BY VAUGHN D. MARTIN and JOHN STABLER
■ FIGURE 2. The magnetic field must be
perpendicular to the Hall effect’s active
In order to create the Hall effect
potential, the magnetic field must be
perpendicular to the Hall element.
The ratio of the voltage created to the
product of the amount of current and
the magnetic field divided by the element thickness is the Hall coefficient.
This characterizes the material comprising the element.
Where Theory Meets Practicality
The Hall effect occurring perpendicular to a magnetic field (see Figure
2) is why you tilt this project’s Hall
effect sensors upward. These 1/4”
diameter magnets in the pawns’ base
produce lines of flux which are most
concentrated directly downward.
Magnetic lines of flux do indeed
emanate in an almost spherical fashion
(see Figure 3). However, as you deviate
farther from the center-line, the flux
intensity diminishes dramatically. This
strategic magnet placement directly
exposes the sensors’ façades to the
strongest magnetic field. This further
ensures that your project’s missing
pawn detection scheme will remain as
reliable as our design intended it to be.
The sensors’ façade has writing on
it to distinguish it from the non-active
side. Notice where it says “branded”
(see Figure 4). The A3214A sensor is
too small for all of that, so you will just
see a 14A nomenclature on it. Use the
white Nylon spacers to ensure these
sensors assume this upward orientation. They will just slightly bend over
the lip of the spacers for proper alignment under their respective magnets
(embedded within the pawns).
circuit board) from the acrylic
sheet. In the six-pawn version,
there will be six shorter spacers to
offset the six sensors to a distance
that is virtually next to the magnets
for reliable activations.
Helping Theory Comply with
The white Nylon mounting
hardware secures and “cradles” the
sensors. It does so without use of
traditional ferrous metallic hardware. This ensures no magnetic field
interaction from metallic hardware that
could diminish the effectiveness of
the magic trick. (Imagine what would
happen if one field coupled from one
sensor to an adjacent sensor!)
■ FIGURE 3. Magnetic lines of force
and their typical convex spherical
pattern. Artwork courtesy of the
Regents of the University of California.
Magnetism: A Historical
this region either attracted or repelled
one another. Touching one of these
stones with an iron needle made it orient itself like the stone; thus, the origin
around 600 BC in the district of Magnesia,
Thessaly (one of the 13
peripheral states of
Greece). Certain stones in
If you decide to purchase a kit, it
will come with two sizes of spacers.
They appear identical until you place
them side-by-side. One is slightly taller
than the other. The taller spacers are
for standing off the PCB (printed
■ FIGURE 4. The Hall
effect sensors’ “branded”
façade where an abbreviated part number would
appear. Artwork courtesy
of Allegro Microsystems.
May 2007 47