HALL-EFFECT SWITCHES

The addition of a Schmitt-trigger threshold detector with built-in
hysteresis, as shown in figure 6, gives the Hall-effect circuit digital
output capabilities. When the applied magnetic flux density exceeds a
certain limit, the trigger provides a clean transition from OFF to ON
without contact bounce. Built-in hysteresis eliminates oscillation
(spurious switching of the output) by introducing a magnetic dead zone
in which switch action is disabled after the threshold value is passed.

An open-collector NPN output transistor added to the circuit (figure
7) gives the switch digital logic compatibility. The transistor is a
saturated switch that shorts the output terminal to ground wherever the
applied flux density is higher than the ON trip point of the device. The
switch is compatible with all digital families. The output transistor can
sink enough current to directly drive many loads, including relays,
triacs, SCRs, LEDs, and lamps.

The circuit elements in figure 7, fabricated on a monolithic silicon
chip and encapsulated in a small epoxy or ceramic package, are
common to all Hall-effect digital switches. Differences between device
types are generally found in specifications such as magnetic parameters,
operating temperature ranges, and temperature coefficients.

OPERATION
All Hall-effect devices are activated by a magnetic field. A mount
for the the devices, and electrical connections, must be provided;
Parameters such as load current, environmental conditions, and supply
voltage must fall within the specific limits shown in the appropriate
documentation.

Magnetic fields have two important characteristics—flux density
and polarity (or orientation). In the absence of any magnetic field,
most Hall-effect digital switches are designed to be OFF (open circuit
at output). They will turn ON only if subjected to a magnetic field that
has both sufficient density and the correct orientation.
Hall switches have an active area that is closer to one face of the
package (the face with the lettering, the branded face). To operate the
switch, the magnetic flux lines must be perpendicular to this face of the
package, and must have the correct polarity. If an approaching south
pole would cause switching action, a north pole would have no effect.
In practice, a close approach to the branded face of a Hall switch by
the south pole of a small permanent magnet will cause the output
transistor to turn ON (figure 8).

A Transfer Characteristics Graph (figures 10 and 11) plots this
information. It is a graph of output as a function of magnetic flux
density (measured in gauss; 1 G = 0.1 mT) presented to the Hall cell.
The magnetic flux density is shown on the horizontal axis. The digital
output of the Hall switch is shown along the vertical axis.

To acquire data for this graph, add a power supply and a pull-up
resistor that will limit current through the output transistor and enable
the value of the output voltage to approach zero (figure 9).
In the absence of an applied magnetic field (0 G), the switch is
OFF, and the output voltage equals the power supply (12 V).

A permanent magnet’s south pole is then moved perpendicularly
toward the active area of the device. As the magnet’s south pole
approaches the branded face of the switch, the Hall cell is exposed to
increasing magnetic flux density. At some point (240 G in this case),
the output transistor turns ON and the output voltage approaches zero
(figure 10). That value of flux density is called the operate point. If we
continue to increase the field’s strength, say to 600 G, nothing more
happens. The switch turns ON once and stays ON.

To turn the switch OFF, the magnetic flux density must fall to a
value far lower than the 240 G “operate point” because of the built-in
hysteresis. For this example we use 90 G hysteresis, which means the
device turns OFF when flux density decreases to 150 G (figure 11).
That value of flux density is called the “release point”.

Allegro Microsystems inc.
Source ( pdf )
http://www.allegromicro.com/en/Products/Design/an/an27701.pdf