IoT sensors are the eyes and ears of smart systems, collecting vital data from the environment. From temperature and humidity to pressure and motion, these sensors convert physical phenomena into electrical signals for processing and analysis.

Understanding sensor types, characteristics, and operating principles is crucial for IoT developers. Factors like accuracy, precision, and power consumption play key roles in selecting the right sensors for specific applications, ensuring reliable data collection and efficient system performance.

Sensor Types in IoT Applications

Types of IoT sensors

• Temperature sensors measure heat energy and convert it into electrical signals
  • Thermocouples generate voltage based on the temperature difference between two dissimilar metals (K-type, J-type)
  • Resistance Temperature Detectors (RTDs) change electrical resistance with temperature, offering high accuracy and stability (Platinum RTD)
  • Thermistors are semiconductor devices that exhibit a large change in resistance with temperature, providing fast response times and cost-effectiveness (NTC, PTC)
  • Semiconductor-based sensors utilize the temperature-dependent properties of semiconductor materials, delivering high accuracy and linearity (LM35, DS18B20)
• Humidity sensors detect and measure the amount of water vapor in the air
  • Capacitive humidity sensors measure the change in capacitance due to moisture absorption by a dielectric material, offering high accuracy and stability (HIH-4000)
  • Resistive humidity sensors measure the change in electrical resistance of a hygroscopic material as it absorbs moisture, providing a wide measurement range (HR202)
  • Thermal conductivity humidity sensors measure the change in thermal conductivity of air due to humidity, suitable for high-temperature applications (HMT330)
• Pressure sensors convert pressure into electrical signals
  • Piezoresistive pressure sensors measure the change in electrical resistance of a material when subjected to pressure, offering high sensitivity and wide pressure range (MPX4250)
  • Capacitive pressure sensors measure the change in capacitance between two plates when pressure is applied, providing high accuracy and low power consumption (BMP280)
  • Piezoelectric pressure sensors generate an electrical charge when subjected to pressure, suitable for dynamic pressure measurements (PCB Piezotronics)
• Motion sensors detect the presence, movement, or position of objects
  • Passive Infrared (PIR) sensors detect infrared radiation emitted by moving objects, ideal for presence detection and security applications (HC-SR501)
  • Ultrasonic sensors measure distance based on the time-of-flight principle, suitable for object detection and proximity sensing (HC-SR04)
  • Microwave sensors detect motion based on the Doppler effect, offering long detection range and immunity to environmental conditions (HB100)
  • Accelerometers measure acceleration and tilt, useful for motion and vibration monitoring (ADXL345)
  • Gyroscopes measure angular velocity, used for orientation and rotation sensing (MPU-6050)

Characteristics of sensors

• Accuracy quantifies how close the sensor's measured value is to the true value
  • Expressed as a percentage of the full-scale range or in absolute terms (±0.5°C, ±2% RH)
  • Determines the sensor's ability to provide precise measurements
• Precision refers to the consistency and repeatability of the sensor's measurements
  • Determined by the sensor's ability to produce the same output for the same input under identical conditions
  • Ensures reliable and consistent data collection over time
• Resolution represents the smallest change in the measured quantity that the sensor can detect
  • Determined by the number of bits used in the sensor's analog-to-digital converter (ADC) (12-bit, 16-bit)
  • Higher resolution enables the detection of smaller changes and provides more detailed data
• Response time defines how quickly the sensor reacts to changes in the measured quantity
  • Measured in units of time, such as milliseconds or seconds (10ms, 1s)
  • Faster response times are crucial for real-time monitoring and control applications

