Key Difference: Sensors are generally classified on the basis of entities which they detect and measure. However, commonly they are divided into active and passive types. Some of the common types of sensors are – mechanical, electrical, radiation, magnetic and chemical sensors.

Sensor refers to a device which identifies a change occurring in the physical stimulus. In addition to the identification, it also converts that change into a form which is usally a signal that can be read by human through some displays at the sensor location or can be sent over a network electronically. Therefore, sensors are capable of detecting and measuring physical properties of the environment like temperature, weight, size, luminance, etc.

Broadly sensors are divided into two groups – Active and Passive.

Active Sensors – These sensors cannot work without external power supply. It is this exteranal source of power which gives majority of the output power of the signal.  Example – LiDAR (Light detection and ranging).

Passive Sensors – These are the ones which do not require external power supply. The output power is mostly described from the measured signal without an excitation voltage.  These are less intrusive but have a dependency on environment.  Example – radiometers.

Sensors can also be divided on the basis of physical variable they measure. These categories are –

Mechanical sensors – These sensors detect changes in mechanical attributes of an entity.  Strain guage is one of the important types of mechanical sensors which is considered as a basic sensing elements for sensors like pressure, sensors, position sensors, torque sensors, etc. In short, strain gauge can be considered as an important tool for sensing force.

Electrical  sensors– This refers to an electronic device which detects the change in electrical or magnetic signals based on an environment input. Metal detectors are examples of electrical sensors. Electrical current sensors detect the flow of current through an electrical wire by measuring the magnetic field which gets created due to flow of current.

Thermal sensors– These sensors detect temperature or heat. There are many different types of temperature sensors. There are basically of two types – Contact temperature sensor types which need to be in physical contact with the object being sensed. It makes use of conduction to track changes occurring in temperature. On the other hand, non-contact temperature types make use of convection and radiation in order to detect changes in temperature.

Radiation sensors– These sensors are used for detecting and measuring radiation. For example, Gamma probe system is used for the measurement of gamma rays in the energy lying between 60 to 600 keV.  Nuclear radiation sensors detect and measure nuclear radiations. There are many different types of radiation sensors. Three basic types of radiation sensors: Ionization sensors, Scintillation sensors and Semiconductor radiation sensors.

Magnetic sensors– These sensors detect and measure magnetic field. However, they can also be used  for measuring vector components of the magnetic field. These sensors are found in many important entities like magnetic storage disks and tape drives. Usually, magnetic fields are sensed through the association between magnetic and electric phenomenon, and therefore they are also covered under electrical sensors sometimes.

Chemical sensors– These sensors wok to detect and measure specific chemicals present in an environment. An ideal chemical sensor is the one which responds perfectly to a specific target chemical substance present in any type of medium. These sensors are usually optimized for usage.  The phenomenon of chemical sensing also relies on various facts like design, constraints, medium in which the analyte is present, etc.

Optical Sensors– This sensor is used to measure the physical quantity of light. It converts the measurement into a human or instrument readable format.  These sensors are built on the principles of light to quantify object attributes. Electric eye is such an optical sensor through which an object is detected by using a beam of light.

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