42 Light and Proximity Labs: Calibrating Sensors

sensors

sensor

proximity

light

42.1 Start With the Decision

Picture a cupboard light that should turn on when a hand reaches inside. It works at noon but fails at dusk because the test used one bright room and one clean target.

42.2 Route Overview

This is part 1 of 2. Continue with Light and Proximity Labs: Touch and Bus Integration.

42.3 Part Objectives

Evaluate light and proximity sensors with range, error, power, and interface evidence.
Evaluate hc-sr04 distance measurement with range, error, power, and interface evidence.

Start With the Measurement Story

Calibrate the Scene Before the Alarm

Picture a cupboard light that should turn on when a hand reaches inside. It works at noon but fails at dusk because the test used one bright room and one clean target. The circuit moved from a bench into a changing scene.

Begin with the action you need. State the closest and farthest target, the darkest and brightest light, and the delay a person will notice. General purpose input output (GPIO) pins are simple digital connections that read or set an on-or-off state. Inter-integrated circuit (I2C) is a shared wired link that lets a controller talk to addressed parts. Check the wiring and power before trusting either kind of reading.

Collect several readings at each edge of the scene. Move one thing at a time: distance, surface, angle, room light, or supply level. Choose a threshold only after the quiet readings and the real events form a clear gap. Then test a hand, an empty space, and a sudden light change.

One short lab cannot cover every material or room. Record the conditions and the uncertain zone. The deeper sections explain the sensor types, circuit choices, and fault checks needed to repeat the result.

A proximity and light lab turns distance and illumination into behavior. Start with the scene: what object or light source is present, how the sensor response changes, and what threshold or curve separates signal from noise.

42.4 In 60 Seconds

Light sensors (BH1750) measure ambient illumination in lux, PIR sensors detect motion via infrared radiation changes, and ultrasonic sensors (HC-SR04) measure distance using sound wave time-of-flight. Each uses a different interface (I2C, digital GPIO, trigger/echo pins) and requires specific setup considerations like pull-up resistors for I2C and temperature compensation for ultrasonic distance calculations.

Chapter Roadmap

Start With the Measurement Story
In 60 Seconds
Light and Proximity Sensors
Prerequisites
Light Sensors
Optional ESP32 Implementation
Try It: Lux Level Explorer
Learning Points: Light Sensing
Checkpoint: Light Thresholds
Proximity & Presence Sensors
Optional ESP32 Implementation
Try It: PIR Detection Zone Simulator
Learning Points: PIR Sensors
Checkpoint: Presence Evidence
Ultrasonic Measurement Flow
Optional ESP32 Implementation
Putting Numbers to It
Try It: Ultrasonic Distance Calculator
Learning Points: Ultrasonic Sensors
HC-SR04 Distance Measurement
First, separate calibrated light readings from analog LDR behavior so threshold choices match the sensor.
Then, test PIR and ultrasonic proximity as different kinds of presence evidence rather than interchangeable detectors.
Next, connect I2C bus checks and analog circuits to the wiring faults that make labs unreliable.
Finally, use the challenges, quizzes, and readout companion to turn working readings into evidence records.

Checkpoint callouts mark places to pause and confirm the story so far; deep-dive and practice sections can be skimmed on a first lab pass and revisited when you need the implementation detail.

42.5 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
Compare touch sensing technologies: Differentiate capacitive and skin-inspired tactile sensor architectures
Diagnose I2C communication: Scan buses, address multiple sensors, and resolve communication issues
Apply circuit fundamentals: Use voltage dividers and RC filters in sensor circuits

Light and Proximity Sensors

Light sensors measure how bright it is (like the automatic brightness on your phone screen), PIR motion sensors detect when a person walks by (like the sensor that turns on a porch light), and ultrasonic distance sensors measure how far away an object is by bouncing sound waves off it (like a bat navigating in the dark). These are some of the most commonly used sensors in smart home and security projects.

42.6 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

42.7 Light Sensors

A digital ambient-light sensor supplies illuminance evidence for brightness decisions rather than a motion or distance reading. The BH1750 example uses an I2C interface and reports the light level in lux. Automatic displays can increase brightness in daylight and use a more moderate setting indoors. The useful decision connects the measured lighting condition to the display or lamp behavior instead of treating a bus address as a light measurement.

