> hello, sorry this is off topic, but i plan to use this with a pic
>
> what is the usual value for the resistor in the following circuit,
> or how can it be calculated?

I've used values around 1 meg with good results.  I was looking for
an infrared LED in standard office lighting, and shining the LED
through an object to analyze its composition.  It would punch
through a piece of bread or a bagel pretty well.  I was making a
gizmo that could tell a piece of bread from a bagel.

I don't calculate it.  Mostly my calculations get me in the
ballpark, and the lab is where I actually pick component values that
are critical.   Just build it and test it.  As Steve Ciarcia Sez:

"Solder is my favorite programming language."


-- Lawrence Lile

 "Nyquist was an optimist."

=> Median Filter Source Code
=> AutoCad blocks for electrical drafting

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