Think it takes 0.6 volts to get a diode to conduct.  Therefore two
diodes/rectifiers in series will drop 1.2 volts.  By putting the two
diodes/rectifiers , biased the same as the opte's diode, in parallel with
the opto, you limit the forward voltage to the opto to 1.2 volts.  And you
may not need a current limiting resistor.

Then a single diode/rectifier biases in the opposite direction across the
circuit will limit the reverse voltage across the opto's diode to 0.6 volts.

Or....  you can replace the single diode/rectifier (to limit reverse
voltage) by putting a second opto/(diode/rectifier pair) across the first
but bias in reverse direction.  This will protect both opto diodes from
excessive forward and reverse voltage.  And give you another pulse.

Bill

----- Original Message ----- 
From: "Chris Loiacono" <chris01@tampabay.rr.com>
To: "'Microcontroller discussion list - Public.'" <piclist@mit.edu>
Sent: Thursday, October 21, 2004 1:15 PM
Subject: RE: [PIC:] AC zero-crossing circuit idea (LM339 hints?)


> Forget the comparator idea, it's fraught with problems. Where'd that come
> from anyway, someone on the Basic Stamp list?
>
> An opto is the way to go. You may also think about using more than one
opto
> in series to increase the life of the PIC.
>
> CL
>
> Subject: [EE:] [PIC:] AC zero-crossing circuit idea (LM339 hints?)
>
>
> I am designing a PIC circuit which needs to stay synchronized
> with the AC line's zero-crossing point.  I want to arrange things
> so that the PIC gets an interrupt every time the AC voltage
> crosses 0V.
>
> My original idea used an H11AA1 optoisolator as shown in this
> schematic:  http://home.quixnet.net/xyzzy/ee/zcopto.gif
>
> The optoisolator connects the INT line on the PIC to ground when
> there's an AC voltage, but when it goes to near 0V, it turns off
> and lets the INT line pull up to +5V, interrupting the PIC.
>
> Someone on another list suggested I use an LM339 comparaotr
> instead of the optoisolator, but I'm not that savvy about this
> component.  His suggestion was like this:
> http://home.quixnet.net/xyzzy/ee/zccomp.gif
>
> Does this look right?  Does it look like a better design than the
> optoisolator?
>
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