> <5349@mmsac.UUCP>
Distribution: usa
Organization: Optilink Corporation, Petaluma, CA
Lines: 18


Perhaps one solution here would be to use a Low Voltage Detector, such as the 
Seiko 8054 series or the Motorola MC34164.  These are typically used to provide 
an open drain pulldown at the output (for driving processor reset lines) 
when the input voltage "browns out" below 4.7 volts or so.  With a divider 
network on the input, this could be adapted to 10 volts or whatever 
threshold you desire.  The open drain output could pull down an LED/limit 
resistor to indicate "in-use".  The system should reset itself automatically 
when the voltage increases again.  One would need to provide a zener clamp or 
resistive current limiting to protect the device from up to 60 volts of open 
circuit "on-hook" voltages.  

Hmmm.  Just might work. 

-- 
 === Marvin Jones === Optilink Corp. === Petaluma, CA === 707.795.9444 X206 === 
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