DIYEFI FAQ

[Stephen Dubovsky]

>(what is a Hall effect sensor?!).

  Ok, imagine a piece of square (or rectangular) silicon semiconductor
placed flat on a table.  If it is doped uniformly either n-type or p-type,
and a voltage is applied across the wafer on the left and right (connections
L & R), the voltage on the edges nearest and farthest away from you will be
0v (connections N & F) (if the connections are made w/ the same x-coordinate
so they dont detect the applied voltage gradient).  Now if a magnetic field
is applied normal to the table (through the chip) there is a neat phenomenon
called the hall effect that says there will be some measureable voltage at
terminals N&F that is somewhat proportional to the applied voltage at L&R,
the geometry of the wafer, the doping concentration, and the magnetic field
strength.  Since the first three are usually fixed, the output of a hall
sensor is proportional to an applied magnetic field.  This means you can
glue a magnets to a flywheel and detect when they go by.  It also allows you
to build things like non-contact DC current meters because you can measure
the magnetic field around a wire when a current passes through it (remember
the right hand rule from physics?).
  In real life they tend to be a little non-linear, but there are meany
creative ways to get around this.  If your looking to do something like
detect magnets, then this doesn't matter anyway.  Hope all this helps.  If
you need more info plz feel free to ask.  The best analogy I can think of
why this happens (w/o going into fields theory) is to remember that a
magnetic field is used to deflect the trajectory of electrons in a TV tube,
something very similar is 'bending' the e paths towards one side of the wafer.
SMD
--
Stephen Dubovsky
dubovsky at vt.edu

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