leo.depalo@POMETIA.IT wrote:
>
> <Cheers writes>
>
> >Why is the capacitor used?  The '84 is perfectly able to time off a
> sampling
> >period.
>
> I would insert the capacitor because my idea is:
>
> set ON a pin to drive the IR LED and load the capacitor, then put OFF the
> pins and read continuosly the light emitted by the IR LED powered by the
> capacitor. When the level of IR drops, in a time windows predefined, iI know
> thath the IR come from the LED and not from the environment.
>
> Obviously if the IR don to light the receiver, I assume that a obiect stay
> between the IR LED and the receiver.
>
> (I hope to have exposed clearly my think, sorry but the englis is not my
> motherlanguage)
>
> Ciao
>
> Leonardo

  So the capacitor's across the LED (in parallel)?  I wouldn't think
that's the best way to do this, you could do this with just a 555 (But
use a PIC!) - if you repeatedly turn the LED ON for a short while (then
read the IR results), turn the LED off for a short while (then read the
IR results), loop through that continuously - you'll see a larger
difference between the LED ON and the LED OFF readings, if no object;
and very little difference, if the object blocks the light.  You want a
decent path for the light between the transmitter & receiver.  (No
Italian here, just some high school Spanish, German, and Russian, years
ago.  Someone?  Probably someone who's done this on a PIC, which I
haven't, too.  <G>)  (I think we diode-ORed this once, or something.)

  Mark, mwillis@nwlink.com, "You'd be Surprised what you can make out of
2 paper grocery bags full of 7410 TTL IC's!"  {High School horror story.
<G>}