Task 4: Ultrasonic Object Detector

3. Circuit Diagram and Wiring

Connect the components according to the table and diagram below. Pay close attention to the pin connections for the HC-SR04 sensor.

(A visual representation of the breadboard setup would look like this: The sensor is placed on the breadboard. Its VCC and GND are connected to the power rails. Trig and Echo pins are connected to Arduino pins 9 and 10. The LED's long leg is connected via a resistor to pin 7, and its short leg is connected to the ground rail.)

4. Procedure

Step 1: Theory - How it Works

The HC-SR04 sensor works like a sonar. It sends out a short burst of high-frequency sound from its Trigger pin. This sound wave travels through the air, hits an object, and bounces back. The sensor's Echo pin then detects this returning wave.

The Arduino measures the time it takes for the echo to return. Since we know the speed of sound (approximately 340 m/s or 0.034 cm/µs), we can calculate the distance.

The formula is: Distance = (Travel Time / 2) * Speed of Sound

We divide the time by 2 because the measured time is for the sound to travel to the object and back. We only want the one-way distance.

Step 2: Writing the Arduino Code

Open the Arduino IDE and enter the following code. Read the comments carefully to understand what each line does.

// Lab 4: Ultrasonic Object Detector
// This program measures distance using an HC-SR04 sensor and turns on an LED
// if an object is detected within 20 cm.

// Define the pin numbers for the ultrasonic sensor and the LED
const int trigPin = 9;
const int echoPin = 10;
const int ledPin = 7;

// Define a variable for the detection distance threshold (in cm)
const int detectionDistance = 20;

// Variables to store the duration of the echo pulse and the calculated distance
long duration;
int distance;

void setup() {
  // Set up the LED pin as an output
  pinMode(ledPin, OUTPUT);
  
  // Set up the sensor pins
  pinMode(trigPin, OUTPUT); // Trig pin sends the pulse, so it's an output
  pinMode(echoPin, INPUT);  // Echo pin receives the pulse, so it's an input
  
  // Start the serial communication to display results on the Serial Monitor
  Serial.begin(9600);
  Serial.println("Ultrasonic Sensor Test");
}

void loop() {
  // --- Step 1: Trigger the sensor ---
  
  // To generate a clean pulse, first make sure the trigPin is low
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  
  // Now, send a 10 microsecond high pulse to trigger the sensor
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  // --- Step 2: Read the echo pulse ---
  
  // The pulseIn() function reads how long the echoPin stays HIGH.
  // This is the time it took for the sound wave to travel to the object and back.
  duration = pulseIn(echoPin, HIGH);
  
  // --- Step 3: Calculate the distance ---
  
  // Using the simplified formula: distance (cm) = duration (µs) / 58
  distance = duration / 58;
  
  // --- Step 4: Print the distance to the Serial Monitor ---
  
  // This is very useful for debugging and seeing the sensor work.
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  
  // --- Step 5: Control the LED based on the distance ---
  
  // Check if an object is detected within our defined threshold
  if (distance < detectionDistance && distance > 0) {
    // If yes, turn the LED ON
    digitalWrite(ledPin, HIGH);
  } else {
    // If no, turn the LED OFF
    digitalWrite(ledPin, LOW);
  }
  
  // Wait for 100 milliseconds before taking the next reading
  delay(100);
}
        

Step 3: Upload and Test

1. Connect your Arduino to your computer via USB.
2. In the Arduino IDE, go to Tools > Board and select "Arduino Uno".
3. Go to Tools > Port and select the correct port for your Arduino.
4. Click the "Upload" button to send the code to your Arduino.
5. Once uploading is complete, open the Serial Monitor (Tools > Serial Monitor or the magnifying glass icon). Make sure the baud rate is set to 9600.
6. Place your hand or an object in front of the sensor. Watch the distance values printed on the Serial Monitor.
7. Observe the LED. It should turn on when the object is closer than 20 cm and turn off when it's farther away.