What are sensors?

Sensors are input devices that provide the output device with the input quantity. They generally detect the variation in the surrounding environment and give output to the relevant system.

Sensors convert any phenomenon into a digital signal, after which the signal is converted into a readable form and then processed further. Sensors respond to physical sensations like heat, sound, pressure, light, etc.

What are the classifications of sensors?

Based on power signal

On the basis of power signal, sensors are classified into two types: active and passive. Active signals need external power signals, whereas passive sensors generate output directly and do not need any external signals.

Based on means of detection

On the basis of means of detection, sensors are divided into electrical, radioactive, biological, etc.

Based on the conversion phenomenon

On the basis of the conversion phenomenon, sensors are fragmented into input and output sensors. Conversion processes signals like electrochemical, thermo-optic, photoelectric, electromagnetic, thermoelectric, etc.

Based on the forms

On the basis of forms, sensors are divided into analog and digital sensors. Analog sensors generate analog signals (output). Digital sensors produce digital information, which are further converted and transmitted.

What are the types of sensors?

There are various types of sensors, some of which are listed below:

  • Ultrasonic sensors
  • Pressure sensors
  • Temperature sensors
  • Color sensors
  • Humidity sensors
  • Tilt sensors
  • Proximity sensors
  • Magnetic sensors
  • Accelerometer
  • Touch sensors
  • Positioning sensors (GPS)
  • Position sensors
  • Digital sensors
  • IR sensors (Infrared sensors)
  • Light sensors
  • Smoke, gas, alcohol sensors
  • Sound or microphone sensors
  • Flow and level sensors
  • Strain and weight sensors
  • LVDT sensors
  • Camera sensors, and many more

Some essential types of sensors are provided below.

Ultrasonic sensors

An ultrasonic sensor is a contactless device employed for distance and velocity measurement. It is based on sound properties. The sound frequency range is higher than the audible range of humans. These sensors use the concept of time of flight for the measurement of distance and the doppler shift effect of sound for the measurement of velocity.


Ultrasonic sensors are applied in spray nozzles, pallet sensing on forklifts, vehicle detection used in barrier systems, fill level measurement for large goods or raw materials, etc.

The diagram represents ultrasonic sensors. It looks rectangular in shape, and two circles are embedded at the extreme ends.
CC BY-SA 3.0 | Image credits: https://commons.wikimedia.org/ | Nevit

Temperature sensors

These are a type of sensing device which senses temperature. Temperature sensors provide the measurement of temperature change. There are various temperature sensors like thermocouples, temperature sensors ICs, even infra-red temperatures detectors, resistance temperature detectors (RTDs), thermistors, etc. An analog type of temperature sensor is LM35, whereas DS18B20 is a digital temperature sensor.


Temperature sensors are applied in computers, air conditioners, mobiles, automobiles, automotive industries, heating pumps, thermometers, domestic water heaters, refrigerators, etc.

The diagram represents temperature sensors. Part (a) represents LM-35 - Temperature sensors IC, and part (b) represents 10K ohm NTC thermistor.
CC BY-SA 2.0 | Image credits: https://commons.wikimedia.org/ | Magnus Manske

Light sensors

Light sensors (photosensors) sense light. The light-dependent resistor (LDR) is an example of a light sensor. LDR works in a way that when its resistance increases, light intensity decreases, and vice versa. (Light intensity and resistance are inversely proportional to each other).

Light sensing devices are classified into analog and digital types. Examples of analog light sensors are photodiode, photodetectors (proximity sensors), and phototransistors. Digital light sensors include TSL2561, BH1750, etc.


Light-sensing devices are used in alarm systems, printers, CD and DVDs, paints, pulse oximetry, automobiles, heart rate monitors, etc.

The diagram represents light sensors. It is circular, and two vertical pins are attached below its surfaces, and a spiral-shaped structure at the top surface can be observed.
CC BY-SA 4.0 | Image credits: https://commons.wikimedia.org/ | Zunter

Proximity sensors

These sensors are contactless, and they detect the availability (presence or absence) of any objects. They are used to detect objects without touching them. Light sensors can detect, count, inspect, and position. They are of multiple types like optical, magnetic, sounds, etc. The varieties involved in optical are infrared or laser, sound is ultrasonic, and magnetic are hall effects.


Proximity sensors are used in car parking, object placement in industrial areas, collision detections in robotic machines, gear tooth detections, ground proximity in airplanes, smartphones, etc.

