7.3 Light or Photo-sensors
Photovoltaic or Solar cells
Solar cells produce direct current electricity from sun light which can be used to power equipment or to recharge a battery. The first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide.
Photoelectric effect
In the photoelectric effect, electrons are emitted from solids, liquids or gases when they absorb energy from light. Electrons emitted in this manner are called photoelectrons. The photoelectric effect requires photons with energies from a few electronvolts to over 1 MeV in high atomic number elements.
In 1905 Albert Einstein published a paper that explained experimental data from the photoelectric effect as being the result of light energy being carried in discrete quantized packets. This discovery led to the quantum revolution. Einstein was awarded the Nobel Prize in 1921 for his discovery of the law of the photoelectric effect.
Infrared automatic door sensors
The principle behind infrared automatic door sensors is the transmission and receiving of infrared light. An element known as a light emitting diode (LED) transmits active infrared light, which is reflected on the floor and received by an optical receiver known as a photo diode (PD). As long as there is no movement or object in the path of the light beam, the light pattern is static and the sensor remains in stand-by.
When a person or object crosses the beam, the reflection of the light is distorted. This is registered by the PD, which gives off an impulse for opening the door. Sensors differ in the number of rows of active infrared spots. These spots are collectively referred to as the detection area. Because objects cause a distortion in the reflected light pattern, active infrared sensors also react to shopping carts and other moving objects.
Active infrared sensors are excellent as a safeguard at the door opening because of their ability to continue recognizing changes that occur in the detection area. As long as there is a person or object in the detection area, the sensor remains active, preventing the door from closing.
Active infrared door sensors are generally immune to the effects of external factors such as rain, snow and falling leaves. Although the sensor registers this type of movement, intelligent software is employed to screen such factors out.

7.4 Magnetic sensors
Magnetic sensors detect changes and disturbances in a magnetic field like flux, strength and direction.There are several types of technologies used to make a magnetic sensor work. Fluxgate, Hall Effect, Magnetoresisitive, Magnetoinductive, nuclear precession, and SQUID (superconducting quantum interference devices) each have a different approach to using magnetic sensors. Magnetoresistive devices record electrical resistance of the magnetic field. Magnetoinductive are coils surrounding magnetic material whose ability to be permeated changes within the Earth's magnetic field. Each type of technology focuses on a particular area for detection, a measurement to be detected and way of recording changes.

Hall Effect Sensor

In the Hall effect sensor, the transport of electrons through an electrical device is affected by the presence of an external magnetic field resulting a measureable voltage across the conductor called Hall voltage. In general, a Hall effect sensor can measure down to about 5% of the earth’s magnetic field.
Hall effect: The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. It was discovered by Edwin Hall in 1879.
For a simple metal where there is only one type of charge carrier (electrons) the Hall voltage VH is given by where I is the current across the plate length, B is the magnetic field, t is the thickness of the plate, e is the elementary charge, and n is the charge carrier density of the carrier electrons. The Hall coefficient is defined as the ratio of the induced electric field to the product of the current density and the applied magnetic field. It is a characteristic of the material from which the conductor is made, since its value depends on the type, number, and properties of the charge carriers that constitute the current. where j is the current density of the carrier electrons, and is the induced electric field. In SI units, this becomes
Fig. Schematic Hall Effect setup

Hall Effect measurement setup for electrons. Initially, the electrons follow the curved arrow, due to the magnetic force. At some distance from the current-introducing contacts, electrons pile up on the left side and deplete from the right side, which creates an electric field ξy. In steady-state, ξy will be strong enough to exactly cancel out the magnetic force, so that the electrons follow the straight arrow (dashed).
Magnetoresistive Sensors
Instead of measuring the build-up of a Hall voltage, it is also possible to measure the increased resistance of the device due to the deflected electrons. In this case, the Hall based sensor is called a Magnetoresistor. Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin-film structures composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR.
The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment. The magnetization direction can be controlled, for example, by applying an external magnetic field. The effect is based on the dependence of electron scattering on the spin orientation.
The main application of GMR is magnetic field sensors, which are used to read data in hard disk drives, biosensors, microelectromechanical systems (MEMS) and other devices. GMR multilayer structures are also used in magnetoresistive random-access memory (MRAM) as cells that store one bit of information.

