Path: utzoo!censor!geac!torsqnt!lethe!yunexus!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!newstop!male!texsun!letni!rwsys!jim
From: jim@rwsys.lonestar.org (James Wyatt KA5VJL)
Newsgroups: sci.electronics
Subject: Re: Zero-crossing help
Summary: use an optoisolator
Keywords: Zero-cross Optoisolator X-10 Schmitt
Message-ID: <12354@rwsys.lonestar.org>
Date: 19 Feb 91 15:05:11 GMT
References: <9102182138.AA22294@en.ecn.purdue.edu>
Reply-To: jim@rwsys.lonestar.org (James Wyatt KA5VJL)
Organization: R/W Systems, Richland Hills, Texas, 76118-5849 (817) 595-0571
Lines: 20

ditz@ECN.PURDUE.EDU (Michael R Ditz) writes:
>I am working on an X-10 based home control system for a senior
>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

Back when I was working on a computer-controlled light dimmer, I found a
simple optoisolator with a *large* dropping resistor worked best. The output
was conditioned by a pull-up resistor into a Schmitt trigger and it produced
a nice square wave (with an occasional glitch). We fed the square-wave into
a PLL, so a minor spike or two didn't hurt us. If you need a pulse, try 
feeding the output into a short delay (sets your pulse width) and feed the
input and output of the delay into an XOR gate. The output will be active
from the time the square-wave changes to the time the change makes it through
the delay. (You might use the rest of a 7414 schmitt tigger for the delay).

Hope this helps - jim
----
James Wyatt (KA5VJL) - Standard disclaimer applies...          (H)817-595-0571
{letni.lonestar.org,merch.tandy.com}!rwsys.lonestar.org!jim    (W)817-390-2864


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