Classification of sensors

By type, sensors can be classified as:

1. Optical,
2. Chemical
3. Electro-mechanical
4. Mass sensitive and
5. Biosensors 

Optical Sensors

Optical sensors can be divided in two main groups and these groups are:

Proximity sensors and

Ambient light sensors 

Proximity Sensors

The proximity sensors are designed for use in detecting the presence of an object or motion detection in various industrial, mobile, electronic appliances and retail automation. Proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance) and looks for changes in the field or return signal. The object that is being sensed by sensor is often referred to as the proximity sensor’s target. Just for information a capacitive or photoelectric sensor might be suitable for plastic target while an inductive proximity sensor always requires a metal target. Examples of proximity sensors usage include the detection of an out-of-paper condition in a printer or a mobile phone screen that dims to save battery life when placed near face. 

Ambient Sensor

Ambient light sensors provide precise light detection for a wide range of ambient brightness and are commonly used in LCD backlight control in mobile phones, LCS TV/panel, and notebook applications.

Chemical Sensors

A chemical sensor is a device that transforms chemical information, such as concentration of a specific sample component, into an analytically useful signal. Chemical sensors contain two basic functional units: a receptor and a transducer. As explained earlier, the chemical information is transformed in receptor into a form of energy which may be measured by the transducer. Based on that definition, electrochemical sensors are a type of chemical sensors. 

Electrochemical Sensors

Electrochemical sensors are the largest group of chemical sensors. They can be used to detect most common toxic gases in a wide variety of safety applications. They operate by reacting with the gas of interest and they produce an electrical signal proportional to the gas concentration. Typical electrochemical sensor consists of a sensing electrode and the counter electrode which are separated by a thin layer of electrolyte. The electrolyte composition and the sensing electrode material are selected based on the chemical reactivity of the target.

Advantages of electrochemical sensors are the following:

• They are compact
• Require very little power
• Exhibit excellent linearity and repeatability
• Generally have long life span, about three years. 

Mass Sensitive Sensors

Mass sensitive sensors can detect molecules by sensing the mass. There are several kinds of mass sensitive sensors.

1. Cantilever sensors utilize beam displacement on a cantilever surface and length of the beam. The surface of the cantilever can be modified, allowing a differential stress to propagate as an analyte of interest is bound to its target on the modified surface.
2. Quartz crystal microbalances are ultra-sensitive mass sensors that rely on a quartz crystal that oscillates at its resonance frequency due to the piezoelectric effect. When adding or removing small masses or molecules from the surface the resonance of quartz crystal is experiencing disturbance which leads to the changes in the measured frequency. This frequency is transformed through a physical relationship to changes in the mass.
3. A surface acoustic wave (Saw) is an acoustic wave traveling along the surface of a material exhibiting elasticity, whit an amplitude that typically decays exponentially with penetration into the substrate. As the acoustic wave propagates through the surface of the material, any changes in the characteristics of the propagation path affect the velocity and/or the amplitude of the wave. Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity that is being measured. 

Biosensors

Biosensor is a device for the detection of an analyte that combines a biological component with a physicochemical detector component. Berkley Lab invented a technique in which DNA or RNA assays can be read and evaluated without necessitating elaborate chemical labeling or sophisticated instrumentation. This biosensor is based on electrostatic repulsion, in which objects with the same electrical charge repel one another. One of the most remarkable aspects of the novel electrostatic detection method is that it requires nothing more than naked eye to read out results that currently require chemical labeling and confocal laser scanners.