Hall effect sensors belong to a family of proximity detectors, used to detect the passing of an object at close range.
What makes the Hall effect sensor unique? The object being sensed needs to be magnetic — either an electromagnet or a permanent magnet.
In this lesson we will be looking at how these sensors work, their advantages and disadvantages, and some common applications.
The Hall effect (from which the sensors get their name) is observed by running a DC current through a conductor — or even better, a semiconductor.
This current fills the conductor with free electrons moving through it.
In the case of a semiconductor, there are holes as well. Electron holes are the places in a material where an electron could comfortably sit, but there's no electron there at the moment. They're a little pocket of positive charge waiting to be filled by a free electron.
When a magnetic field is introduced to the conductor, the free electrons are pushed toward one side (and in a semiconductor, the holes are pushed toward the opposite side).
The separation causes one side of the conductor to have a slightly negative charge and the opposite side to have a slight positive charge.
By attaching a voltmeter to the sides of the conductor, we can see that a voltage drop has been induced.
The voltage we measure will be always less than that of the power supply, and the amount will depend on the material that the sensor is made out of.
We can use the magnitude of this voltage to tell how far away the magnetic field is from the sensor. The closer the magnet gets, the greater the voltage.
When a magnetic field interacts with an object (like our conductor), it produces something called magnetic flux. That's the direction and force with which the magnetic field will push a charge in the object. This is related to the polarity of the magnet — the north pole and south pole.
That means the orientation of the magnet will affect the magnitude and polarity of the induced voltage. The more perpendicular the magnetic field is to the face of the sensor, the greater the magnetic flux pushing electrons aside, and thus the greater the magnitude of the voltage.
Notice how the polarity of the induced voltage reverses when the polarity of the magnetic field is flipped?
That means a spinning magnet will create a sine wave.
This is a property we can take advantage of later!
The most common Hall effect sensors come in the form of a three pin integrated chip.
One pin will be for the voltage supply, one is ground, and the third is the output signal.
Inside the chip are things like an amplifier, some type of signal conditioner, and the actual Hall effect device.
It is important to keep in mind that with these chips, only one face is able to do the actual sensing. It will be indicated by the manufacturer.
There are fancier Hall effect sensors available that can track a magnetic field in all three dimensions.
These use multiple Hall effect devices on a single chip to triangulate the position of the magnet.
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