Active Sensors

Though a lot of electrical and electronic devices need an external power supply to operate, some gifted devices can work on their own without any external power supply. Some sensors also fall under this category. Such sensors are known as Active sensors and it will be explained in the following article.


  1. Introduction to Active Sensors
  2. Commonly Used Active Sensors
  3. Light Detection and Ranging
  4. Global Positioning System
  5. Infrared Sensors
  6. Seismic Sensors
  7. Radio Detection and Ranging
  8. Comparison of various sensors

Introduction to Active Sensors

Active sensors are devices that work by emitting radiation or light and waiting for the reflected light. Once there is reflected light, the reflected light is absorbed and is analyzed by the sensor itself. In general, the sun can be compared to an active sensor. Though it doesn’t sense anything, it is a source of light and doesn’t depend on any foreign matter for illumination. The below diagram is a pictorial representation of working of active sensors.

working of active sensors
  • As shown in the above figure, if there is a process to sense a parameter, the first step would be done by the active sensor itself.
  • Most of the time, an active sensor consists of two LED’s or LED like elements – one for transmitting signals and one for receiving signals.
  • First, the transmitter LED emits radiation, which gets transmitted to the surrounding like a sound wave.
  • When there is an obstacle in the path of the radiation, it touches the obstacle and gets reflected. The reflected rays then reach the sensor again and are received by the receiver LED.
  • From the received rays, property of the obstacle, the distance between the obstacle and the sensor can be calculated.
  • The above mentioned are the basic working of an active sensor and gets changed as the sensor’s complexity level increases.

Commonly Used Active Sensors

Because of the complexity in the construction and working of active sensors, it is not used frequently in day-to-day life. Most of the active sensors are used in advanced applications such as remote sensing, automotive, etc. which will be explained further in this article. But below are some of the examples of active sensors.

  • LiDAR (Light Detection and Ranging)
  • RADAR (Radio Detection and Ranging)
  • GPS (Global Positioning System)
  • X-Ray sensing
  • Infrared sensors
  • Seismic sensors
  • Electron microscopes


Light Detection and Ranging

The LiDAR or the Light Detecting and Ranging is one of the best examples of active sensors. The construction and working of a LiDAR system are explained below.

  • A LiDAR system consists of a source of light that emits pulsed light waves to the environment. The light waves hit the nearby object and bounces back to the detector of the LiDAR.
  • There will be an encoder in the system which analyses and calculate the distance between the LiDAR and the object.
  • As the distance varies from object to object, the data is stored and finally, an image like structure will be produced in which the distance will be differentiated by different colors.

Global Positioning System

The GPS or the Global positioning system is used to navigate a persons’ accurate location. Though it was initially used by the military and civil service for navigation, now it is also used for various purposes like mining and agriculture. The working of a GPS is explained below.

  • A GPS consists of 77 satellites which are operated by the space force of the United States of America. These satellites will be circling the earth and constantly send out signals.
  • When position of a certain person or a vehicle is required, some of those satellites, generally four satellites, will arrive at that area and receive signals supplied by other satellites.
  • By further analyzing the received signal, the exact location can be determined.

Infrared Sensors

The infrared sensors are the simplest form of active sensors. They are handy and can be used in many embedded systems. These sensors are used in applications such as motion detectors, burglar alarms, etc.

  • An infrared sensor has the same working as a basic active sensor.
  • It consists of a transmitter LED and a receiver LED.
  • The transmitter LED sends infrared signals to the atmosphere and the reflected signal is detected by the receiver LED.
  • An IR sensor is often paired up with some microprocessors like Arduino which controls the sensor based on the code written. The distance can be thus viewed in the console of the Arduino itself.

Seismic Sensors

A seismic sensor or vibration sensor is a special kind of active sensor. It is used to measure the vibration of the ground and predict natural disasters like earthquakes. The working of a seismic sensor is explained below.

  • This sensor consists of a mass that is suspended in a string. This mass is placed above a motion sensor and is also connected to a beam and a damping device. The whole setup is then placed on ground level.
  • When there is a vibration in the ground, the stable state of the mass is disturbed and it starts to move. This movement is detected by the motion sensor and the intensity of the vibration is proportional to the amount of motion detected by the sensor.
  • By using this method, earthquakes and tsunamis can be predicted.


Radio Detection and Ranging

A RADAR or Radio Detection and Ranging is a versatile tool that is also an application of active sensors. RADAR can be used in any technological field such as automobile location detention, speed measurement and control in automobiles, climate prediction, properties of migration of animals and birds analysis etc. The below is the step-by-step analysis of the working of a RADAR.

  • The RADAR also follows the same principle of a basic active sensor except that it uses the echo principle and the basic technique.
  • They send out electromagnetic radiation to the atmosphere. These radiations resemble short pulses like echo and they are scattered.
  • The scattered echo waves touch the nearby object and get bounced back. The reflected waves undergo precipitation and are detected and analyzed by the radar.

Comparison of various Sensors

The working of the above-mentioned sensors is compared in the table given below.

Sensors Type of ray/light emitted Application
LiDAR Lightwave Creation of elevation map
GPS Satellite signals Position detection
IR sensor Infrared waves Obstacle detection
Seismic sensor The vibration of the ground (Input) Earthquake sensing
RADAR Radio waves Location detection, speed control etc.

Key points to remember

Here is the list of key points we need to remember about “Active Sensors”.

  • All active sensors follow a basic algorithm that emits light or a wave and analyses the reflected light or wave is analyzed.
  • The type of wave that the transmitter sends decide the working and the application of the sensor.
  • A LiDAR uses light waves for distance detection.
  • A GPS is a larger-scale process involving several satellites for location detection.
  • Infrared sensors use IR waves for obstacle detection.
  • Seismic sensors measure the vibration of the ground using a spring and mass.
  • The RADAR use pulses like radio waves and is used in various applications.
  • X-ray sensor uses X-rays for medical imaging and scanning.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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