IoT is composed of several technology layers which enable things to share the data they collect over the Internet to ultimately deliver intelligence, autonomous actions, and value which largely depend on the quality of the data itself. Therefore, sensors and actuators are an essential part of the IoT tech stack. 

A sensor, also called a transducer, is a device whose task is to detect events or changes in its immediate environment and convert these physical phenomena (like temperature, light, air humidity, movement, presence of chemical substances, and many others) into electrical impulses which then can be meaningfully interpreted. A microphone is a sensor that takes vibrational energy (sound waves), and converts it to electrical energy. Another type of transducer is an actuator. An actuator operates in the reverse direction of a sensor. It takes an electrical input and turns it into physical action, An electric motor, a hydraulic system, and a pneumatic system are all different types of actuators. In a typical IoT system, a sensor may collect information and route it to a control center where a decision is made and a corresponding command is sent back to an actuator in response to that sensed input.

Types of Sensors 

Sensors can be either standalone devices or devices embedded in ordinary objects or machines to make them smart, and they can be divided into categories in terms of the physical phenomenon they are intended to measure.

Moisture IoT sensors

moisture iot sensors - types of sensors
Fig. Moisture IoT Sensors 

Moisture and Humidity sensors are used in agriculture, environment monitoring, food supply chain, and health monitoring. 

Hair Tension Moisture Sensor

This is the traditional and oldest type of moisture sensor. The design of the device is based on the different properties of human or horsehair. Though currently, synthetic or cotton fibers are also used. These fibers change their length upon contact with moisture. The pointer, which shows the reading on the scale, is connected to the hair (or fiber) and reacts to a change in its length. The advantage of a moisture sensor is its simple and cheap construction, which is also resistant to damage. 


The construction of this device is based on two thermometers. They are called dry and wet. The dry thermometer defines the current temperature and the humidity can be calculated from the temperature difference. The wet thermometer has a mercury container lined with a moisture-wicking material. It uses the phenomenon of damping evaporation by moist air and accelerating it with dry air. The higher the humidity, the lower the temperature of the wet thermometer. 

Light IoT Sensors 

light iot sensors - types of sensors

Fig. Light IoT Sensors

Smart TV and mobile computer screens are able to adjust their brightness depending on the light intensity because of the light sensors. They are increasingly used for adapting strict light or urban lighting levels for increased economy. 
  • Photoresistor: A photoresistor is a photosensitive component, whose resistance changes through radiation. It can casiƂy be connected for example Arduino as an analog light sensor. Photoresistors can also be used as simple proximity sensors or temperature 6'7 sensors, 
  • Photodiode: Photodiode is based on the photoelectric effect. When photons reach the junction of a photodiode they are absorbed, which results in the electron being transferred to the conductivity band to create an electron-hole pair. Photodiodes are widely used in industrial automation, telecommunications (optocouplers, optoelectronic links), and many more industries.

Acoustic and Noise IoT Sensors

Acoustic and Noise IoT Sensors - types of sensors
Fig: Acoustic and Noise IoT Sensors

Smart acoustic sensors are used to monitor the level of noise in a given environment. Acoustic IoT sensor systems are gaining ground in smart city solutions. 

  • Hydrophone: Hydrophones are a basic structural element of passive sonars. A microphone is used to pick up sounds propagating in water or other liquids. for example to detect fish in various aquatic environments. 
  • Geophone: It is a sensor that converts ground vibrations into electrical voltage. This is a type of seismometer and it has been used as such on the moon in the Active Seismic Experiment by Apollo 16.

Water Level IoT Sensors 

Data gathered by the water level monitoring sensors can be used in flood warning systems for analytics and prediction to prevent natural disasters. This sensor is also used in a variety of industrial applications to control and optimize manufacturing processes.
water level iot sensor - types of sensors
Fig. Water Level IoT Sensors

Hydrostatic Pressure Sensors

This is used to measure the level of the liquid filling. These sensors operate based on level measurements using hydrostatic-the hydrostatic pressure measured at the tank measure point is proportional only to the height of the liquid filling, regardless of the shape volume of the tank. 

Optical Sensor 

A sensor that detects the water level implicit by the refraction of light in the prism after contact with the liquid, Optical sensors have an advantage over water level sensors in that they are limited by mechanical and moving parts that can break. 

