Re: [CADCAD_CAM_EDM_DRO] RPM Meter

John,
Well I'm doing a timed gate at 0.999 seconds and counting the number of
pulses in the gate period. As you noticed there are 4 pulses per rev. so
to obtain a 1 second update rate I simply multiply the current gate
count by 15, hence the 15 RPM lowest resolution. I was not looking to
anything below 500 RPM anyway.... I just wanted to know that I'm at
about 2500 RPM and not 3000 .. I added the extra two digits just to
make the readout easier.
Max RPM is 9999 ...It does display 15 RPM but the next value would be 30
RPM and 45 etc
Your suggested approach of measuring time between pulse for a better low
end resolution is a pretty standard way of do things and you should have
no problem with it.

BTW that little circuit I made for the sensor is very immune to 60 Hz
fluorescent ambient light, and responds as extremely low RPM, it should
be ideal fro your sub 1 RPM version. Heck I could just change the code
on what I have, but I've no need for it at the moment.

Is there that much of a need for a precise low RPM measuring device ?

The source is in "C" and requires the PCM compiler, I did not post it as
most people would not have the compiler. However the HEX file is on the
web site.
There's not magic in the "C" code, it simple sets the 16 bit counter and
then timer 0 for 66.6mS X 15 as the gate counter and extracts the
1000's,100's 10's and 1's for the display. Each of these four values is
a pointer into an array of seven segment display values... all pretty
much run of the mill code. Heck I made the entire thing yesterday
afternoon including the circuit boards and fitting to the mill. Notice I
did not make a case for the thing, it will serve me well as it is.

regards
Hans Wedemeyer

John Beidl wrote:

> Hi Hans,
>
> I would be interested in the source for the 4 digit RPM meter. What
> are the upper and lower limits of RPM? It appears that you
> generate 4 pulses per revolution with the sensor you are using.
>
> Are you measuring frequency or time? I have been thinking about build
> a low RPM tach (less than 5000 RPM) using time
> measurement between two pulses to get very fast readings of low RPM
> (down to 10 RPM or so).
>
> John
>
> hansw wrote:
>
> > From: hansw <hansw@...>
> >
> > If any is interested I posted a simple design for a 4 digit RPM
> meter at
> > my web site. It uses an optical sensor and a PIC Microcontroller.
> >
> > The entire design can be built for about $20.- . If anyone needs to
> PCB
> > layout files and HEX code let me know. However a PCB is not needed
> as
> > it's such a simple design it could be wired on a perf-board.
> >
> > http://home.swbell.net/hans_w
> >
> > regards
> > Hans Wedemeyer
> >
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