Sensor Principles and Suitability

Principles of sensor operation

• Temperature sensors
  1. Thermocouples: Based on the Seebeck effect, where a voltage is generated when two dissimilar metals are subjected to a temperature gradient, suitable for wide temperature ranges and harsh environments (Type K for -200°C to 1250°C)
  2. RTDs: Measure temperature by correlating the change in electrical resistance of a metal (usually platinum) with temperature, offering high accuracy and stability (Pt100 for -200°C to 850°C)
  3. Thermistors: Semiconductor devices that exhibit a large change in resistance with temperature, providing fast response times and cost-effectiveness (NTC for -50°C to 150°C)
  4. Semiconductor-based sensors: Utilize the temperature-dependent properties of semiconductor materials (bandgap voltage) to generate an output voltage proportional to temperature, delivering high accuracy and linearity (LM35 for -55°C to 150°C)
• Humidity sensors
  1. Capacitive humidity sensors: Measure the change in capacitance of a dielectric material (polymer or ceramic) as it absorbs moisture from the air, offering high accuracy and stability (HIH-4000 for 0-100% RH)
  2. Resistive humidity sensors: Measure the change in electrical resistance of a hygroscopic material (conductive polymer or salt) as it absorbs moisture, providing a wide measurement range (HR202 for 20-95% RH)
  3. Thermal conductivity humidity sensors: Measure the change in thermal conductivity of air due to the presence of water vapor, suitable for high-temperature applications (HMT330 for -70°C to 180°C)
• Pressure sensors
  1. Piezoresistive pressure sensors: Measure the change in electrical resistance of a material (silicon or polysilicon) when subjected to pressure, offering high sensitivity and wide pressure range (MPX4250 for 20-250 kPa)
  2. Capacitive pressure sensors: Measure the change in capacitance between two plates (one fixed and one movable) when pressure is applied, providing high accuracy and low power consumption (BMP280 for 30-110 kPa)
  3. Piezoelectric pressure sensors: Generate an electrical charge when subjected to pressure due to the piezoelectric effect in certain materials (quartz or ceramics), suitable for dynamic pressure measurements (PCB Piezotronics for 0.07 kPa to 100 MPa)
• Motion sensors
  1. PIR sensors: Detect the infrared radiation emitted by moving objects, using a pyroelectric material that generates an electrical signal when exposed to heat, ideal for presence detection and security applications (HC-SR501)
  2. Ultrasonic sensors: Measure distance by emitting high-frequency sound waves and calculating the time taken for the waves to bounce back from an object, suitable for object detection and proximity sensing (HC-SR04 for 2cm to 400cm)
  3. Microwave sensors: Detect motion by emitting microwave radiation and measuring the frequency shift of the reflected signal due to the Doppler effect, offering long detection range and immunity to environmental conditions (HB100 for 10.525 GHz)
  4. Accelerometers: Measure acceleration and tilt by detecting the displacement of a proof mass using capacitive, piezoresistive, or piezoelectric sensing methods, useful for motion and vibration monitoring (ADXL345 for ±16g)
  5. Gyroscopes: Measure angular velocity by detecting the Coriolis effect on a vibrating structure, used for orientation and rotation sensing (MPU-6050 for ±2000°/s)

Active vs passive sensors

• Active sensors require an external power source to operate and generate their own signal or energy for measurement
  • Examples include ultrasonic sensors (generate sound waves), radar sensors (emit radio waves), and LiDAR sensors (emit laser light)
  • Active sensors typically have higher power consumption compared to passive sensors, as they need to generate and transmit energy
• Passive sensors do not require an external power source for operation and rely on the energy of the measured quantity or environmental conditions
  • Examples include thermocouples (generate voltage from temperature difference), PIR sensors (detect infrared radiation), and photoresistors (change resistance based on light intensity)
  • Passive sensors generally have lower power consumption compared to active sensors, as they do not need to generate energy for measurement
• Power requirements are a critical consideration in IoT systems, as sensors can significantly impact the overall power budget
  • Active sensors require a stable and sufficient power supply to function properly, which may necessitate the use of mains power or large batteries
  • Passive sensors have minimal power requirements, making them suitable for battery-powered or energy-harvesting IoT applications with limited power resources
  • Power management techniques, such as duty cycling (periodic wake-sleep cycles) and low-power modes (sleep, standby), can be employed to optimize the power consumption of both active and passive sensors in IoT systems, extending battery life and enabling long-term deployment