Humidity Sensor 

A humidity sensor (or hygrometer) senses, measures, and reports both moisture and air temperature. The ratio of moisture in the air to the highest amount of moisture at a particular air temperature is called relative humidity. Humidity sensors work by detecting changes that alter electrical currents or temperature in the air. 

These sensors usually follow the use of temperature sensors, as many manufacturing processes require perfect working conditions. By measuring humidity, you can ensure that the whole process runs smoothly, and when there is any sudden change, action can be taken immediately, as sensors detect the change almost instantaneously. 

These sensors are used in the industrial and residential domain for heating, ventilating, and air conditioning systems control. They can also be found in Automotive, museums, industrial spaces and greenhouses, meteorology stations, paint and coatings industries, hospitals, and pharma industries to protect medicines. Due to this fact, these sensors have found use in healthcare, environment monitoring, energy, aerospace, and many more industries. With their presence oil companies, pharmaceutical companies and mining companies are in a much better position to track environmental changes while keeping their employees safe. Their main use can be found in ambient light detection, digital optical switches, and optical fiber communications, due to electrical isolation best suited for oil and gas applications, civil and transportation fields, high-speed network systems, elevator door control, assembly line part counters, and safety systems.

There are three basic types of humidity sensors: 
  1. Capacitive 
  2. Resistive 
  3. Thermal 

Capacitive: A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes. The metal oxide's electrical capacity changes with the atmosphere's relative humidity. Weather, commercial and industries are the major application areas, The capacitive type sensors are linear and can measure relative humidity from 0% to 100%, These are the only types of full-range relative humidity measuring devices down to 0% relative humidity.
Resistive: Resistive humidity sensors utilize ions in salts to measure the electrical protection of atoms. As humidity changes, so do the resistance of the electrodes on either side of the salt medium. 
Thermal: Two thermal sensors conduct electricity based on the humidity of the surrounding air. One sensor is wrapped in dry nitrogen while the other measure moving air. The difference between the two measures the humidity. 

Parameters of Humidity Sensor 

Accuracy: Every sensor has its own calibration curve, based on a 9-point system. It basically pitches the pros against the cons of the particular sensor. 
Linearity: It indicates the voltage deviation from the BFSL value and the measured output voltage value, converted to relative humidity. 
Reliability: The measurements often cause the sensor to fall out of sync. However for a sensor to be useful, it has to provide reliable measurements. 
Repeatability: The measurements from a sensor, have to be so that they don't drift apart. Repeatability is the measurement of drift among measurements of a single quantity. 
Response Time: Typically, the time taken by a sensor rises to 66% (rise time) or falls to 33% (fall time) of maximum output voltage, which is known as the response time. 

Applications of Humidity Sensors

A humidity sensor is also found as part of home heating, ventilating and air conditioning systems (HVAC systems). These are also used in offices, cars, humidors, museums, industrial spaces, and greenhouses and are also used in meteorology stations to report and predict the weather. 

Examples of Humidity Sensor 

Humidity Indicator

In industries such as textile, the change in moisture content has a direct impact on the properties of the fabric, such as tensile strength, elasticity, fiber diameter, and friction. Cotton and linen require high Relative Humidity (RH) levels of around 70-80 percent since they are very brittle. Wool requires RH levels of around 65 percent. While silk requires between 65 and 70 percent. With this circuit, you can not only monitor humidity levels between 30 and 90 percent RH but also control it. 

Fridge Temperature and Humidity Indicator

The small sniffer device picks up temperature and humidity from inside the fridge and transmits it on an RF link to a nearby receiver unit. The receiver unit checks the received code, identifies the right sniffer device and displays live temperature and humidity, w can measure temperature and humidity inside the fridge using a normal temperature humidity indicator but relative humidity could be inaccurate in that case. 

IoT-Enabled Air Pollution Meter With Digital Dashboard On Smartphone

Air pollution meter to monitor air quality on smartphone using Blynk application and Arduino board. Blynk is an Internet of Things (IoT) platform to control Arduino, Raspberry Pi, and the like over the Internet. Blynk provides a digital dashboard on your smartphone that displays real-time air quality readings for th immediate surroundings. 

Examples of humidity sensors are LM35 and DHT11 and DHT22. The automation application used the DHT11 sensor because of the output in digital form. The digital data comes from the data pin every two seconds. Now we explain the specification of DHT11. 

