553  Sensor Applications

Real-World IoT Implementation Examples

Keywords

smart home, environmental monitoring, servo control, relay switching, MQTT, automation

553.1 Learning Objectives

By the end of this chapter, you will be able to:

• Build a complete smart home environmental monitoring system with MQTT integration
• Implement sensor-actuator feedback loops for automated control
• Design temperature-controlled relay switching with hysteresis to prevent rapid cycling
• Select appropriate sensors for common IoT applications
• Apply real-world control logic patterns to sensor-driven automation

553.2 Introduction

This chapter brings together sensor interfacing, data processing, and actuation into complete, practical IoT applications. Each example demonstrates the full sensor-to-action pipeline, from reading physical measurements to triggering real-world responses.

553.3 Smart Home Environmental Monitoring

25 min | Intermediate | P06.C09.U05a

A complete environmental monitoring system combining multiple sensors with cloud connectivity and automated responses.

// Complete environmental monitoring system
#include <WiFi.h>
#include <PubSubClient.h>
#include <DHT.h>
#include <Wire.h>
#include <BH1750.h>

// Sensors
DHT dht(4, DHT22);
BH1750 lightMeter;

// Wi-Fi and MQTT
WiFiClient espClient;
PubSubClient mqtt(espClient);

// Thresholds
#define TEMP_HIGH 28.0
#define TEMP_LOW 18.0
#define HUMIDITY_HIGH 70.0
#define LIGHT_THRESHOLD 200.0

void setup() {
  Serial.begin(115200);

  // Initialize sensors
  dht.begin();
  Wire.begin();
  lightMeter.begin();

  // Connect to Wi-Fi
  connectWiFi();

  // Connect to MQTT
  mqtt.setServer("mqtt.example.com", 1883);
  connectMQTT();
}

void loop() {
  if (!mqtt.connected()) {
    connectMQTT();
  }
  mqtt.loop();

  // Read sensors
  float temp = dht.readTemperature();
  float humidity = dht.readHumidity();
  float light = lightMeter.readLightLevel();

  // Check conditions and take actions
  if (temp > TEMP_HIGH) {
    Serial.println("Temperature too high! Turning on AC");
    mqtt.publish("home/ac", "ON");
  } else if (temp < TEMP_LOW) {
    Serial.println("Temperature too low! Turning on heater");
    mqtt.publish("home/heater", "ON");
  }

  if (humidity > HUMIDITY_HIGH) {
    Serial.println("Humidity too high! Turning on dehumidifier");
    mqtt.publish("home/dehumidifier", "ON");
  }

  if (light < LIGHT_THRESHOLD) {
    Serial.println("Dark! Turning on lights");
    mqtt.publish("home/lights", "ON");
  } else {
    mqtt.publish("home/lights", "OFF");
  }

  // Publish sensor data
  String payload = "{\"temp\":" + String(temp) +
                   ",\"humidity\":" + String(humidity) +
                   ",\"light\":" + String(light) + "}";
  mqtt.publish("home/sensors", payload.c_str());

  delay(60000);  // 1 minute
}

void connectWiFi() {
  WiFi.begin("SSID", "password");
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("\nWi-Fi connected");
}

void connectMQTT() {
  while (!mqtt.connected()) {
    if (mqtt.connect("ESP32_Home_Sensor")) {
      Serial.println("MQTT connected");
    } else {
      delay(5000);
    }
  }
}

553.4 Sensor-Controlled Servo Motor

20 min | Intermediate | P06.C09.U05b

Servo Motors in IoT: Servo motors provide precise angular positioning, making them ideal for automated control systems. Combined with sensors, they create feedback-controlled actuators.

Specifications (SG90 Servo):

• Rotation Range: 0 to 180 degrees
• Control: PWM signal (1000-2000us pulse width)
• Torque: 1.8kg/cm at 5V
• Speed: 0.1s/60 degrees at 5V
• Power: 5V, 100-250mA (moving), 10mA (idle)

553.4.1 Application Example: Automated Blinds Control

#include <ESP32Servo.h>

#define SERVO_PIN 18
#define LDR_PIN 34

Servo blindsServo;

void setup() {
  Serial.begin(115200);

  // Attach servo
  blindsServo.attach(SERVO_PIN, 1000, 2000);  // min, max pulse width