7.5 Fluid Flow Sensors
Flow sensors are used in many monitoring and control applications, to measure both air and liquid flows. There are many ways of defining flow (mass flow, volume flow, laminar flow, turbulent flow). Usually the amount of a substance flowing (mass flow) is the most important, and if the fluid’s density is constant, a volume flow measurement is a useful substitute that is generally easier to perform. There are numerous reliable technologies and sensor types used for this purpose. Some technologies have been applied to both air and liquid flow measurements, as their principles of operation hold true in either application. Other technologies lend themselves to being airflow or liquid flow specific.
 What accuracy, range, linearity, repeatability, and piping requirements must be
considered?
 Is the liquid to be measured clean, viscous, or a slurry?
 Is the liquid to be measured electrically conductive?
 What is the specific gravity or density of the liquid to be measured?
 What flow rates are involved in the application?
 What are the process's operating temperatures and pressures?

Differential Pressure Measurement Sensors
Differential pressure measurement sensor technologies can be used for both airflow and liquid flow measurements. A variety of application-specific sensors used for both air flow and pressure measurements are on the market, as well as differential pressure sensors used for liquid measurements. Differential pressure flow-meters are the most common type of unit in use, particularly for liquid flow measurement.
The operation of differential pressure flow-meters is based on the concept that the pressure drop across the meter is proportional to the square of the flow rate; the flow rate is found by measuring the pressure differential and taking the square root.
Differential pressure flow devices, like most flow-meters, have a primary and secondary element. The primary element causes a change in the kinetic energy, to create the differential pressure in the pipe. The unit must be correctly matched to the pipe size, flow conditions, and the properties of the liquid being measured. The secondary element measures the differential pressure and outputs the signal that is converted to the actual flow value.
Venturi tubes are the largest and most expensive differential pressure device. They work by gradually narrowing the diameter of the pipe, and measuring the pressure drop that results. An expanding section of the differential pressure device then returns the flow to close to its original pressure. As with the orifice plate, the differential pressure measurement is converted into a corresponding flow rate. Venturi tubes can typically be used only in those applications requiring a low pressure drop and a high accuracy reading. They are often used in large diameter pipes.

Electromagnetic Flow Sensors
Operation of these sensors is based upon Faraday’s Law of electromagnetic induction, which says that a voltage will be induced when a conductor moves through a magnetic field.The liquid is the conductor, and the magnetic field is created by energized coils outside the flow tube. The voltage produced is proportional to the flow rate. Electrodes mounted in the pipe wall sense the induced voltage, which is measured by the secondary element.
Electromagnetic flow meters are applied in measuring the flow rate of conducting liquids (including water) where a high quality, low maintenance system is needed. The cost of magnetic flow meters is high relative to other types of flowmeters. They do have many advantages, including: they can measure difficult and corrosive liquids and slurries, and they can measure reverse flow.

7.6 Metal detectors

Metal detectors are useful for finding metal inclusions hidden within objects, or metal objects buried underground. They often consist of a handheld unit with a sensor probe which can be swept over the ground or other objects. If the sensor comes near a piece of metal this is indicated by a changing tone in earphones, or a needle moving on an indicator. Usually the device gives some indication of distance; the closer the metal is, the higher the tone in the earphone or the higher the needle goes. Another common type are stationary "walk through" metal detectors used for security screening at access points in prisons, courthouses, and airports to detect concealed metal weapons on a person's body. The simplest form of a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. If a piece of electrically conductive metal is close to the coil, eddy currents will be induced in the metal, and this produces a magnetic field of its own. If another coil is used to measure the magnetic field (acting as a magnetometer), the change in the magnetic field due to the metallic object can be detected.