With a tag

> Why wait half and half. You could paint the flywheel all black with a
> white stripe

I do this to measure light aircraft propellor speed, up to about 3000rpm.
One version uses a Hall Effect IC and 6 magnets, the other a reflective
optical sensor with 6 strips of aluminium adhesive tape on a black
background. The PIC is a 10MHz 16F628A and it has no problem
counting up to 300 pulses per second on INT0 and doing the maths.

Additonally, one of the PIC's comparators is used to determine if power
to a 12VDC motor has been stopped by an end switch. A 0R47 resistor
goes from the cold side of the motor to 0V. Although this is used as a
go / no go detector, the motor current could be determined by measuring
the voltage across the sense resistor. eg as is done with stepper=20
controllers
in chopped mode

At 10,000 rpm and one event per rev that would be only 167Hz. Piece
o' cake. You wouldn't even need to do any maths, just compare times
between events to a table. How you do this and how big the table will
be depends on what resolution you want. At the top end it's going to get
a little cramped. At 10rpm resolution for example,  9990rpm is 166.500Hz,
10000rpm is 166.666Hz.

If you did use tables, perhaps have several, for particular ranges

Joe=20



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