530  Light and Proximity Sensor Labs

530.1 Learning Objectives

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

• Interface light sensors: Configure BH1750 digital lux meter and LDR analog sensors
• Build proximity detection: Use PIR motion sensors and HC-SR04 ultrasonic distance sensors
• Understand touch sensing: Learn capacitive and skin-inspired tactile sensor technologies
• Master I2C communication: Scan buses, address multiple sensors, and debug communication issues
• Apply circuit fundamentals: Use voltage dividers, RC filters, and transistor switching in sensor circuits

530.2 Prerequisites

Required Knowledge:

• Motion & Environmental Sensors - I2C basics
• Electronics Basics - Circuit fundamentals
• Sensor Circuits - Signal conditioning

Hardware Requirements:

• ESP32 development board
• BH1750 light sensor or LDR (photoresistor)
• PIR motion sensor (HC-SR501)
• HC-SR04 ultrasonic distance sensor
• Resistors (10kOhm for voltage divider, 4.7kOhm for I2C)
• Breadboard and jumper wires

530.3 Light Sensors

530.3.1 BH1750 (Digital Light Intensity)

The BH1750 is a digital ambient light sensor with spectral response close to the human eye, making it ideal for automatic brightness adjustment.

Specifications:

• Range: 1-65535 lux
• Interface: I2C
• Resolution: 1 lux
• Spectral response close to human eye
• Power: 120uA active, 0.01uA power down

ESP32 Implementation:

#include <Wire.h>
#include <BH1750.h>

BH1750 lightMeter;

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

  if (lightMeter.begin(BH1750::CONTINUOUS_HIGH_RES_MODE)) {
    Serial.println("BH1750 initialized");
  } else {
    Serial.println("Error initializing BH1750");
  }
}

void loop() {
  float lux = lightMeter.readLightLevel();

  Serial.print("Light: ");
  Serial.print(lux);
  Serial.print(" lux");

  // Classify light levels
  String classification;
  if (lux < 1) {
    classification = "Darkness";
  } else if (lux < 50) {
    classification = "Very dim (moonlight)";
  } else if (lux < 200) {
    classification = "Dim (hallway)";
  } else if (lux < 400) {
    classification = "Normal (office)";
  } else if (lux < 1000) {
    classification = "Bright (retail)";
  } else if (lux < 10000) {
    classification = "Very bright (overcast day)";
  } else {
    classification = "Extremely bright (direct sun)";
  }

  Serial.print(" - ");
  Serial.println(classification);

  delay(1000);
}

TipLearning Points: Light Sensing

Lux Reference Values:

Condition	Lux Level
Full moon	0.1-1
Street lighting	10-50
Home lighting	150-300
Office	300-500
Overcast sky	1,000-2,000
Full daylight	10,000-25,000
Direct sunlight	100,000+

Real-World Applications: - Smart street lights: Dim when natural light is sufficient - Display backlighting: Adjust screen brightness based on ambient light - Greenhouse automation: Supplement natural light with artificial lighting - Energy efficiency: Reduce power when full brightness isn’t needed

530.4 Proximity & Presence Sensors

530.4.1 PIR Motion Sensor (HC-SR501)

PIR (Passive Infrared) sensors detect motion by measuring changes in infrared radiation from warm bodies (humans, animals).

Specifications:

• Detection Range: 3-7 meters (adjustable)
• Detection Angle: 110 degree cone
• Output: Digital HIGH when motion detected
• Delay Time: 0.3 to 200 seconds (adjustable)
• Power: 5V, 65mA
• Trigger Modes: Single trigger or repeatable trigger

ESP32 Implementation:

#define PIR_PIN 13  // GPIO13 for PIR sensor

bool motionDetected = false;
unsigned long lastMotionTime = 0;

void setup() {
  Serial.begin(115200);
  pinMode(PIR_PIN, INPUT);

  Serial.println("PIR Motion Sensor Test");
  Serial.println("Warming up sensor (60 seconds)...");
  delay(60000);  // PIR needs 60s warm-up time
  Serial.println("Ready!");
}

void loop() {
  int pirState = digitalRead(PIR_PIN);

  if (pirState == HIGH) {
    if (!motionDetected) {
      motionDetected = true;
      lastMotionTime = millis();

      Serial.println("MOTION DETECTED!");
      Serial.print("Time: ");
      Serial.println(millis() / 1000);

