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From: ftpam1@acad3.fai.alaska.edu (MUNTS PHILLIP A)
Newsgroups: sci.electronics
Subject: Re: Light Dimmer
Message-ID: <1990Nov27.201653.4865@hayes.ims.alaska.edu>
Date: 27 Nov 90 20:16:53 GMT
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In article <3450@exodus.Eng.Sun.COM>, bender@oobleck.Eng.Sun.COM (Michael Bender) writes...
> 
>A few years ago I built an 8-channel digitally-controlled dimmer.  I used an
>8749 for the CPU, and Intel 8254 (right number??) counter/timer chips to
>actually control the firing of the Triacs.  A zero-cross detector would feed
>the CT chips' reset (clear?) input, a higher-frequency clock would actually
>clock the CT chips, and the 8749 would load values into the CT registers,
>which determined the duty cycle of the Triac-controlled load.  It worked
>quite well.  The CT chips would be used in auto-reload mode, so that every
>time they got a reset (clear??) from the zero-cross detector, they would
>start counting again; after a programmed number of counts, they would
>trigger the Triac and it would turn on for the remainder of the cycle.
>

     Better yet, use an 8751 and do it all in software.  An 11 MHz 8751 can
support 16 channels at 128 brightness levels.  It also has a serial port for
easy interfacing to a host computer.

     What did you use for a zero-cross detector?  My latest is an AC input
optocoupler, but the current transfer ratio is critical for proper operation.
(8751 port pins require a fair bit of current to pull down.)

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