2.5.1 Arduino Program Structure

A basic Arduino program normally contains two important functions:

setup()

The setup() function runs once when the Arduino starts or resets.

It is normally used for:

• Pin configuration
• Starting serial communication
• Initializing sensors
• Initializing displays
• Setting initial values

loop()

The loop() function runs repeatedly as long as the Arduino is powered.

For example:

setup()
   ↓
loop()
   ↓
loop()
   ↓
loop()
   ↓
loop()
   ↓
...

One of the most common first Arduino programs is the Blink program.

void setup()
{
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop()
{
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);

  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

void setup()

void setup()

Defines the setup function.

pinMode()

pinMode(LED_BUILTIN, OUTPUT);

Configures the built-in LED pin as an output.

digitalWrite()

digitalWrite(LED_BUILTIN, HIGH);

Turns the LED ON.

digitalWrite(LED_BUILTIN, LOW);

Turns the LED OFF.

delay()

delay(1000);

Pauses the program for 1000 milliseconds, which is equal to 1 second.

Therefore:

LED ON  → 1 second
LED OFF → 1 second

The process repeats continuously.

2.5.4 Steps for Uploading the Program

Step 1: Connect the Arduino

Connect the Arduino Uno to the computer using USB.

Step 2: Open Arduino IDE

Start the Arduino IDE.

Step 3: Create a New Sketch

Select:

File → New Sketch

Step 4: Enter the Program

Type or paste:

void setup()
{
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop()
{
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

Step 5: Select Board

Select the correct Arduino board.

Tools → Board → Arduino Uno

Step 6: Select Port

Select the port connected to the Arduino.

Tools → Port → COMx

The exact port number depends on the computer.

Step 7: Verify the Program

Click Verify.

The IDE compiles the program and reports whether errors were found.

Step 8: Upload

Click Upload.

The IDE transfers the compiled program to the Arduino.

Step 9: Observe the LED

After successful uploading, the built-in LED should:

ON
 ↓
1 second
 ↓
OFF
 ↓
1 second
 ↓
ON
 ↓
Repeat

2.5.5 Basic Arduino Programming Commands

An external LED can be connected to an Arduino through an appropriate current-limiting resistor.

Arduino Pin 13
     │
     │
   Resistor
     │
     │
    LED
     │
     │
    GND

Example program:

int ledPin = 13;

void setup()
{
  pinMode(ledPin, OUTPUT);
}

void loop()
{
  digitalWrite(ledPin, HIGH);
  delay(500);

  digitalWrite(ledPin, LOW);
  delay(500);
}

The LED turns ON for 0.5 seconds and OFF for 0.5 seconds repeatedly.

2.5.7 Example: Reading a Push Button

Arduino can read the state of a push button.

Basic operation:

Push Button
     │
     ↓
Arduino Input
     │
     ↓
Arduino Processing
     │
     ↓
LED Output

Example:

int buttonPin = 2;
int ledPin = 13;

void setup()
{
  pinMode(buttonPin, INPUT);
  pinMode(ledPin, OUTPUT);
}

void loop()
{
  int buttonState = digitalRead(buttonPin);

  if (buttonState == HIGH)
  {
    digitalWrite(ledPin, HIGH);
  }
  else
  {
    digitalWrite(ledPin, LOW);
  }
}

This program reads the push button and controls the LED according to the button state.

Operation

This demonstrates the fundamental embedded-system principle:

3. Automatic Night Light Using LDR

Components: Arduino Uno, LDR, LED, resistor.

Purpose: Learn analog sensor reading.

int ldrPin = A0;
int ledPin = 13;

void setup()
{
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop()
{
  int lightValue = analogRead(ldrPin);

  Serial.println(lightValue);

  if (lightValue < 500)
  {
    digitalWrite(ledPin, HIGH);
  }
  else
  {
    digitalWrite(ledPin, LOW);
  }

  delay(500);
}

Result: When the environment becomes dark, the LED turns ON. When there is enough light, the LED turns OFF.

The value 500 can be adjusted depending on the LDR circuit and room lighting.

4. Temperature Monitoring Using LM35

Components: Arduino Uno, LM35 temperature sensor.

Purpose: Learn analog sensor measurement and Serial Monitor.

int sensorPin = A0;

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  int sensorValue = analogRead(sensorPin);

  float voltage = sensorValue * (5.0 / 1023.0);
  float temperature = voltage * 100;

  Serial.print("Temperature: ");
  Serial.print(temperature);
  Serial.println(" °C");

  delay(1000);
}

Result: The temperature is displayed on the Serial Monitor.

Set the Serial Monitor to 9600 baud.

5. Ultrasonic Distance Measurement

Components: Arduino Uno, HC-SR04 ultrasonic sensor.

Purpose: Learn sensor measurement and serial communication.

Connections

Code

int trigPin = 9;
int echoPin = 10;

void setup()
{
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);

  Serial.begin(9600);
}

void loop()
{
  long duration;
  float distance;

  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);

  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);

  digitalWrite(trigPin, LOW);

  duration = pulseIn(echoPin, HIGH);

  distance = duration * 0.034 / 2;

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

  delay(500);
}

Result: The distance between the HC-SR04 sensor and an object is displayed in centimeters on the Serial Monitor.

Arduino Motion Detection Project Using PIR Sensor

Project: Motion Detector with PIR Sensor and LED
Level: Beginner
Board: Arduino Uno
Sensor: PIR motion sensor (such as HC-SR501)

Components

• Arduino Uno
• PIR motion sensor
• LED
• 220 Ω resistor
• Jumper wires
• Breadboard

Connections

LED:

Simple Code

int pirPin = 2;
int ledPin = 13;

void setup()
{
  pinMode(pirPin, INPUT);
  pinMode(ledPin, OUTPUT);

  Serial.begin(9600);
}

void loop()
{
  int motion = digitalRead(pirPin);

  if (motion == HIGH)
  {
    digitalWrite(ledPin, HIGH);

    Serial.println("Motion Detected!");
  }
  else
  {
    digitalWrite(ledPin, LOW);

    Serial.println("No Motion");
  }

  delay(500);
}

2.5.8 Arduino Programming Workflow

The general Arduino development process is:

Identify Project Requirement
          ↓
Select Arduino Board
          ↓
Connect Hardware
          ↓
Write Program
          ↓
Verify / Compile
          ↓
Correct Errors
          ↓
Select Board and Port
          ↓
Upload Program
          ↓
Test the System
          ↓
Troubleshoot
          ↓
Improve the Program