Sensors and Actuators Guide: Hardware Meets Software

Sensors Convert the Physical World to Data

A sensor is a transducer — it converts one form of energy into another. Temperature sensors convert thermal energy to voltage. Accelerometers convert acceleration forces to capacitance changes. Microphones convert sound pressure to voltage. All sensors ultimately produce an electrical signal that a microcontroller can read.

Two broad categories:

• Analog sensors — Output a continuous voltage proportional to the measured quantity. Require an Analog-to-Digital Converter (ADC) to read. Example: LM35 temperature sensor (10mV per °C).
• Digital sensors — Output a digital signal. Either discrete (GPIO high/low) or serial protocol (I2C/SPI data). Example: DHT22 temperature/humidity sensor (custom serial protocol).

Interface Protocols: How Sensors Talk to Microcontrollers

I2C example with Arduino:

#include <Wire.h>
#include <Adafruit_BME280.h>  // I2C temperature/humidity/pressure

Adafruit_BME280 bme;

void setup() {
  Serial.begin(9600);
  if (!bme.begin(0x76)) {  // 0x76 is the I2C address
    Serial.println("Could not find BME280 sensor!");
    while(1);
  }
}

void loop() {
  Serial.print("Temperature: ");
  Serial.print(bme.readTemperature());
  Serial.println(" °C");
  delay(2000);
}

Common Sensor Types and When to Use Them

Temperature & Humidity:

• DHT22 — Digital, ±0.5°C accuracy, 0-100% humidity. Easy to use, single-wire protocol. Good for home automation.
• BME280 — I2C, adds barometric pressure. ±1°C, ±3% RH. Better accuracy and additional pressure reading for weather monitoring.
• DS18B20 — 1-Wire waterproof temperature sensor. Multiple sensors on one wire. Great for liquid temperature or outdoor use.

Distance & Proximity:

• HC-SR04 — Ultrasonic, 2cm-400cm range. Two GPIO pins (trigger and echo). Works by measuring time-of-flight of sound. Good for 5-300cm.
• VL53L1X — ToF (Time-of-Flight) laser, I2C, 4m range, ±1mm accuracy. Compact, more accurate than ultrasonic.
• IR proximity sensor (GP2Y0A21YK) — Analog output, 10-80cm. Affected by object color and surface reflectivity.

Motion & Orientation:

• MPU-6050 — 6-DOF IMU (3-axis gyroscope + 3-axis accelerometer), I2C. Used in drones, robots, wearables.
• BNO055 — 9-DOF IMU with integrated sensor fusion. Outputs Euler angles directly — saves you the math.
• PIR sensor (HC-SR501) — Passive infrared motion detection. Digital output, adjustable sensitivity and timeout. Simple security and occupancy sensing.

Actuators: Converting Electrical Signals to Physical Action

Actuators do the opposite of sensors — they take electrical input and produce physical output: movement, light, sound, heat.

• LED — Light emitting diode. GPIO high/low for on/off; PWM for brightness control.
• Buzzer — Piezoelectric: digital signal (tone() in Arduino). Electromagnetic: DC voltage for constant tone.
• Relay — Electrically controlled switch. A 5mA GPIO signal switches a circuit carrying amps (motor, light, appliance). Always use a flyback diode with relay coils.
• Solenoid — Electromagnetic plunger. Linear motion (lock mechanisms, valve actuators). Needs a transistor driver and flyback diode.

Motor Types and Control

Servo control example:

#include <Servo.h>

Servo myServo;

void setup() {
  myServo.attach(9);  // Servo on pin 9
}

void loop() {
  myServo.write(0);    // 0 degrees
  delay(1000);
  myServo.write(90);   // 90 degrees (center)
  delay(1000);
  myServo.write(180);  // 180 degrees
  delay(1000);
}

Integrating Sensors with Cloud and AI

The real power of sensor data comes from processing it at scale. Common integration patterns:

• MQTT to cloud — Publish sensor readings to an MQTT broker (Mosquitto, HiveMQ, AWS IoT Core). Subscribe and process on the backend.
• Edge inference — Run a TensorFlow Lite model on the microcontroller itself. Classify sensor data locally without a network. Good for latency-sensitive or offline applications.
• REST API — Send sensor data via HTTP POST to a cloud endpoint. Simple, works everywhere, not real-time.
• InfluxDB + Grafana — Time-series database for sensor data. Grafana for visualization. Standard IoT monitoring stack.