      // Trigger action (turn on light, send alert, etc.)
      digitalWrite(LED_BUILTIN, HIGH);
    }
  } else {
    if (motionDetected) {
      unsigned long motionDuration = (millis() - lastMotionTime) / 1000;
      Serial.print("Motion ended. Duration: ");
      Serial.print(motionDuration);
      Serial.println(" seconds");

      motionDetected = false;
      digitalWrite(LED_BUILTIN, LOW);
    }
  }

  delay(100);
}

TipLearning Points: PIR Sensors

Key Characteristics: - PIR sensors need a 30-60 second warm-up period after power-on - Output is digital (HIGH/LOW), making interfacing very simple - The sensor stays HIGH as long as motion is detected - Adjustable potentiometers control sensitivity and hold time

Real-World Applications: - Smart lighting: Turn on lights when someone enters a room - Security systems: Send alerts when motion detected while away - Energy saving: Power down devices when no one is present - Occupancy counting: Track room usage patterns

530.4.2 Ultrasonic Distance Sensor (HC-SR04)

Ultrasonic sensors measure distance by timing the echo return of a 40kHz sound pulse.

Specifications:

• Range: 2cm to 400cm
• Accuracy: +/-3mm
• Measuring angle: 15 degrees
• Trigger pulse: 10us
• Power: 5V, 15mA

ESP32 Implementation:

#define TRIG_PIN 5
#define ECHO_PIN 18

void setup() {
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
}

void loop() {
  long distance = measureDistance();

  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");

  // Object detection
  if (distance > 0 && distance < 50) {
    Serial.println("Object detected nearby!");
  }

  delay(100);
}

long measureDistance() {
  // Send 10us pulse
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Measure echo pulse duration
  long duration = pulseIn(ECHO_PIN, HIGH, 30000);  // 30ms timeout

  // Calculate distance (speed of sound = 343 m/s)
  // distance = (duration * 0.0343) / 2
  long distance = duration * 0.034 / 2;

  return distance;
}

TipLearning Points: Ultrasonic Sensors

Key Concepts: - Time-of-Flight: distance = (time * speed_of_sound) / 2 - Speed of Sound: 343 m/s at 20C (0.0343 cm/us) - Temperature Compensation: Speed varies ~0.6 m/s per degree C - Blind zone: Cannot measure objects closer than 2cm

Real-World Applications: - Parking sensors: Alert drivers to obstacles - Robotics: Obstacle avoidance and navigation - Liquid level sensing: Measure tank fill levels - People counting: Detect presence at doorways

Temperature Compensation Formula:

float speed_of_sound = 331.3 + (0.606 * temp_celsius);  // m/s
float distance_cm = (pulse_duration_us * speed_of_sound) / 20000;

530.5 Touch and Skin Sensing

Figure 530.1: Skin-inspired touch sensing technology

Figure 530.2: SKIN sensor array configuration

Figure 530.3: SKIN sensing mechanics and response

Figure 530.4: Skin mechanics and tactile sensing

530.6 I2C Sensors and Bus Management

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sequenceDiagram
    participant MCU as ESP32/Arduino<br/>Master
    participant SDA as SDA Line<br/>Data
    participant SCL as SCL Line<br/>Clock
    participant Sensor as I2C Sensor<br/>Slave (0x76)

    Note over MCU,Sensor: I2C Read Transaction (BMP280 Temperature)

    MCU->>SDA: START condition
    MCU->>SCL: Generate clock pulses
    MCU->>SDA: Device address (0x76) + WRITE
    Sensor->>SDA: ACK

    MCU->>SDA: Register address (0xFA)
    Sensor->>SDA: ACK

    MCU->>SDA: REPEATED START
    MCU->>SDA: Device address (0x76) + READ
    Sensor->>SDA: ACK

    Sensor->>SDA: Data byte 1 (MSB)
    MCU->>SDA: ACK
    Sensor->>SDA: Data byte 2 (LSB)
    MCU->>SDA: NACK

    MCU->>SDA: STOP condition

    Note over MCU,Sensor: Multiple sensors on same bus<br/>BMP280 (0x76), MPU6050 (0x68), BH1750 (0x23)

Figure 530.5: I2C Communication Protocol: Master-Slave Transaction Sequence

I2C Bus Scanner:

#include <Wire.h>

#define I2C_SDA 21
#define I2C_SCL 22

void setup() {
  Serial.begin(115200);
  Wire.begin(I2C_SDA, I2C_SCL);

