[sdiy] diy frequency counter

[Dave Manley]

David G. Dixon wrote:
>> 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.

-Dave