  // Configure ADC
  analogReadResolution(12);
  analogSetAttenuation(ADC_11db);

  Serial.println("Automated Blinds Control");
}

void loop() {
  // Read light sensor
  int lightLevel = analogRead(LDR_PIN);
  float lightPercent = (lightLevel / 4095.0) * 100;

  // Map light level to servo angle
  // High light = close blinds (180 degrees)
  // Low light = open blinds (0 degrees)
  int servoAngle = map(lightLevel, 0, 4095, 0, 180);

  // Move servo
  blindsServo.write(servoAngle);

  // Display status
  Serial.print("Light: ");
  Serial.print(lightPercent, 1);
  Serial.print("% | Servo Angle: ");
  Serial.print(servoAngle);
  Serial.print(" deg | Blinds: ");

  if (servoAngle < 45) {
    Serial.println("Fully Open");
  } else if (servoAngle < 135) {
    Serial.println("Partially Open");
  } else {
    Serial.println("Fully Closed");
  }

  delay(1000);
}

TipLearning Points

Observe:

• Sensor-Actuator Feedback Loop: Light sensor to decision logic to servo control
• PWM Servo Control: Pulse width (1000-2000us) determines angle (0-180 degrees)
• Map Function: Converts sensor range to actuator range
• Proportional Control: Servo angle proportional to light level
• Real-time Response: System continuously adjusts to changing conditions

Control Logic:

High Light (bright)  -> Angle 180 deg -> Blinds CLOSED (shade interior)
Medium Light         -> Angle 90 deg  -> Blinds HALF (moderate light)
Low Light (dark)     -> Angle 0 deg   -> Blinds OPEN (maximize daylight)

Real-World Applications:

• Automated Blinds/Curtains: Adjust based on time of day and sunlight
• Solar Panel Tracking: Follow sun position for maximum efficiency
• Smart Vents: Open/close based on temperature readings
• Robotic Arms: Position based on distance sensor feedback
• Camera Pan/Tilt: Track objects detected by motion sensors

Experiment:

• Add hysteresis: only move servo if light changes by >10%
• Implement smooth transitions instead of immediate movements
• Add manual override with buttons
• Create schedules: fully open in morning, close at noon
• Use temperature sensor to control ventilation servo

553.5 Temperature-Controlled Relay Switching

20 min | Intermediate | P06.C09.U05c

Relays in IoT: Relays are electrically operated switches that allow low-power microcontrollers to control high-power AC/DC devices safely. Essential for home automation and industrial control.

Relay Specifications:

• Control: 3.3V or 5V DC signal
• Switch: 10A @ 250VAC or 10A @ 30VDC (typical)
• Isolation: Optical/electrical isolation between control and load
• Types: SPST, SPDT, DPDT
• Response Time: 10-15ms

553.5.1 Application Example: Temperature-Controlled Fan

#include <DHT.h>

#define DHT_PIN 4
#define RELAY_PIN 13

#define TEMP_THRESHOLD_ON 28.0   // Turn fan ON above 28C
#define TEMP_THRESHOLD_OFF 26.0  // Turn fan OFF below 26C (hysteresis)

DHT dht(DHT_PIN, DHT22);
bool fanState = false;

void setup() {
  Serial.begin(115200);

  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW);  // Fan initially OFF

  dht.begin();

  Serial.println("Temperature-Controlled Fan System");
}

void loop() {
  float temperature = dht.readTemperature();
  float humidity = dht.readHumidity();

  if (isnan(temperature) || isnan(humidity)) {
    Serial.println("Failed to read from DHT sensor!");
    delay(2000);
    return;
  }

  // Control logic with hysteresis to prevent rapid switching
  if (!fanState && temperature > TEMP_THRESHOLD_ON) {
    // Turn fan ON
    fanState = true;
    digitalWrite(RELAY_PIN, HIGH);
    Serial.println("Temperature HIGH - Fan turned ON");
  }
  else if (fanState && temperature < TEMP_THRESHOLD_OFF) {
    // Turn fan OFF
    fanState = false;
    digitalWrite(RELAY_PIN, LOW);
    Serial.println("Temperature OK - Fan turned OFF");
  }