42.7.1 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:

Start by Range: 1-65535 lux. Then Interface: I2C. Next Resolution: 1 lux. After that I2C Address: 0x23 (ADDR pin LOW/floating) or 0x5C (ADDR pin HIGH). Continue by Spectral response close to human eye. Finally Power: 120uA active, 0.01uA power down.

What to Observe Before Coding:

Lux Range	Real-World Meaning	Device Decision Example
Below 50 lux	Dark or very dim	Turn on night lighting
50-500 lux	Indoor lighting	Keep display brightness moderate
Above 2000 lux	Bright daylight	Increase display brightness or ignore PIR shadows

The mathematical gist. The chapter’s 3.3 V supply and 10 kΩ fixed resistor give $V_{out}=3.3R_{fixed}/(R_{LDR}+R_{fixed})$ . Its bright, midpoint, and dark cases therefore map 1 kΩ, 10 kΩ, and 1 MΩ to 3.00 V (code 3723), 1.65 V (code 2048), and 0.0327 V (code 41). The same lab keeps its 50 cm proximity threshold empirical because target colour, angle, sunlight, and optics break a universal inverse-square calibration.

Optional ESP32 Implementation

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

BH1750 lightMeter;

void setup() {
  Serial.begin(115200);
  Wire.begin();  // Uses default ESP32 I2C pins: SDA=GPIO21, SCL=GPIO22

  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 (approximate ranges)
  String classification;
  if (lux < 1) {
    classification = "Darkness (moonlight)";
  } else if (lux < 50) {
    classification = "Very dim (street lighting)";
  } else if (lux < 200) {
    classification = "Dim (hallway)";
  } else if (lux < 500) {
    classification = "Normal (home/office)";
  } else if (lux < 2000) {
    classification = "Bright (retail/overcast)";
  } else if (lux < 25000) {
    classification = "Very bright (daylight)";
  } else {
    classification = "Extremely bright (direct sun)";
  }

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

  delay(1000);
}

Try It: Lux Level Explorer

Adjust the lux reading to see how the BH1750 classifies different light levels. Compare with the reference table below to understand real-world lighting conditions.

Learning Points: Light Sensing

A light reading supports different actions as the available natural illumination changes during the day. Street lighting can dim when natural light is sufficient, while a display adjusts backlighting for its viewing conditions. A greenhouse can supplement natural illumination with artificial light when the crop needs it. The energy benefit comes from avoiding unnecessary full brightness rather than collecting lux values without changing the load.

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:

Start with Smart street lights: Dim when natural light is sufficient. Then Display backlighting: Adjust screen brightness based on ambient light. Next Greenhouse automation: Supplement natural light with artificial lighting. Finally Energy efficiency: Reduce power when full brightness isn’t needed.

Checkpoint: Light Thresholds

You now know:

A BH1750 reports calibrated lux over I2C, while an LDR needs a divider and ADC interpretation.
Thresholds are decisions about scenes: dark, indoor, daylight, and direct sunlight lead to different device actions.
Hysteresis belongs in the control rule when one threshold would make an output flicker.

The light lab established how illumination becomes a stable decision. The next question is how a device should notice a nearby person or object when brightness is not the signal.

42.8 Proximity & Presence Sensors

42.8.1 PIR Motion Sensor (HC-SR501)

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

Different bodies can provide the same PIR motion capability: the module, Fresnel lens, adjustment controls, terminals, and enclosure change how the sensor is mounted and commissioned without changing the fact that it reports infrared change rather than identity or distance.