  // Scan I2C bus
  scanI2C();
}

void scanI2C() {
  Serial.println("Scanning I2C bus...");
  int devices = 0;

  for(byte address = 1; address < 127; address++) {
    Wire.beginTransmission(address);
    byte error = Wire.endTransmission();

    if (error == 0) {
      Serial.print("I2C device found at 0x");
      if (address < 16) Serial.print("0");
      Serial.println(address, HEX);
      devices++;
    }
  }

  Serial.print("Found ");
  Serial.print(devices);
  Serial.println(" devices");
}

void loop() {
  // Empty loop
}

530.7 Circuit Fundamentals for Sensors

530.7.1 Voltage Divider with LDR

For analog light sensors (LDR/photoresistor), use a voltage divider circuit:

VCC (3.3V) ----[LDR]----+----[10kOhm]---- GND
                        |
                      ADC Pin

How it works: - LDR resistance decreases with more light (200 Ohm bright, 10k Ohm dark) - Voltage at midpoint varies with light level - ADC reads this varying voltage

530.7.2 RC Low-Pass Filter

Filter high-frequency noise from sensor readings:

Sensor Output ----[10kOhm]----+---- ADC Pin
                              |
                         [100nF]
                              |
                             GND

Cutoff Frequency: f_c = 1 / (2 * PI * R * C) = 159 Hz

TipLearning Points: Circuit Fundamentals

Voltage Divider Formula:

V_out = V_in * (R2 / (R1 + R2))

RC Filter Time Constant:

tau = R * C
5*tau = time to reach steady state

Why Use Pull-up Resistors for I2C: I2C uses open-drain outputs that can only pull LOW. Pull-up resistors (4.7k Ohm typical) pull the line HIGH when no device is transmitting.

530.8 Interactive Simulator: Sensor-Controlled Servo

Try it yourself! See how sensors control actuators for automated systems.

What This Simulates: An LDR light sensor controlling a servo motor to automatically adjust window blinds based on sunlight.

How to Use: 1. Click Start Simulation 2. Click on the LDR sensor to adjust light levels 3. Watch the servo motor rotate as light changes 4. Observe the Serial Monitor showing light levels and servo angles

NoteLearning Points: Sensor-Actuator Integration

Control Logic:

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

Real-World Applications: - Automated blinds/curtains based on time of day and sunlight - Solar panel tracking following sun position - Smart vents opening/closing based on temperature - Camera pan/tilt tracking objects detected by motion sensors

530.9 Knowledge Check

Question 1: Why are pull-up resistors required for I2C communication?

Explanation: I2C devices use open-drain outputs. They can pull the line LOW but cannot drive it HIGH. Pull-up resistors (typically 4.7k Ohm) are needed to pull the line HIGH when no device is transmitting.

Question 2: Your smart parking system uses ultrasonic sensors (HC-SR04). In summer (35C), the sensor reports cars 10cm closer than reality. In winter (0C), cars appear 10cm farther. What’s wrong?

Explanation: Speed of sound varies with temperature: v = 331.3 + (0.606 * T) m/s. At 0C: 331.3 m/s. At 35C: 352.5 m/s (+6.4%). Without temperature compensation, distance calculations will be incorrect. Solution: Add a temperature sensor and compensate in software.

Question 3: You’re choosing between a thermistor (NTC, analog output) and DS18B20 (digital, 1-Wire) for an outdoor weather station. The ESP32 is in a weatherproof box, sensor is 10m away on exposed pole. Which is better and why?

Explanation: Digital sensors are far superior for long-distance deployment due to noise immunity. Over 10m cable, analog signals pick up EMI, suffer voltage drops, and ground potential differences. Digital 1-Wire sends binary data that’s either correctly received or rejected via CRC.

530.10 Summary

This chapter covered light and proximity sensor implementation:

• BH1750 provides calibrated lux measurements with spectral response matching human vision
• PIR sensors detect motion via infrared radiation changes with simple digital output
• Ultrasonic HC-SR04 measures distance using time-of-flight with temperature compensation needed
• I2C bus scanning identifies connected sensors and verifies communication
• Voltage dividers convert variable resistance (LDR) to measurable voltage
• RC filters reduce high-frequency noise from analog sensor readings

530.11 What’s Next?

Continue to best practices and hands-on labs for complete sensor implementation guidance.

Continue to Best Practices & Labs →

530.12 See Also

• Temperature Sensor Labs - Temperature sensing fundamentals
• Motion & Environmental Sensors - IMU and barometric sensing
• Sensor Calibration Lab - Hands-on calibration techniques