AVR-Chat

On Tue, Dec 30, 2003 at 08:18:21PM -0000, William Nachefski wrote:

> I am just getting started into AVR's.  I am seeking help on a 
> circuit.  I need to build a tachometer with opitcal pick up.  The max 
> RPM to be counted will be around 200,000 RPM.  I was told that pretty 
> much all of the tasks could be handled in code with a 
> microcontroller.  My pickup is a Phototransistor/LED its set up to 
> give one pulse per revolution.  So basically all I need is a 
> frequency counter with a 1 sec gate time, and then multiply the value 
> by 60,(my guess)Then to display the data on a 6 digit LED display.  

By that do you mean you want 1ppm resolution?

That's a tough task. And you won't get it by multiplying a simple
count by 60. You'd have to count at least 60 pulses per rev.

> I don't know really anything about M.C's but bought a few 28 pin 
> Mega8 chips to experiment with.  What should the circuit look like?

Schmitt trigger to "square-up" the signal, feeding to an interrupt
pin. LED display is probably easiest, given the number of required
digits. You can MUX and drive the digits from the Mega8; unless the
display is huge.

Depending on the accuracy and range of speeds you really need, you
could measure the period between pulses and invert. Display the
result, reset counter and start counting on the next pulse; stop
when the next pulse arrives. etc. Because one pulse per rev results
in timing of much less than a millisecond, you'll probably want to
average over a set of results spanning at least a tenth of a second.
(Moving average.)

If the machine stops spinning faster than that, you have a bigger
problem than speed measurement anyway.

> I am thinking the only thing I need on the board would be the display 
> + multiplexing components and the Mega8 with a connector runing from 
> a input pin(interupt pin?)Let me know if I am on the right track.  Or 
> if someone else has built one of these.  Any help would be greatly 
> appreciated!

-- 
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