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From: ftpam1@acad3.alaska.edu (MUNTS PHILLIP A)
Newsgroups: sci.electronics
Subject: Re: Zero-crossing help
Message-ID: <1991Feb19.023908.4743@ims.alaska.edu>
Date: 19 Feb 91 02:39:08 GMT
References: <9102182138.AA22294@en.ecn.purdue.edu>
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In article <9102182138.AA22294@en.ecn.purdue.edu>, ditz@ECN.PURDUE.EDU (Michael R Ditz) writes...
> 
>I am working on an X-10 based home control system for a senior
>project.  We have decided to make our own X-10 transmitter for
>specific reasons.  The part that I am having trouble with is the
>zero-cross detector.  What we need is the AC line as input, and 
>some "nice" voltage output pulse at each zero-cross.  One of
>the problems is that a transformer CANNOT be used, because it
>will shift the phase.  I would GREATLY appreciate any help on 
>how to do this.

     The best method I have found is to use an optoisolator.  I like to use
an AC input device like the H11AA4 with about a 10 Kohm series current limiting
resistor on the input side.  I tie the output emitter to ground, and collector
to +V thru a pullup resistor.  The voltage across the transistor will be
a recified sinusoid, which can be squared up with a Schimdt trigger gate like
the 7414.

     Another possibility is to use a logic output optoisolator with a small
bridge rectifier on the input.

Philip Munts N7AHL
NRA Extremist, etc.
University of Alaska, Fairbanks