  // Display status
  Serial.print("Temp: ");
  Serial.print(temperature, 1);
  Serial.print("C | Humidity: ");
  Serial.print(humidity, 1);
  Serial.print("% | Fan: ");
  Serial.println(fanState ? "ON" : "OFF");

  delay(2000);
}

TipLearning Points

Observe:

• Hysteresis Control: Different ON/OFF thresholds (28C ON, 26C OFF) prevent rapid switching
• Relay Switching: GPIO pin controls relay coil, relay contacts switch high-power load
• Electrical Isolation: Relay physically separates low-voltage control from high-voltage load
• State Management: Boolean flag tracks fan state between loop iterations
• Safety: Microcontroller never directly touches high-voltage circuits

Hysteresis Explanation:

Temperature Rising: 24C -> 26C -> 28C -> [FAN TURNS ON]
Temperature Falling: 30C -> 28C -> 26C -> [FAN TURNS OFF]

Without Hysteresis (bad):
27.9C -> OFF, 28.1C -> ON, 27.9C -> OFF... (rapid switching damages relay!)

With Hysteresis (good):
Temperature must fall to 26C before turning off again

Real-World Applications:

• HVAC Control: Heating/cooling systems with thermostats
• Irrigation Systems: Pump control based on soil moisture
• Lighting Control: Turn on/off high-wattage lights
• Industrial Automation: Motor control, conveyor systems
• Safety Systems: Emergency shutoffs, fire suppression
• Appliance Automation: Coffee makers, heaters, refrigerators

Safety Considerations:

• Never work on AC circuits while powered
• Use proper gauge wire for current load
• Ensure relay rating exceeds load requirements
• Add flyback diodes for inductive loads (motors, solenoids)

Experiment:

• Add multiple temperature zones with different thresholds
• Implement time delays: keep fan on for minimum 5 minutes
• Add manual override button
• Log switching events to detect relay wear
• Implement “smart” scheduling: different thresholds for day/night

553.6 Sensor Selection Guide

15 min | Intermediate | P06.C09.U04

553.6.1 Comparison of Common Temperature Sensors

Sensor	Range	Accuracy	Interface	Power	Cost	Use Case
DHT22	-40 to 80C	0.5C	1-Wire digital	1.5mA	$	General purpose, humidity too
DS18B20	-55 to 125C	0.5C	1-Wire	1.5mA	$	Multiple sensors, waterproof
LM35	-55 to 150C	0.5C	Analog	60uA	$	Simple, low power
BME280	-40 to 85C	1C	I2C/SPI	3.6uA	\[ | Pressure, humidity, altitude | | TMP117 | -55 to 125C | 0.1C | I2C | 3.5uA | \]$	High precision applications

553.6.2 Sensor Interface Decision Tree

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flowchart TD
    Start[Sensor Selection Decision]
    Q1{Need multiple<br/>sensors?}
    Q2{Speed critical?<br/>MHz needed?}
    Q3{Just ON/OFF?}
    Q4{Digital output<br/>available?}

    I2C[I2C Protocol<br/>2 wires, multiple devices]
    SPI[SPI Protocol<br/>4 wires, high speed]
    Digital[Digital GPIO<br/>Simple HIGH/LOW]
    UART[UART/Serial<br/>Async communication]
    Analog[Analog ADC<br/>Voltage reading]

    Start --> Q1
    Q1 -->|Yes| Q2
    Q1 -->|No| Q3
    Q2 -->|Yes| SPI
    Q2 -->|No| I2C
    Q3 -->|Yes| Digital
    Q3 -->|No| Q4
    Q4 -->|Yes| UART
    Q4 -->|No| Analog

    style Start fill:#2C3E50,stroke:#16A085,color:#fff
    style I2C fill:#16A085,stroke:#2C3E50,color:#fff
    style SPI fill:#16A085,stroke:#2C3E50,color:#fff
    style Digital fill:#16A085,stroke:#2C3E50,color:#fff
    style UART fill:#16A085,stroke:#2C3E50,color:#fff
    style Analog fill:#16A085,stroke:#2C3E50,color:#fff
    style Q1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Q2 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Q3 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Q4 fill:#E67E22,stroke:#2C3E50,color:#fff

Figure 553.1: Sensor Interface Protocol Selection: Decision Tree for I2C, SPI, or Analog

553.7 Practice Exercises

NoteExercise 1: I2C Multi-Sensor Integration

Objective: Build a multi-sensor system using I2C bus and resolve address conflicts.

