[sdiy] diy frequency counter

[David M. Ingebretsen]

As a teenager in the 70's, my brother and I bought a He-Ne laser to  
play around with holograms. An electronics whiz friend of ours added a  
audio modulation input to it. We took the laser a couple of miles  
away, pointed it back at the house, set up a telescope, fed it into a  
solar cell and pulled the audio back out. Very cool for a bunch of  
teenagers...

David.
-- 

David M. Ingebretsen, M.S., M.E.
Collision Forensics & Engineering, Inc.
2469 East Fort Union Boulevard, Suite #114
Salt Lake City, UT 84121

(801) 733-5458
dingebre at 3dphysics.net


Quoting Dave Manley <dlmanley at sonic.net>:

> 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
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