In a message dated 98-01-16 19:38:18 EST, you write:

<<
 A raw hall sensor won't but try using one of the auto spec devices with the
 magnet built in. The not produces a disturbance in the field which .....

 Alternatively, you could use a reluctor sensor from an old Volvo or the
 rear wheel speed sensor from a BMW K100 motorbike. This only requires a peg
 in the flywheel (use 2 to keep balance) to trigger. These units have
 inbuilt magnets, respond at low revs (eg, starting) and are super reliable
 (can't kill it with dirt or oil). Watch the bi-polar pulse at revs. It bites!

 Bill
  >>
Bill
Its DULEY  not DUDLEY
All the original post said was Hall effect sensor.  I have used the Interupter
type hall sensors.  They are very expensive!
Around $80 US.  I know because we killed a few because the metallic flag we
were using wasn't aligned right and we kept cutting the sensor in half.  Cool
little cubic rare earth magnets inside!  For a $1 Hall sensor I will stick to
rolling my own.BYOM  (Bring your own magnet!)

Dave DULEY