Exposed PIR development module with Fresnel lens and adjustment controls — An adjustable development module leaves the board, sensitivity control, delay control, and header exposed. It is easy to tune on a bench, but those same exposed parts need protection and stable orientation in a deployed product. Photo: Sjilla, CC BY-SA 4.0

Compact PIR board module with a small white Fresnel dome and pin interface — A compact dome module packages the same passive-infrared principle into a smaller board-level body. Its lens still defines optical zones, while the pins and mounting position define how reliably those zones face the intended scene. Photo: Kszapsza, CC BY-SA 4.0

Disassembled flush wall PIR detector showing its housing, Fresnel lens, circular circuit board, and terminals — A flush architectural detector integrates lens, circular PCB, terminals, and wall hardware as one installation system. The enclosure changes the service and coverage geometry, while the detector still responds only to changing thermal-infrared zones. Photo: Dmitry G, CC BY-SA 3.0

Enclosed wall-mounted security PIR detector with a broad lens window and indicator — An enclosed security detector presents a finished wall-mount body with a broad lens window and status indicator. It hides the electronics from the learner, so commissioning evidence must record height, aim, warm-up, sensitivity, and the scene it covers. Photo: Geom, CC BY-SA 4.0

Read across the four forms from exposed prototype to finished wall unit. The lens and housing determine optical zones, adjustment access, weather and tamper protection, and mounting repeatability; none of them changes the PIR evidence boundary examined next.

Specifications:

Detection Range: 3-7 meters (adjustable)
Detection Angle: 110 degree cone
Output: Digital HIGH when motion detected
Hold Time: ~2.5 to 200 seconds (adjustable via potentiometer)
Power: 5V, ~65µA quiescent
Trigger Modes: Single trigger or repeatable trigger

What to Observe Before Coding:

PIR sensors do not identify people. They detect changes in infrared energy across their detection zones. In the simulator below, focus on how distance, angle, and environment affect false triggers.

Optional 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);
  pinMode(LED_BUILTIN, OUTPUT);

  Serial.println("PIR Motion Sensor Test");
  Serial.println("Warming up sensor (30-60 seconds)...");
  delay(60000);  // PIR needs 30-60s 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);
}

Try It: PIR Detection Zone Simulator

Configure the PIR sensor parameters to visualize its detection zone. Adjust mounting height, sensitivity, and hold time to see how they affect coverage.

Learning Points: PIR Sensors

A PIR motion sensor provides a digital state for a motion-triggered light or alert. The sensor needs a warm-up interval after power-on before the test should judge its normal behavior. Sensitivity and hold-time controls change detection and output persistence rather than converting the output into an illuminance reading. A useful test connects motion in the detection area to the expected light or security response.

Key Characteristics:

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

Real-World Applications:

Start by Smart lighting: Turn on lights when someone enters a room. Then Security systems: Send alerts when motion detected while away. Next Energy saving: Power down devices when no one is present. Finally Occupancy counting: Track room usage patterns.

Checkpoint: Presence Evidence

You now know:

PIR output is a digital motion signal, so it is good for presence changes but not for distance.
Mounting height, hold time, trigger mode, and environment all shape whether a motion event is useful evidence.
A warm-up period and confirmation delay are part of the lab design, not optional cleanup.

PIR answers whether motion changed in a detection zone. Ultrasonic ranging answers a different question: how far away the reflecting surface is, and whether the timing math still holds under the current air temperature.

42.8.2 Ultrasonic Distance Sensor (HC-SR04)

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

Before applying the echo-time equation, inspect Figure to assign the module’s two acoustic paths and four electrical pins. Distance is inferred from a timed round trip, not read directly by either metal can.

HC-SR04 ultrasonic distance-sensor board with two round metal transducers and four labeled connection pins — The HC-SR04's two metal cans are separate 40 kHz transmitter and receiver transducers. The four-pin header exposes power, trigger, and echo; that echo output and the module's blind zone are the electrical and geometric constraints the timing calculation must respect. Photo: Suyash Dwivedi, CC BY-SA 4.0

Read Figure, identify the transmitter and receiver first, then trace trigger and echo at the header alongside power and ground. The outgoing 40 kHz burst and returning echo define the measured interval, connecting target geometry, blind zone, and sound speed to the distance calculation.

Specifications:

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

The HC-SR04 operates at 5V logic, but the ESP32 GPIO pins are 3.3V. The ECHO pin outputs a 5V HIGH signal that can damage the ESP32. Use a voltage divider (two resistors, e.g., 1kOhm + 2kOhm) on the ECHO line to reduce it to ~3.3V, or use a 3.3V-compatible variant like the HC-SR04P.

