An IoT sensor is a device that converts a physical quantity (temperature, pressure, presence, gas, vibration, current) into an electrical signal that a controller can read and send over a network. In industry, the sensor connects to a PLC or a gateway through a digital input, a 4-20 mA or 0-10 V analog input, a bus such as Modbus RTU, or a wireless link such as LoRaWAN, and the PLC sends the data to SCADA or the cloud with MQTT or Modbus TCP.
Below are 25 types of sensors used in IoT and IIoT projects, what each one measures, the signal it usually gives and where it is used. After the list you will find how to connect each kind of signal to an industrial PLC.
What are the main types of IoT sensors?
| # | Sensor | What it measures | Typical signal | Typical industrial use |
|---|---|---|---|---|
| 1 | Temperature (RTD, thermocouple, NTC, digital) | Temperature of air, liquids or surfaces | Resistance (PT100), mV, 4-20 mA, 1-Wire | Ovens, cold rooms, process lines |
| 2 | Relative humidity | Water vapour in the air, in %RH | Single-wire digital, I2C, 4-20 mA, Modbus | Warehouses, greenhouses, clean rooms |
| 3 | Pressure transmitter | Pressure of a liquid or gas in a pipe or tank | 4-20 mA, 0-10 V | Pumps, compressors, hydraulics |
| 4 | Barometric pressure | Atmospheric pressure | I2C, SPI | Weather stations, altitude compensation |
| 5 | Level (ultrasonic, radar, hydrostatic) | Height of a liquid or solid in a tank or silo | 4-20 mA, Modbus | Water tanks, silos, wells |
| 6 | Flow | Volume of liquid or gas per unit of time | Pulses, 4-20 mA, Modbus | Water networks, dosing, energy balance |
| 7 | Inductive and capacitive proximity | Presence of an object near the sensor without contact | Digital 24 V (PNP or NPN) | Conveyors, machine positions, part counting |
| 8 | Photoelectric | Presence of an object that interrupts or reflects a light beam | Digital 24 V | Packaging lines, doors, safety areas |
| 9 | Ultrasonic distance | Distance to an object by the echo time of a sound pulse | Pulse width, 4-20 mA | Level, presence, car parks |
| 10 | Contact and limit switch | A door, a lid or an axis at its end position | Dry contact, digital 24 V | Machine guards, valves, cabinets |
| 11 | Motion (PIR) | Movement of people by the change of infrared radiation | Digital output | Lighting, intrusion, occupancy |
| 12 | Vibration and acceleration | Acceleration of a machine on one to three axes | Analog, IEPE, I2C, 4-20 mA | Condition monitoring of motors, fans and pumps |
| 13 | Inclination | Angle of a surface or a structure | CANopen, Modbus, analog | Solar trackers, cranes, platforms |
| 14 | Gas (CO, combustible, O2) | Concentration of a gas in the air | 4-20 mA, relay output | Car parks, boiler rooms, confined spaces |
| 15 | CO2 (NDIR) | Carbon dioxide in ppm | Modbus, UART, I2C, 0-10 V | Ventilation, silos, culture rooms |
| 16 | Particulate matter | PM2.5 and PM10 dust in the air | UART, Modbus | Air quality in workshops and cities |
| 17 | Smoke | Smoke particles from a fire | Relay or dry contact | Fire alarms in cabinets and rooms |
| 18 | Water leak | Water on the floor or in a tray | Digital contact | Data centers, pump rooms, basements |
| 19 | pH and water quality | pH, conductivity, dissolved oxygen, turbidity | 4-20 mA, Modbus | Water treatment, aquaculture, food |
| 20 | Current | AC or DC current in a conductor | Analog voltage, 4-20 mA | Motor load, consumption, broken belt detection |
| 21 | Voltage and energy meter | Voltage, power and energy of a line | Modbus RTU, pulses | Energy monitoring and cost per machine |
| 22 | Light (lux) | Illuminance | Analog, I2C | Street lighting, greenhouses, buildings |
| 23 | Sound level | Sound pressure level | Analog, Modbus | Noise at workstations, machine diagnosis |
| 24 | RFID reader | The identifier of a tag or a card | UART, Wiegand, Ethernet | Access control, traceability of parts |
| 25 | GPS receiver | Position, speed and time | Serial (NMEA) | Vehicle tracking, mobile assets |
The list is a map, not a catalogue: each type has many models with different ranges, accuracy and outputs. Choose the output first by the controller you have, then by the cable length and the noise of the place.
