Hall Effect Sensor

The Hall effect sensor is a semiconductor device that generates an output voltage when exposed to a magnetic field. The basic construction consists of a slab of semiconductor material through which a current is passed, as shown in Fig. 1(a). If a magnetic field is applied as shown in the figure perpendicular to the direction of the current, a voltage $V_H$ will be generated between the two terminals, as indicated in Fig. 1(a).
Fig. 1: Hall effect sensor: (a) orientation of controlling parameters; (b) effect on electron flow.
The difference in potential is due to the separation of charge established by the Lorentz force first studied by Professor Hendrick Lorentz in the early eighteenth century. He found that electrons in a magnetic field are subjected to a force proportional to the velocity of the electrons through the field and the strength of the magnetic field. The direction of the force is determined by the left-hand rule. Simply place the index finger of the left hand in the direction of the magnetic field, with the second finger at right angles to the index finger in the direction of conventional current through the semiconductor material, as shown in Fig. 1(b). The thumb, if placed at right angles to the index finger, will indicate the direction of the force on the electrons. In Fig. 1(b), the force causes the electrons to accumulate in the bottom region of the semiconductor (connected to the negative terminal of the voltage VH), leaving a net positive charge in the upper region of the material (connected to the positive terminal of $V_H$). The stronger the current or strength of the magnetic field, the greater the induced voltage $V_H$.
In essence, therefore, the Hall effect sensor can reveal the strength of a magnetic field or the level of current through a device if the other determining factor is held fixed. Two applications of the sensor are therefore apparent-to measure the strength of a magnetic field in the vicinity of a sensor (for an applied fixed current) and to measure the level of current through a sensor (with knowledge of the strength of the magnetic field linking the sensor).
The Hall effect sensor has a broad range of applications that are often quite interesting and innovative. The most widespread is as a trigger for an alarm system in large department stores, where theft is often a difficult problem. A magnetic strip attached to the merchandise sounds an alarm when a customer passes through the exit gates without paying for the product. The sensor, control current, and monitoring system are housed in the exit fence and react to the presence of the magnetic field as the product leaves the store. When the product is paid for, the cashier removes the strip or demagnetizes the strip by applying a magnetizing force that reduces the residual magnetism in the strip to essentially zero.
Obtaining a speed indication for a bicycle using a Hall effect sensor
Fig. 2: Obtaining a speed indication for a bicycle using a Hall effect sensor: (a) mounting the components; (b) Hall effect response.
The Hall effect sensor is also used to indicate the speed of a bicycle on a digital display conveniently mounted on the handlebars. As shown in Fig. 2(a), the sensor is mounted on the frame of the bike, and a small permanent magnet is mounted on a spoke of the front wheel. The magnet must be carefully mounted to be sure that it passes over the proper region of the sensor. When the magnet passes over the sensor, the flux pattern in Fig. 2(b) results, and a voltage with a sharp peak is developed by the sensor. Assuming a bicycle with a $26 in.$ diameter wheel, the circumference will be about $82 in$. Over 1 mi, the number of rotations is $$5280 ft ({12 in. \over 1 ft} ({1 rotation \over 82 in.}) = 773 rotations$$ If the bicycle is traveling at 20 mph, an output pulse will occur at a rate of 4.29 per second. $$ \begin{split} speed &= 20 mph = 20 miles/hour\\ &= {20 \times 773 rotations \over 60\times 60 \,sec} = 4.29 rotations/sec\\ \end{split} $$