Hall-Effect Sensors

Another type of positioning sensor is the Hall-effect sensor. This type of sensor works on a simple electromagnetic principle. In 1879 Edwin Hall observed that when a thin sheet of conducting material such as gold foil has current flowing through it, a separate small voltage could be induced in the foil by passing a magnetic field at right angles to the foil. This voltage is called the Hall-effect voltage. From the diagram in ill. 1 notice that a current will flow through the thin foil from the power supply current at the terminals attached to the left and right sides. These terminals are identified as I and ground. When a perpendicular magnetic field is brought close to the foil, a very small Hall-effect voltage will be available at the top and bottom terminals.

Above: ill. 1: Diagram of the Hall effect. A current is conducted through a thin piece of foil from terminal I and ground. When a magnetic field is brought perpendicular to the foil, a small voltage called the Hall-effect voltage is produced at the terminals attached to the opposite sides of the foil.

When Edwin Hall did his experiment at Johns Hopkins University, the amount of voltage produced by the Hall effect was in the range of 20-30 µV. This very small voltage was not useful until amplifiers such as the op amp were designed to increase the signal to a useful range. ill. 2 shows an electrical block diagram of a current-sourcing circuit for a Hall-effect sensor, and ill. 3 shows an electrical block diagram of a current-sinking, Hall-effect sensor. The basic concepts of both diagrams are very similar. From the diagrams notice that both sensors are three-wire sensors. This means that two wires, the + V, and the ground provide dc voltage for the power supply portion of the sensor. Terminal 0 and ground are used as the output terminals for the sensor. Since this is a three-wire sensor, the ground terminal is part of the power supply and part of the output circuit. The power supply uses a voltage regulator to provide the initial current for the Hall-effect element and voltage for the op amp. The small sensor terminals are connected to the op amp input terminals. When a magnetic field is sensed, a small voltage is sent to the op amp and the output of the op amp is sent to a Schmitt trigger and then to the base of an NPN transistor. When the base of the transistor is biased, it will go into saturation and current will flow through its emitter-collector circuit to provide a digital (on/off) output signal. In the current-sinking circuit, notice that the transistor provides a path to ground when the transistor is biased to saturation.

Above: ill. 2: Electronic block diagram for a Hall-effect sensor used in a current-sourcing circuit. An op amp, Schmitt trigger, and NPN transistor are used to provide an output signal when a magnetic field is brought close to the Hall-effect sensor.

Above: ill. 3: Electronic diagram of current-sinking output for a Hall-effect sensor.

ill. 4 shows examples of several Hall-effect sensors. In some applications where the Hall-effect sensor is used to determine if a door is open or closed, the magnet is built into the Hall effect. When the door is closed, a piece of ferrous metal is placed between the magnet and the Hall-effect sensor to interrupt the magnetic field. When the door is open, the piece of metal is removed, and the magnet will cause the sensor to conduct the Hall-effect voltage.

Another application for this type of sensor is in membrane-type keyboards. The membrane-type keyboard is a sealed keyboard used in industrial applications the keyboard for computers and color graphic operator screens must be sealed so they can be washed down. Each key on the keyboard has a small magnet and Hall-effect sensor. When a key is depressed, the magnet is brought close enough to the sensor to activate the Hall effect.

The Hall-effect sensor is also useful in analog signal applications. This type of circuit uses a semiconductor material as the Hall element The amount of Hall voltage will be proportional to the strength of the magnetic field. This means that the sensor can produce an analog signal to indicate how close the magnetic field is to the Hall-effect sensor. The distance can easily be converted into a signal that indicates position or distance traveled.