How a sensor becomes an IoT sensor
IoT (Internet of Things) is the connection of devices to a network so that their data can be read and used from another place. A sensor alone only gives a signal. It becomes part of an IoT system when something reads that signal, gives it a meaning and sends it: a PLC, a gateway or a module with a radio.
An example: a camera next to a motion sensor. Without IoT, someone has to connect the camera to a computer to copy the photos. With IoT, the motion sensor triggers the camera, and the photo goes to the cloud at once.
IIoT (Industrial Internet of Things) is the same idea in factories, utilities and infrastructure. The difference is in the requirements: industrial signals (24 V, 4-20 mA, RS-485), supply ranges and noise of a cabinet, and data that also drives control actions, not only dashboards.
Connecting IoT sensors to an industrial PLC
| Sensor signal | PLC input | Guide |
|---|---|---|
| On/off at 24 V (proximity, photoelectric, contacts) | Digital input | Inductive proximity sensor with M-Duino Arduino PLC |
| 4-20 mA (transmitters) | Analog input with a current input or a conversion resistor | 4-20 mA to 0-10 V in an Arduino PLC |
| 0-10 V or 0-5 V | Analog input | ACS712 current sensor with an Arduino PLC |
| RTD (PT100) | RTD input or transmitter | PT100 temperature sensor |
| 1-Wire or single-wire digital | A direct pin of the PLC (5 V on Arduino PLCs, 3.3 V on ESP32 PLCs) | OneWire with Arduino and ESP32 based PLCs, DHT22 |
| Modbus RTU (RS-485) | RS-485 port | Modbus RTU with Arduino and ESP32 based PLCs |
| Wireless (LoRaWAN, Wi-Fi) | Gateway or radio in the PLC | LoRaWAN with a PLC |
In an Arduino PLC, an ESP32 PLC or a Raspberry PLC, the same controller reads the inputs and publishes the values with MQTT, Modbus TCP or HTTP. That makes it the IoT node of the machine, without a separate gateway for the wired sensors.
See sensors in real projects. In the grain silo case study, solar radio nodes read CO2 in each silo every 30 minutes and a Raspberry PLC sends the values to the cloud. In the wireless sensor network simulator, you can break a network of battery nodes and a gateway (4G loss, weak links, flat batteries) and see the alarms.
Read the grain silo case studyOpen the wireless sensor network simulator
FAQ about IoT sensors
What is the difference between a sensor and an IoT sensor?
The sensing element is the same. An IoT sensor has, or is connected to, something that sends its data over a network: a radio, an Ethernet port or a PLC that publishes the values.
Which IoT sensors are used most in industry?
Temperature, pressure, level, flow, proximity and current or energy sensors. They cover most process and machine monitoring tasks.
Is 4-20 mA or Modbus better for an IoT sensor?
4-20 mA gives one value per pair of wires and is simple to diagnose: 0 mA means a broken wire. Modbus RTU carries many values and the diagnostics of the device on one RS-485 bus. Use 4-20 mA for a few critical signals and Modbus when you need many values per device.
Can a PLC send sensor data to the cloud?
Yes. A PLC with Ethernet, Wi-Fi or 4G reads the sensors and publishes the values with MQTT or HTTP, or serves them over Modbus TCP to a SCADA.
More about sensors and PLCs
- Dallas DS18B20 temperature sensor with an Arduino PLC
- Detecting moving objects with an HC-SR04 ultrasonic sensor
- FMR10 radar level sensor with an ESP32 PLC 14
- 4-20 mA pressure and temperature sensors with an ESP32 PLC
- Warehouse temperature and humidity monitoring case study
- Industrial applications: 50 real examples by sector
Compare the controllers that read these sensors in choosing an industrial controller.