monitoring RPM

[Frederic Breitwieser]

>For example, a 16-bit counter running at 1MHz will give you 4614
>counts at 13000 RPM but will overrun (200000 counts) at a startup
>RPM of 300.  In this case, we need to slow the clock down by
>a third to eliminate overrun (only have 65536 counts possible),
>which reduces our high RPM reading to 1538 counts.  This still
>gives us very good resolution of:

This is becoming rather complex.  If you are making your own ECM or using a
parallel port on the PC to monitor RPM, I have a much simpler solution -
you connect either a hall effect transistor or a phototransistor, depending
what you prefer, to +5V and to a TTL counter, such as the 74LS193.  This
will give you a count 0-15.  Connect the output of that chip to another
74LS193, and your resolution doubled, 0-255.  If you have 1 magnet (or
hole) every 2 degress, not only can you determine RPM by counting the time
it takes to change numbers, but you also now have a position sensors.  On
TDC of the number 1 cyl, you have one additional hole that triggers a reset
to both counters, thus resetting it back to zero.  The parallel port (or
ECM) just has to read the 8-bit output of the cascaded LS193's and you'll
know exactly where your crank is, and how fast its going.

	RPM = abs(position two - position one)



Frederic Breitwieser
Bridgeport, CT 06606
http://www.xephic.dynip.com/

1993 Supercharged Lincoln Continental
1989 4-Door Softtop Humvee (Hummer)
2000 Buick GTP (Mid-engine track car)

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