Hall Effect Gear Tooth Sensor

[Jim Pieronek]

Jay Monkman writes:
 > Previously MTN-KAT babbled:
 > 
 > > I don't know that you'd be able to magnetize the ring-gear teeth in a uniform
 > > manner that would result in all of the teeth having the proper polarity.
 > 
 > Is that really necessary? I was looking through Allegro's application 
 > notes, and they showed a hall-effect sensor between a metal gear and
 > a magnet. 

The following bizarre ASCII-gram shows the way the sensors are usually
used.  A magnet (MM) is slapped right on the face of the sensor (HH)
and a ferrous metal clip ("|+-") is routed from the other face of the
magnet to make a gap in the vicinity of the other side of the sensor.
As the ferrous teeth (Fe) of the gear pass by the gap they "complete"
the magnetic circuit which increases the flux through the sensor.  A
similar arrangement is used for distributor sensors.

                   FeFe
                 FeFeFe
       +-----  FeFeFeFe
       |MMHH-    FeFeFe
                   FeFe
                     Fe
                   FeFe
                 FeFeFe
               FeFeFeFe
                 FeFeFe

I thought I had seen sensors that had the magnet built in and the
whole thing was encapsulated in a threaded tube about 3/8" in
diameter.  The gap was visible at one end of the tube, and wires came
from the other end.  You just screwed the thing into an appropriate
hole, being careful to orient the gap correctly, and tighten a control
nut behind it to hold it in place.

Anyone else seen such a thing?  Do I need to have my medication
changed again?

Jim