Sensors - PhysisLab

Overview

PhysisLab uses various sensors for physics measurements including IR sensors for free fall detection, ultrasonic sensors for distance measurement, and laser-based ToF sensors for high-precision ranging.

IR Sensors (Free Fall Detection)

Overview

Infrared (IR) break-beam sensors are used in the free fall experiment to detect when an object passes through specific points, enabling precise time-of-flight measurements.

Specifications

Type: Reflective or break-beam IR sensor
Operating Voltage: 3.3V - 5V
Output: Digital (HIGH when blocked, LOW when clear)
Response Time: < 1 ms
Detection Range: 2-30 cm (depending on model)

Wiring Configuration

From freeFall3Sensores.ino:3-5:

#define PIN_INICIO      2   // INT0
#define PIN_INTERMEDIO  3   // INT1  
#define PIN_FINAL       4   // PCINT20 (PORTD)

Arduino Connection:

IR Sensor 1 (START)       → Arduino Pin 2 (with internal pull-up)
IR Sensor 2 (INTERMEDIATE)→ Arduino Pin 3 (with internal pull-up)
IR Sensor 3 (FINAL)       → Arduino Pin 4 (with internal pull-up)

ESP32 Connection:

IR Sensor START → GPIO 18 (with internal pull-up)
IR Sensor END   → GPIO 5  (with internal pull-up)

Setup Code (Arduino)

From freeFall3Sensores.ino:78-103:

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

  pinMode(PIN_INICIO, INPUT_PULLUP);
  pinMode(PIN_INTERMEDIO, INPUT_PULLUP);
  pinMode(PIN_FINAL, INPUT_PULLUP);

  // Interrupciones externas
  attachInterrupt(digitalPinToInterrupt(PIN_INICIO), isrInicio, FALLING);
  attachInterrupt(digitalPinToInterrupt(PIN_INTERMEDIO), isrIntermedio, FALLING);

  // PCINT para PIN_FINAL
  PCICR  |= (1 << PCIE2);
  PCMSK2 |= (1 << PCINT20);

  Serial.println("Sistema listo: 3 PIR con seguridad y Serial");
}

Interrupt Handling

From freeFall3Sensores.ino:30-48:

// PIR INICIO
void isrInicio() {
  if (estado == ESPERANDO_INICIO) {
    noInterrupts();
    contadorMs = 0;       // reinicia conteo
    tInicio = 0;
    estado = ESPERANDO_INTERMEDIO;
    interrupts();
  }
}

// PIR INTERMEDIO
void isrIntermedio() {
  if (estado == ESPERANDO_INTERMEDIO) {
    noInterrupts();
    tIntermedio = contadorMs;
    estado = ESPERANDO_FINAL;
    interrupts();
  }
}

Typical Values

Separation Distance (S1): 0.68 m (start to intermediate)
Separation Distance (S2): 1.75 m (start to final)
Time Resolution: 1 ms (using Timer1)
Expected Fall Time: ~370 ms for 0.68 m drop

VL53L0X Laser Distance Sensor

Overview

The VL53L0X is a Time-of-Flight (ToF) laser-ranging sensor that provides accurate distance measurements from 30mm to 2000mm.

Specifications

Range: 30 mm - 2000 mm
Accuracy: ±3% typical
Field of View: 25°
Measurement Rate: Up to 50 Hz
Interface: I2C (400 kHz)
Operating Voltage: 2.6V - 3.5V
I2C Address: 0x29 (default)

Pin Connections

From VL53_filtros.ino:100:

Wire.begin(21, 22);  // SDA = GPIO21, SCL = GPIO22

VL53L0X Pin  →  ESP32 Pin
-------------------------
VIN          →  3.3V
GND          →  GND
SDA          →  GPIO 21
SCL          →  GPIO 22
GPIO1 (INT)  →  GPIO 27

Initialization Code

From VL53_filtros.ino:98-116:

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);

  pinMode(VL53_INT_PIN, INPUT_PULLUP);

  if (!lox.begin()) {
    Serial.println("Error VL53L0X");
    while (1);
  }

  lox.configSensor(Adafruit_VL53L0X::VL53L0X_SENSE_HIGH_ACCURACY);

  attachInterrupt(digitalPinToInterrupt(VL53_INT_PIN), vl53ISR, FALLING);

  timer = timerBegin(0, 80, true);           // 1 µs tick
  timerAttachInterrupt(timer, &onTimer, true);
  timerAlarmWrite(timer, 200 * 1000, true);  // 200 ms
  timerAlarmEnable(timer);
}

Reading Distance

From VL53_filtros.ino:126-141:

VL53L0X_RangingMeasurementData_t measure;
lox.rangingTest(&measure, false);

if (measure.RangeStatus == 0) {
  // Tiempo
  tNow = micros();
  float dt = (tPrev == 0) ? 0.2 : (tNow - tPrev) * 1e-6;
  tPrev = tNow;

  // Distancia cruda (convertir mm a cm)
  float raw = measure.RangeMilliMeter / 10.0;
  
  // Aplicar filtros...
}

Signal Filtering

The VL53L0X readings are filtered using three different methods:

1. Exponential Moving Average (EMA)

From VL53_filtros.ino:33-36, 146-151:

const float alphaEMA = 0.2;
float ema = 0.0;

if (!emaInit) {
  ema = raw;
  emaInit = true;
} else {
  ema = alphaEMA * raw + (1.0 - alphaEMA) * ema;
}

2. Butterworth Low-Pass Filter (2nd Order)

From VL53_filtros.ino:40-50, 78-95:

// Butterworth 2º orden (fs=5Hz, fc≈0.8Hz)
const float b0 = 0.2066;
const float b1 = 0.4131;
const float b2 = 0.2066;
const float a1 = -0.3695;
const float a2 = 0.1958;

float butterworth(float x) {
  if (!bwInit) {
    bw_x1 = bw_x2 = x;
    bw_y1 = bw_y2 = x;
    bwInit = true;
    return x;
  }

  float y = b0*x + b1*bw_x1 + b2*bw_x2
            - a1*bw_y1 - a2*bw_y2;

  bw_x2 = bw_x1;
  bw_x1 = x;
  bw_y2 = bw_y1;
  bw_y1 = y;

  return y;
}

3. Alpha-Beta (α-β) Filter

From VL53_filtros.ino:53-59, 161-170:

float alphaAB = 0.85;
float betaAB  = 0.02;
float posAB = 0.0;
float velAB = 0.0;

if (!abInit) {
  posAB = raw;
  velAB = 0.0;
  abInit = true;
} else {
  float pred = posAB + velAB * dt;
  float err  = raw - pred;
  posAB = pred + alphaAB * err;
  velAB = velAB + (betaAB * err) / dt;
}

HC-SR04 Ultrasonic Sensor

Overview

The HC-SR04 is an ultrasonic distance sensor that uses sonar to determine distance to an object (2cm to 400cm).

Specifications

Range: 2 cm - 400 cm
Accuracy: ±3 mm
Measuring Angle: 15°
Ultrasonic Frequency: 40 kHz
Operating Voltage: 5V
Trigger Pulse: 10 μs (minimum)

Pin Connections

HC-SR04 Pin  →  ESP32 Pin
--------------------------
VCC          →  5V
TRIG         →  GPIO 18
ECHO         →  GPIO 19 (via level shifter or voltage divider)
GND          →  GND

Important: The ECHO pin outputs 5V. Use a voltage divider or level shifter to protect the ESP32 GPIO (3.3V max).Voltage divider:

R1 = 1kΩ (from ECHO to GPIO)
R2 = 2kΩ (from GPIO to GND)

Pin Definitions

From ultraSonido_filtros.ino:4-7:

#define TRIG_PIN 18
#define ECHO_PIN 19

Initialization

From ultraSonido_filtros.ino:109-121:

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

  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);

  attachInterrupt(digitalPinToInterrupt(ECHO_PIN), echoISR, CHANGE);

  timer = timerBegin(0, 80, true); // 1 µs tick
  timerAttachInterrupt(timer, &onTimer, true);
  timerAlarmWrite(timer, TRIGGER_INTERVAL_MS * 1000, true);
  timerAlarmEnable(timer);
}

Measurement Process

Trigger Signal

From ultraSonido_filtros.ino:81-85:

void IRAM_ATTR onTimer() {
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
}

Echo Detection

From ultraSonido_filtros.ino:69-76:

void IRAM_ATTR echoISR() {
  if (digitalRead(ECHO_PIN) == HIGH) {
    echoStart = micros();
  } else {
    echoEnd = micros();
    echoReceived = true;
  }
}

Distance Calculation

From ultraSonido_filtros.ino:24, 143:

const float SOUND_SPEED = 0.0343;   // cm/us

unsigned long duration = echoEnd - echoStart;
float raw = (duration * SOUND_SPEED) / 2.0;

Signal Filtering

The ultrasonic sensor uses the same three filtering methods as the VL53L0X:

EMA (α = 0.3)
Butterworth (fs=10Hz, fc≈1Hz)
α-β Filter (α = 0.85, β = 0.05)

From ultraSonido_filtros.ino:40-64:

Sample Output

From ultraSonido_filtros.ino:186-197:

Serial.print("tm,");
Serial.print(dt,3);
Serial.print("RAW,");
Serial.print(raw,2);
Serial.print("EMA,");
Serial.print(ema,2);
Serial.print("BW,");
Serial.print(bw,2);
Serial.print("POSAB,");
Serial.print(posAB,2);
Serial.print("Vel,");
Serial.println(velAB,2);

Sensor Calibration

VL53L0X Calibration

High Accuracy Mode: Use VL53L0X_SENSE_HIGH_ACCURACY for best precision
Sampling Rate: 5 Hz (200 ms intervals) for stable readings
Filter Tuning: Adjust α values based on motion speed

Ultrasonic Sensor Calibration

Minimum Range: Keep objects at least 5 cm away
Sampling Rate: 10 Hz (100 ms intervals) recommended
Temperature Compensation: Sound speed varies with temperature (~0.6 m/s per °C)

// Adjust for temperature
float temp_celsius = 20.0;  // measure actual temperature
float sound_speed = 331.3 + (0.606 * temp_celsius);  // m/s
float sound_speed_cm_us = sound_speed / 10000.0;     // cm/us

IR Sensor Calibration

Alignment: Ensure sensors are perpendicular to falling object path
Height Measurement: Measure actual distance between sensors precisely
Threshold Adjustment: Tune detection sensitivity to avoid false triggers

Troubleshooting

VL53L0X Issues

Problem: Sensor not detected (begin() fails)

Check I2C wiring (SDA/SCL)
Verify 3.3V power supply
Try different I2C speed: Wire.setClock(100000) for 100 kHz

Problem: Noisy readings

Increase sampling interval (reduce rate)
Use Butterworth filter for smoothing
Avoid reflective or transparent surfaces

HC-SR04 Issues

Problem: Reading 0 or timeout

Check TRIG pulse is at least 10 μs
Verify ECHO pin voltage level (should be 3.3V after level shifter)
Ensure object is within 2-400 cm range

Problem: Unstable readings

Add validation: ignore readings < 2 cm or > 400 cm
Use median filter for outlier rejection
Check for acoustic interference

IR Sensor Issues

Problem: Missed detections

Increase pull-up resistor to 10kΩ
Add debouncing (10-50 ms delay)
Check alignment and focus

Problem: False triggers

Shield from ambient IR sources (sunlight, incandescent bulbs)
Add hysteresis to detection logic
Use interrupt-driven detection

Best Practices

Power Supply: Use stable, low-noise 3.3V/5V supply
Filtering: Always filter sensor data for physics measurements
Sampling Rate: Match to expected motion speed (faster motion = higher rate)
Validation: Implement range checks to reject invalid readings
Calibration: Measure and verify physical distances with ruler/caliper

Components

Circuits

​Overview

​IR Sensors (Free Fall Detection)

​Overview

​Specifications

​Wiring Configuration

​Setup Code (Arduino)

​Interrupt Handling

​Typical Values

​VL53L0X Laser Distance Sensor

​Overview

​Specifications

​Pin Connections

​Initialization Code

​Reading Distance

​Signal Filtering

​1. Exponential Moving Average (EMA)

​2. Butterworth Low-Pass Filter (2nd Order)

​3. Alpha-Beta (α-β) Filter

​HC-SR04 Ultrasonic Sensor

​Overview

​Specifications

​Pin Connections

​Pin Definitions

​Initialization

​Measurement Process

​Trigger Signal

​Echo Detection

​Distance Calculation

​Signal Filtering

​Sample Output

​Sensor Calibration

​VL53L0X Calibration

​Ultrasonic Sensor Calibration

​IR Sensor Calibration

​Troubleshooting

​VL53L0X Issues

​HC-SR04 Issues

​IR Sensor Issues

​Best Practices

​Related Documentation

Build docs developers (and LLMs) love

Overview

IR Sensors (Free Fall Detection)

Overview

Specifications

Wiring Configuration

Setup Code (Arduino)

Interrupt Handling

Typical Values

VL53L0X Laser Distance Sensor

Overview

Specifications

Pin Connections

Initialization Code

Reading Distance

Signal Filtering

1. Exponential Moving Average (EMA)

2. Butterworth Low-Pass Filter (2nd Order)

3. Alpha-Beta (α-β) Filter

HC-SR04 Ultrasonic Sensor

Overview

Specifications

Pin Connections

Pin Definitions

Initialization

Measurement Process

Trigger Signal

Echo Detection

Distance Calculation

Signal Filtering

Sample Output

Sensor Calibration

VL53L0X Calibration

Ultrasonic Sensor Calibration

IR Sensor Calibration

Troubleshooting

VL53L0X Issues

HC-SR04 Issues

IR Sensor Issues

Best Practices

Related Documentation