Pin Definition 

VCCInput Voltage
GNDGround Pin
NCNon-Connection Pin (in this pin need to add SOIC14 fabrication)
OutDigital Data Output


Operating Voltage3.3 to 5V DC
Measuring Range20-95%RH(Relative Humidity) : 0-50?
Resolution8-bit Humidity, 8-bit Temperature
Interfaces3-Pin interfaces, 4-pin interfaces
PCB Size22.0mm X 20.5mm X 1.6mm

Ultrasonic Sensors

ultrasonic sensors - types of sensors
Fig. Ultrasonic Sensor

An ultrasonic sensor is a device that detects an object and measures the distance to it. It measures the distance by emitting ultrasound and receiving the wave that the object reflects. 

ultrasonic sensors device - types of sensors
Fig. 2.11: Ultrasonic Sensor device

Ultrasonic transducers convert ultrasound waves to electrical signals and vice versa, working of this sensor is similar to that used by transducers in radar and sonar systems which check out the attributes of the target object by processing the echo signals from radio or sound waves. Ultrasonic sensors consist of two parts: a transmitter and receiver which create a transducer that converts ultrasound waves into electrical signals (A/C and vice versa, The transceiver vibrates and creates an ultrasonic wave that is transmitted and travels until it hits an object and reflected back to the receiver. The interval between the signal being sent and received is typically referred to as time-of-flight (ToF) and is dependent on the distance the ultrasonic wave travels until it is reflected. 

Ultrasonic Transducer Technology 

The transceiver vibrates and creates an ultrasonic wave using piezoelectric transducers or capacitive transducer technologies. When a voltage is applied: the capacitive transducer can change shape and size, and A/C voltage makes them oscillate at the same frequency and produce ultrasonic sound. Capacitive transducers use electrostatic fields between a conductive diaphragm and a backing plate. A single ultrasonic transducer can both generate and receive a signal, but the two functions are often separated in order to optimize the performance of each task. 

Distance Measurement 

The ultrasonic sensor is a powerful method of measuring small distances. This is an ideal tool for measuring distance without requiring contact with the object. As the distance from the object is proportional to the time interval between transmitting and receiving signals, a simple analysis of this data can reveal changes in the sensor's distance to the object. 

Warking of an HC-SR04 Sensor 

HC-SR04 sensor - types of sensor
Fig. HC-SR04 Sensor

HC-SR04 and like other ultrasonic sensor modules use a single transducer to send a pulse and receive the echo. The sensor determines the distance to the target by measuring the time lapse between sending and receiving of the ultrasonic pulses.

Ultrasonic waves travel faster than the speed of audible sounds. Ultrasonic sensors have two main components: the transmitter (which emits the sound using piezoelectric crystals) and the receiver (which encounters the sound after it has traveled to and from the target). 

In order to calculate the distance between the sensor and the object, the sensor measures the time it takes between the emission of the sound by the transmitter to its contact with the receiver. The formula for this calculation is D T C (where D is the distance, T is the time and C is the speed of sound - 343 meters/second). For example, if a scientist set up an ultrasonic sensor aimed at a box and it took 0.025 seconds for the sound to bounce back, the distance between the ultrasonic sensor and the box would be: 

D = 0.5 x 0.025 x 343 
or about 4.2875 meters. 

Ultrasonic sensors are used primarily as proximity sensors. They can be found in automobile self-parking technology and anti-collision safety systems. Ultrasonic sensors are also used in robotic obstacle detection systems, as well as manufacturing technology. In comparison to infrared (IR) sensors in proximity sensing applications, ultrasonic sensors are not as susceptible to interference of smoke, gas, and other airborne particles (though the physical components are still affected by variables such as heat). Ultrasonic sensors are also used as level sensors to detect, monitor, and regulate liquid levels in closed containers 

Pin Definition 

VCC5V Power Supply
TrigThe Trigger is an input pin. it will have kept high for 10uS
EchoOutput pin
GNDGround pin


Theoretical Measuring Distance2cm to 450cm
Practical Measuring Distance2cm to 80cm
Operating Current15ma

Temperature Sensor 

A device, used to measure the amount of heat energy that allows to detection of a physical chane in temperature from a particular source and converts the data for a device or user, is known as a "Temperature Sensor." 