What to Observe Before Coding:

The HC-SR04 measures time, not distance directly. The microcontroller sends a trigger pulse, waits for the echo pulse, then converts travel time into distance using the speed of sound.

Ultrasonic Measurement Flow

An ultrasonic distance measurement starts with a trigger and uses the returning echo pulse as timing evidence. The measured echo interval includes the outward and return travel of the sound. The distance calculation divides that round-trip path by two to obtain the one-way range. Temperature affects the speed of sound, so compensation changes the conversion from elapsed time to the reported distance.

Start by Send a 10 microsecond trigger pulse. Then Measure the echo pulse width. Finally Convert round-trip echo time into one-way distance using the speed of sound.

Optional 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() {
  float distance = measureDistance();

  Serial.print("Distance: ");
  Serial.print(distance, 1);
  Serial.println(" cm");

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

  delay(100);
}

float 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 at 20°C)
  // distance = (duration * 0.0343) / 2
  float distance = duration * 0.0343 / 2.0;

  return distance;
}

Putting Numbers to It

Ultrasonic sensors measure distance using the time-of-flight formula where sound travels to the object and back.

d = \frac{t \times v_{\text{sound}}}{2}

where $d$ is distance, $t$ is echo pulse duration in microseconds, and $v_{\text{sound}}$ is 343 m/s (0.0343 cm/µs) at 20°C.

Worked example: The HC-SR04 measures an echo pulse duration of 1,750 µs. Calculate the distance:

d = \frac{1750 \, \mu\text{s} \times 0.0343 \, \text{cm/}\mu\text{s}}{2} = \frac{60.025}{2} = 30.01 \, \text{cm}

Temperature compensation: At 0°C, speed of sound is 331.3 m/s (3.4% slower), so the same 1,750 µs pulse would calculate as 29.0 cm instead of 30.0 cm, introducing 1 cm error per 30 cm distance without compensation.

Run it: Before you read echo time as distance, switch the Light and Proximity Sensing Workbench to Proximity mode. Set an object distance and an air temperature and watch the speed of sound (v = 331.3 + 0.606 x T) and the round-trip echo time update together, so a warmer day visibly shortens the echo for the same distance. Then move the detect and release thresholds and toggle hysteresis to see the dead band absorb chatter as an object approaches the stop line.

Try It: Ultrasonic Distance Calculator

Adjust the echo pulse duration and ambient temperature to see how they affect the calculated distance. Notice how temperature changes the speed of sound and therefore the distance measurement.

Learning Points: Ultrasonic Sensors

Key Concepts:

Start with Time-of-Flight: distance = (time ** speed_of_sound) / 2. Then Speed of Sound: 343 m/s at 20°C (0.0343 cm/µs). Next Temperature Compensation: Speed varies ~0.6 m/s per °C. Finally Blind zone: Cannot measure objects closer than 2cm.

Real-World Applications:

Start by Parking sensors: Alert drivers to obstacles. Then Robotics: Obstacle avoidance and navigation. Next Liquid level sensing: Measure tank fill levels. Finally People counting: Detect presence at doorways.

Temperature Compensation: Sound travels faster in warmer air. Add about 0.606 m/s to the 331.3 m/s baseline for each degree Celsius, then divide the round-trip echo distance by two.

HC-SR04 Distance Measurement

The HC-SR04 uses time-of-flight measurement with these precise steps:

Start with Send trigger pulse - 10µs HIGH signal on TRIG pin activates the sensor. Then Sensor emits ultrasound - 8 cycles of 40 kHz sound burst (inaudible to humans). Next Wait for echo - ECHO pin goes HIGH when sound is transmitted. After that Measure echo duration - ECHO pin goes LOW when reflected sound returns. Finally Calculate distance - distance_cm = (echo_duration_µs × 0.0343) / 2.

Why divide by 2? The sound travels to the object AND back, so the total time covers twice the distance.

42.9 Continue to the Next Part

Carry this evidence into Light and Proximity Labs: Touch and Bus Integration, which begins with Touch and Skin Sensing.