Scenario: Connect BMP280 (pressure, 0x76), BH1750 (light, 0x23), and two MPU6050 IMUs (both default to 0x68) to an ESP32.

Tasks:

1. I2C Address Scanning: Write code to scan I2C bus and detect all connected devices
2. Address Conflict Resolution: MPU6050 has AD0 pin for address selection (0x68 or 0x69). Modify hardware to use different addresses
3. Read All Sensors: Implement code to read from all 4 sensors in a single loop
4. Timing Analysis: Measure read time at 100kHz vs 400kHz I2C speed
5. Error Handling: Implement timeout and disconnection detection

Implementation Starter:

#include <Wire.h>

void scanI2C() {
    Serial.println("Scanning I2C bus...");
    for(byte addr = 1; addr < 127; addr++) {
        Wire.beginTransmission(addr);
        if (Wire.endTransmission() == 0) {
            Serial.printf("Found device at 0x%02X\n", addr);
        }
    }
}

// Exercise: Add code to read from all 4 sensors
// Exercise: Add error handling for sensor failures

NoteExercise 2: Power-Optimized Sensor Reading Strategy

Objective: Minimize power consumption for battery-powered sensor node.

Scenario: Battery-powered weather station (ESP32 + BME280 + BH1750) transmitting via LoRa every 15 minutes. Battery: 2000 mAh. Target lifetime: 1 year.

Tasks:

1. Power Budget Analysis: Calculate current consumption for each component
2. Lifetime Calculation: With sensors always on and ESP32 waking every 15 minutes, calculate battery lifetime
3. Sensor Power Management: Implement code to power sensors on/off using GPIO-controlled MOSFET
4. Buffering Strategy: Buffer 8 readings and transmit every 2 hours. Calculate power savings
5. Adaptive Sampling: If temperature changes < 0.5C in 1 hour, reduce sampling to 30 minutes

Implementation Starter:

#define SENSOR_POWER_PIN 13

void powerOnSensors() {
    digitalWrite(SENSOR_POWER_PIN, HIGH);
    delay(100);  // Sensor stabilization time
}

void powerOffSensors() {
    digitalWrite(SENSOR_POWER_PIN, LOW);
}

void loop() {
    powerOnSensors();
    // Read sensors
    float temp = bme.readTemperature();
    powerOffSensors();

    // Buffer and transmit logic
    esp_deep_sleep(15 * 60 * 1000000);  // 15 minutes
}

NoteExercise 3: Sensor Calibration and Data Validation Pipeline

Objective: Build complete data pipeline with calibration, filtering, validation, and anomaly detection.

Scenario: Industrial temperature monitoring using 10 DS18B20 sensors. Sensors require calibration and readings must be validated before sending to cloud.

Tasks:

1. Two-Point Calibration: Calibrate all 10 sensors using ice bath (0C) and boiling water (100C). Store calibration coefficients in EEPROM/SPIFFS
2. Data Validation: Implement checks:
   • Range validation: Reject readings outside -40C to 125C
   • Rate-of-change: Flag readings changing > 5C/minute (sensor malfunction)
   • Cross-sensor validation: If one sensor differs > 10C from average of others, flag as outlier
3. Median Filtering: Apply 5-sample median filter to each sensor to remove spikes
4. Anomaly Detection: If 3 consecutive readings from same sensor fail validation, send maintenance alert
5. Data Logging: Format validated data as JSON with timestamps, calibrated values, and quality flags

553.8 Summary

This chapter covered real-world sensor applications:

• Smart Home Monitoring: Multi-sensor environmental monitoring with MQTT integration
• Servo Control: Light-responsive automated blinds using PWM servo control
• Relay Switching: Temperature-controlled fan with hysteresis to prevent rapid cycling
• Sensor Selection: Comparison of common temperature sensors and interface selection
• Practical Patterns: Sensor-actuator feedback loops, state management, safety isolation

553.9 What’s Next

The next chapter provides a hands-on Sensor Calibration Lab using the Wokwi simulator, where you will implement two-point calibration, signal conditioning, and EEPROM storage.

NoteRelated Chapters

• Sensor Communication Protocols - I2C, SPI interfaces
• Sensor Data Processing - Filtering and calibration
• Sensor Power Management - Battery optimization
• Actuators - Motor and relay control