This sensor is used to measure the temperature of its surroundings and the output voltage linearly proportional to the Centigrade temperature. The operating voltage is 4 - 30V, and it is an analog sensor, based on an application designed by various transistors. 
temperature sensor - types of sensors
Fig. Temperature Sensor
temperature sensor device - types of sensors
Fig. Temperature Sensor device


Operating Voltage4-30V
Operating Temperature (C)-40 to 110, -55 to 150, 0 to 100, 0 to 70
Supply current (uA)114
OutputAnalog Output

These sensors have been deployed for a long time in a variety of devices. These sensors mostly included A/C control, refrigerators, and similar devices used for environmental control. In this IoT scenario, the role of this sensor is found in the manufacturing processes, agriculture, and health industry. In the manufacturing process, many machines require specific environmental temperatures, as well as device temperatures. With this kind of measurement, the manufacturing process can always remain optimal.

Temperature sensors are used in manufacturing plants, warehouses, and weather reporting systems, and in agriculture, the temperature of the soil is crucial for crop growth. This helps with the production of plants, maximizing the output. 

Categories of Temperature Sensors

  • Thermocouples: The electric circuit consists of two different conductors. The electromotive force that arises between the connectors of the thermocouple is proportional to the temperature difference. These are voltage devices that express temperature measuring with a change in voltage. As the temperature goes up, the output voltage of the thermocouple rises. 
  • Resistor temperature detectors (RTD): The resistance of the device is directly proportional to the temperature, increase in a positive direction when the temperature rises resistance goes up.
  • Thermistors: A thermistor is a resistor whose resistance depends on temperature. These are used as a sensor in electronics. For example a temperature sensor in an electronic thermometer.
  • IC (Semiconductor): They are linear devices were the conductivity of the semiconductor increases linearly and it takes advantage of the variable resistance properties of semiconductor materials. It can provide a direct temperature reading in digital form, especially at low temperatures. 
  • Infrared sensors: It detects temperature by intercepting a portion of emitted infrared energy of the object or substance, and sensing its intensity, can be used to measure the temperature of solids and liquids only, not possible to use it on gases because of their transparent nature. 

Proximity Sensor 

This sensor detects the presence of nearby objects without requiring physical contact. The sensor is able to detect a range of objects with the help of emitting an electromagnetic field or a beam of electromagnetic radiation without any physical contact. It can also be used to recognize air gestures and hover manipulations. Ultrasonic sensors are used as proximity sensors, setting a threshold distance that can determine whether an object is an obstacle. These types of proximity sensors are used in the robotics industry. 

Proximity sensors are largely used in the retail industry, as they can detect motion and the correlation between the customer and the product they might be interested in. A user i immediately notified of discounts and special offers of nearby products, They are also used for parking availability in places such as malls, stadiums, or airports.

Pin Diagram 
  • Vcc - 5V Power supply 
  • Out - Data output 
  • GND - Ground 

There are four types of proximity sensors: 

Inductive Proximity Sensor 

This is an electromagnetic sensor used to detect metallic objects without touching them and an operating principle coil and high-frequency oscillator. The coil and oscillator create a magnetic field in the surrounding surface. The range of detection is based on coil size. It will detect only metallic objects. The applications of this sensor are car parking areas and industrial applications. 

Capacitive Proximity Sensor 

Capacitive proximity sensors can detect both metallic as well as for non-metallic targets. Nearly all other materials are dielectric and different from the air. It can be used to sense very small objects through a large portion of the target. So, generally used in difficult and complicated applications. The applications of this sensor are mobile phones, laptops, and computer displays. 

Optical Proximity Sensor

It's a non-metallic sensor used to detect the object and measure the distance between the object and the sensor. The working principle of an optical sensor is the action of the light move is made up of Transmitter (Light source) and Receiver (Light detector). When the transmitter transmits light sources and the object reflects the light source, the object reflects to be a receiver (Photodiode). The applications of this sensor are security, automobile, and level sensing. 

Ultrasonic Proximity Sensor

Ultrasonic sensors are also used to detect the presence or to measure the distance of targets similar to radar or sonar. This makes a reliable solution for harsh and demanding conditions. The applications of this sensor are robots and vehicles.