Hall Effect Sensor

A spacecraft working out which way it's pointing, or a car's engine controller counting how fast the crankshaft is spinning, often relies on the same small device: a sensor that turns a magnetic field into a voltage, with no physical contact needed at all.

What it does, not how it works

The underlying physics of the Hall effect — how charge carriers moving through a current-carrying conductor get deflected sideways in the presence of a magnetic field, building up a small transverse voltage — is not needed here. This lesson treats the Hall effect sensor purely as a practical device: what it does, and how that behaviour is put to use in two specific applications.

Functionally, a Hall effect sensor produces an output voltage that is proportional to the strength of the magnetic field it is exposed to, with the sign of that voltage depending on the field's direction (polarity). That's the entire behaviour that matters for using the device: point it at a magnetic field, and it hands back an electrical voltage that tracks how strong that field is, and roughly which way it points, without ever touching whatever is producing the field.

Application 1: magnetic field sensing for attitude monitoring

Attitude, in this context, means the orientation of an object — which way it's pointing — rather than its location. A Hall effect sensor can be used to monitor attitude by measuring how the local magnetic field it senses changes as the object rotates or tilts relative to some reference field.

That reference field might be the Earth's own magnetic field (as in a simple electronic compass), or a fixed magnet mounted elsewhere within a larger system. As the object carrying the sensor rotates, the component of the reference field along the sensor's sensitive axis changes, and so does the sensor's output voltage — giving a continuously updating electrical signal that reflects the object's orientation. Because the sensing is entirely non-contact and the sensor itself has no moving parts, this kind of arrangement is valued in navigation and orientation-sensing systems where a robust, low-maintenance sensor is important.

Application 2: use in a tachometer

A tachometer is a device that measures rotational speed. A common way of building one uses a Hall effect sensor together with a rotating source of magnetic field: a small magnet (or several magnets spaced evenly) fixed to a rotating shaft, or a toothed wheel made of ferromagnetic material, with a fixed Hall sensor mounted close to the path the magnet or teeth sweep past.

Each time a magnet (or tooth) passes the sensor, the local magnetic field it detects rises then falls again, producing a pulse in the sensor's output voltage. As the shaft turns continuously, the sensor produces a steady stream of these pulses — one for each magnet or tooth that passes. Counting the number of pulses over a known time interval, or measuring the time between successive pulses, gives a direct way of finding the shaft's rotational speed electronically, with no mechanical linkage to the rotating part itself.

A real tachometer built this way can respond quickly to changes in speed, has no mechanical wear (unlike a sensor requiring physical contact with the rotating shaft), and works reliably even in dirty or harsh environments where an optical sensor might be obscured.

Bringing this topic together

Across the five devices in this topic, a pattern emerges. The MOSFET and the Zener diode are devices that control or regulate voltage and current — one acting as a voltage-controlled switch, the other holding a steady reference voltage. The photodiode and the Hall effect sensor, by contrast, are devices that convert an external physical quantity — light intensity, magnetic field strength — into an electrical signal that the rest of a circuit can use. Together, these five devices form a basic toolkit of switching, regulation, and sensing components that real electronic systems are built from — the raw ingredients that the next topic, on processing and combining signals, builds directly on top of.