[sdiy] diy frequency counter

[Dave Manley]

David G. Dixon wrote:
> 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
LOL!  Back to the original question- seems like a handful of BCD 
counters, a set of latches, feeding BCD to 7-segment decoders, to a 
display seems pretty simple.  You then need the trigger/gate circuit 
used to reset the counters and latch their outputs.  The gate circuit is 
a crystal osc followed by another bcd counter whose max value sets the 
gate time.  Of course to measure low frequencies you need a long gate 
time, but you could couple the gate time to the position of the decimal 
point and only have gate times that are powers of 10.

-Dave