The diagram represents an inductive proximity sensor. It appears like a nut-bolt structure with a wire attached at one of its extreme ends.
CC BY-SA 4.0 | Image credits: https://commons.wikimedia.org/ | Ekbsensor

Infrared Sensors (IR sensors)

This type of sensor is also employed for detection and proximity. IR sensors are highly used in mobiles as proximity sensors.

They are classified into two types: transmissive and reflective IR sensors. In transmissive IR sensors, the IR transmitter and detector are placed parallel facing each other. If any object passes in between them, then it is easily detected.

In reflective IR sensors, the reflector and transmitter are held adjacent to each other. When any object is placed in front of a reflective type sensor, then infrared light from the transmitter is reflected from that object. Then the object is detected with the help of the receiver.


IR sensors are applied in burglar alarms, thermometers, gas analyzers, IR imaging devices, flame monitors, rail safety, optical power meters, airborne measurements, etc.

The diagram represents the infrared sensor (reflective type).  Its appears like a combination of rectangular and trapezoidal shapes. It looks like an electrical circuit board.
CC BY-SA 4.0 | Image credits: https://commons.wikimedia.org/ | Raymond

Application of sensors in aircraft

Sensors are applied in the autopilot or automatic flight control system in airplanes or aircraft. They sense the temperature, position, height, speed, location, etc., and transfer the inputs to the computer system. The control signals are generated with the help of a computer.

The computer device takes information from the sensors and processes them further. Then the output can be obtained via the functioning of engines, wings, motors, flaps, etc. The control signal is given to these parts of the aircraft.

Common Mistakes

  • Proximity sensors are not noiseless. They produce electrical as well as mechanical noises.
  • Sensors are not suitable for all ranges of frequencies. Some are limited to a particular range of frequencies.
  • Accelerometers can withstand very high temperatures.
  • Thermocouples are not highly stable and with high sensitivity. Rather, the converse is true.
  • RTDs and thermistors are not directly powered but require external power.
  • Piezo-electric does not give the exact position of vehicles. Due to displacement, the information gets lost, and hence it is not exact.

Context and Applications

The topic of sensors is studied in many technical and professional, graduation, post-graduation, Doctor of Philosophy, and other courses. For example:

  • Bachelor of Technology in Mechanical Engineering
  • Bachelor of Technology in Electrical Engineering
  • Bachelor of Technology in Electronics Engineering
  • Master of Technology in Mechanical Engineering
  • Master of Technology in Electrical Engineering
  • Doctor of Philosophy in Mechanical Engineering
  • Doctor of Philosophy in Electrical Engineering
  • Sensors overview
  • Types of sensors
  • Proximity sensors
  • Applications of sensors
  • Sensors examples
  • Sensor uses

Practice Problems

Q1: Which of the following are linear types of sensors?

(a) Resistance temperature detector (RTDs)

(b) Thermocouple

(c) Thermistor

(d) Infra-red temperature detector

Correct option: (a)

Explanation: Resistance temperature detectors are also called resistance thermometers. They are linear types of sensors. They have good linear characteristics at various temperatures.

Q2: Which of the following sensors are suitable for higher frequencies?

(a) Proximity sensors

(b) Velocity sensors

(c) Accelerometer sensors

(d) Resistive linear position sensors

Correct option: (c)

Explanation: Accelerometers sensors are feasible for the higher or lower range of frequencies. Applications of higher frequency accelerometers are machinery having gear arrangements.

Q3: Which of the following sensors can withstand higher temperatures?

(a) Non-contact

(b) Thermistor 

(c) RTDs

(d) Thermocouple

Correct option: (d)

Explanation: Thermocouples are suitable for higher temperatures. They generate a temperature-dependent potential difference. They can measure a wide range of temperatures.

Q4: Proximity sensors have good ________and high ________.

(a) Stability, speed

(b) Speed, cost

(c) Power, cost

(d) Stability, cost

Correct option: (a)

Explanation: Proximity sensors are highly stable and have higher speeds. They have lower costs. They have good power. It measures the presence of any type of object.

Q5: Infrared sensors are _______ and ________.

(a) Less responsive, costly

(b) Responsive, stable

(c) Non-repeatable, non-stable

(d) Stable, less responsive

Correct option: (b)

Explanation: Infrared sensors are more complex, but they are highly stable. They are responsive and less costly. They are highly repeatable. They are highly responsive over time. They are smaller and affordable.

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