MOTM

--- In motm@y..., mark@i... wrote:

> Then it says this circuit generally has "problems with noise and
> instabilities at high frequencies because of the op-amp's high gain 
> and internal phase shifts".  

Well, how high? 100kHz? 1 mHz? You will be running more or less 1 
kHz - 2 kHz tops, so I'd guess you don't even have to worry about it. 
Try it on a breadboard, and see if it works.
 
> Even if I can get the differentiator to work, I would end up with 
> both negative and positive voltages.  So after this 
> differentiatior, I would need an "absolute value circuit" 
> 
> Do they make comparators that will give a positive output
> regardless of the polarity of the input voltage?

You won't be able to get by with just one differentiator and a 
comparator. That's not enough to deal with 3 voltage states - high, 
zero, and low. You'll still need the absolute value circuit, OR you 
could build a second comparator to trigger on the zero to low 
transition, then sum the two comparator outputs. You may also need to 
add a one shot to insure that you have a clean output pulse of known 
duration.

Moe