Lab 12.1: Ultrasonic Distance Sensor

~90 min. Wire the HC-SR04 ultrasonic distance sensor. Read distance in centimeters. Trigger an LED or buzzer when something gets too close.

Goal: read a specialized sensor with a protocol of its own; convert sensor data to a real-world quantity (centimeters)

Estimated time: 90 minutes

Prerequisites: lab 8.1 (digital I/O); lab 11.1 (Serial debug)

Steps

Step 1: Identify the HC-SR04 pins (5 min)

The HC-SR04 has 4 pins: VCC (+5 V), Trig, Echo, GND. Label them visually; they are usually printed on the board

Step 2: Wire the sensor (15 min)

• VCC → R4's 5V pin
• GND → R4's GND
• Trig → R4's pin 7
• Echo → R4's pin 8

Pins 7 and 8 are arbitrary digital pins; pick others if you prefer. Just make sure they are digital-only (no PWM concerns; this is purely digital pulse timing)

Step 3: Write the basic read sketch (25 min)

const int TRIG_PIN = 7;
const int ECHO_PIN = 8;

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  Serial.begin(9600);
}

void loop() {
  // Send 10us trigger pulse
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Read echo pulse duration (in microseconds)
  unsigned long duration = pulseIn(ECHO_PIN, HIGH, 30000);  // 30 ms timeout

  // Convert to centimeters: speed of sound = 343 m/s = 29.1 us/cm
  // pulse is round-trip, so divide by 2
  float distance_cm = duration / 58.3;

  Serial.print("Duration: ");
  Serial.print(duration);
  Serial.print(" us  Distance: ");
  Serial.print(distance_cm);
  Serial.println(" cm");

  delay(250);
}

Upload. Open Serial Monitor. You should see distance readings. Move your hand toward and away from the sensor; the distance updates

Step 4: Calibrate against a ruler (15 min)

Place objects at known distances (10 cm, 20 cm, 50 cm, 100 cm). Compare the sensor's reading to the ruler. The HC-SR04 is typically accurate to ±1 cm at short ranges, ±2-3 cm at longer ranges

Note any failure modes: very close (< 2 cm; sensor's minimum), very far (> 400 cm; sensor's maximum), oblique surfaces (echo bounces away), soft surfaces (echo is absorbed)

Step 5: Add a "too close" alert (20 min)

Add an LED on pin 9. Light it when distance < 20 cm:

const int LED_PIN = 9;

void setup() {
  // ... existing code ...
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  // ... existing read code ...

  if (distance_cm > 0 && distance_cm < 20) {
    digitalWrite(LED_PIN, HIGH);
    Serial.println("  ALERT: too close!");
  } else {
    digitalWrite(LED_PIN, LOW);
  }

  delay(250);
}

Upload. Move objects in front of the sensor; the LED turns on inside 20 cm

Step 6: Document (10 min)

Lab notebook: distance vs reading calibration table. Failure modes you observed. Reflect on what changing the threshold (10 cm? 50 cm? 5 cm?) would mean for an actual application

Expected output

• Working distance sensor reading distance in cm
• Calibration against ruler
• "Too close" LED alert at threshold

Common pitfalls

• Wrong pin assignment for Trig vs Echo: Trig is OUTPUT (you send the pulse); Echo is INPUT (you receive the response). Reversed = sketch hangs or gets garbage
• Timeout too short: pulseIn(ECHO_PIN, HIGH, X) where X is too small will return 0 when the target is far away. 30000 (30 ms) handles up to ~5 m
• Sound interference: ultrasonic sensors can interfere with each other or pick up echoes from walls. Test in an open area first

Stretch (optional)

• Replace LED with the buzzer (with transistor switch if buzzer is too loud for direct pin drive). The buzzer beeps faster as the object gets closer (proximity alarm)
• Use the distance to control LED brightness via PWM (close = bright, far = dim). Map distance (0-200 cm) to brightness (255-0)
• Read three different sensors from your kit (temperature, humidity, motion, etc.) following the same pattern: protocol → read → convert → use

Lab 12.1 v0.1. The first specialized sensor. The pattern generalizes to every other sensor you will encounter.