AVR-Chat

I Need a pretty broad range, from say 15,000 rpm to 200,000.  
Accuracy doesn't have to be that percise as long as its within a 
couple of thousand actuall rpms.  What I meant was With each complete 
rotation of the turbine rotar shaft I get one pulse from the 
Phototransistor.  So that equates to one pulse to the M.C. per 
revolution.  Thanks for the help guys keep the suggestions comming!
William

--- In AVR-Chat@yahoogroups.com, Bernd Felsche <bernie@i...> wrote:
> 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!
> 
> -- 
> /"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
> \ /  ASCII ribbon campaign | I'm a .signature virus!
>  X   against HTML mail     | Copy me into your ~/.signature
> / \  and postings          | to help me spread!