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All magnets have both a North and South pole. A Standard Hall Switch produces a digital output that changes states only with the presence of a South Pole field. An Either Pole Hall Switch (EHS, EPPHS, MRS or EHS1) will trigger with the presence of either a North or a South Pole. Here is a representation of the flux lines emitted from a 2 pole magnet:
If a Hall sensor is pointed at the South pole as shown, the vector of the flux line in the -X direction, into the face is what the sensor element sees. If the sensor was at location A, most of the flux will be South Pole positive. If the sensor was at location B, the vector is completely in the Y direction, so the sensor element sees no field whatsoever.
If the sensor were at location C, the vector is mostly in the + X direction. So, this would represent the sensor element seeing a North Pole field.
So, if a magnet moves by a sensor as shown here, the sensor will see a field in this pattern: 0, then North pole, 0, South pole for a longer period, 0, North pole, then back to 0.
This will produce 3 pulses instead of one.
To resolve this issue, either use a normal South pole Hall Switch (HS, HS1), or increase the gap between the sensor and magnet far enough that the fringe North poles are not strong enough to trigger the sensor.