Now I understand better, thanks.  

You probably know all this stuff cold, but for me I found this link that figures out the trigger voltages based on the resistor and Vref values.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/schmitt.html#c2



> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Gerhard Fiedler
> Sent: Thursday, April 07, 2005 2:35 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Fundamental frequency from a whistle
> 
> Jinx wrote:
> 
> >> By Schmitt trigger do you mean one that was built using an op-amp
> >> and not an IC?  I got stuck because I was looking at Schmitt-Trigger
> >> IC datasheets (like a monkey with a math puzzle) and all I could see
> >> was A1 in and Y1 out and no way so adjust the trigger value.  Is this
> >> correct?
> >
> > You can make an ST, eg with a couple of transistors, that has a movable
> > trigger voltage. ST ICs (4093, 74xx14, PIC i/ps etc) are made with fixed
> > thresholds. A comparator or op-amp with high gain should be OK as an
> > ST in an analogue application
> 
> I meant a ST made with an op amp and positive feedback. National has a few
> app notes with 100s of op amp circuits, and you find them in there. These
> have the advantage over logic ST inputs that you can control both the
> threshold and the hysteresis. This is desirable in most analog
> applications. And by selecting the op amp (and the circuit and the other
> components) properly, you can also control the precision of threshold and
> hysteresis -- again something that isn't really possible with digital ST
> inputs.
> 
>  Vcc
>   |
>   |
>  R1     input signal---- inv in opamp
>   |                                 opamp out--+--- ST output
>   +--------------------- noninv in opamp       |
>   |                                            |
>   |                                            |
>   +-------------------------R2-----------------+
>   |
>  R3
>   |
>  Gnd
> 
> That's the basic opamp ST circuit. R1 and R3 determine the threshold, and
> R2 (together with R1 and R3) determine the hysteresis.
> 
> Gerhard
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