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Every IoT device is some combination of sensors (which observe the physical world and produce data) and actuators (which take data and change the physical world). The split matters because the two halves have fundamentally different design constraints: sensors care about sample rate, noise floor, calibration, and power-per-reading; actuators care about response time, current capacity, failure safety, and feedback. A motor controller and a thermometer ostensibly use the same MCU and the same Wi-Fi chip — but everything around those choices is different. Get the categorisation wrong on day one (e.g., treating a valve as a 'data point' instead of a safety-critical actuator) and you'll fight it for the entire product lifecycle.
The two sides of the I/O loop, with common examples.
SENSORS — read the world, output a signal
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Environmental temperature (thermistor, DS18B20, BME280)
humidity, pressure, gas (BME680, MQ-series)
light/UV (BH1750, VEML7700, TSL2591)
Motion accelerometer (MPU6050, LSM6DSO, ADXL345)
gyro / IMU / magnetometer (BNO055, ICM-20948)
GPS (u-blox NEO-6M, NEO-M9N)
Distance ultrasonic (HC-SR04), IR (Sharp GP2Y), ToF (VL53L1X), LiDAR
Vision camera modules (OV2640 on ESP32-CAM, Pi Camera)
Audio MEMS mic (SPH0645, INMP441), analog mic + ADC
Biometric PPG (MAX30102 — heart rate, SpO2), ECG (AD8232), GSR
Current/voltage INA219, INA226, ACS712
Touch/proximity capacitive (MPR121), Hall effect, reed switches
ACTUATORS — take a signal, change the world
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Visual LED (digital), addressable LED strip (WS2812), display
(OLED SSD1306, e-paper, LCD via SPI/I²C)
Motion DC motor + H-bridge (L298N, DRV8833, TMC driver)
servo (SG90, MG996R), stepper (NEMA 17 + driver)
brushless motor + ESC (drones, RC)
Switching relay (mechanical, SSR), MOSFET (high-side / low-side)
triac (AC dimming)
Pneumatic/fluid solenoid valve, peristaltic pump, vacuum pump
Audio buzzer (passive — needs PWM; active — just power)
amplifier + speaker (MAX98357A I²S DAC)
Thermal heater element + relay, Peltier cooler, fan PWM
KEY DIFFERENCE
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Sensors fail soft: a bad reading is wrong data. You ignore, filter, or recalibrate.
Actuators fail hard: a bad signal opens a valve, spins a motor, or starts a fire.
Always design actuator paths with a known safe state (off, neutral, last-good).