[sdiy] diy frequency counter

[David G. Dixon]

OK, Dave, I'm sold!  Now how do you build a high-speed photo-tachometer?


> >> Howdy,
> >>  Get a high speed photo, not laser tachometer. Set up a pulse wave
> output
> >> to an LED. Point photo tachometer at it and note reading. A little
> >> arithmetic will get you from RPM to CPS.
> >> Rig
> >>
> >
> > Wouldn't the LED basically just stay on beyond a certain frequency?
> >
> >
> Way back in the 70's I modulated a incandescent bulb with audio and
> detected it with a solar cell.  It sounded as good as the crappy
> transistor AM radio that was feeding it.  If that works, I'm sure a LED
> is much better.  Off to "the google"... I'm guessing the capacitance of
> the LED sets the modulation rate, and capacitance can't that high.  Look
> at the rise/fall times here (measured in nS):
> 
> http://www.vishay.com/docs/80097/physics.pdf
> 
> Not sure exactly how those number relate to optical output though.
> Another datapoint - IR LEDs are used in remote controls, and those are
> modulated up to 50kHz.