Lesson 4: Conditional Statements with Arduino

Teach your program how to make decisions with `if`, `else`, and comparisons

32-Lesson Course • Semester 1

You already know variables, calculations, and Serial output. Now it is time to make your programs choose between different actions. Conditional statements let Arduino decide what to do based on a value or situation.

Learning Objectives

By the end of this Lesson, you will:

Understand what `if` and `else` do in Arduino code
Use comparison operators like `==`, `!=`, `<`, `>`, `<=`, and `>=`
Write simple decision-making code with the Serial Monitor
Control an LED differently depending on a button state or a value

Skills Developed:

Tracing program flow step by step
Testing different outcomes using values and inputs
Reading conditions carefully
Preparing for loops, functions, and hardware control later

Section 1: What Is a Conditional Statement?

A conditional statement tells the program to check whether something is true. If it is true, one block of code runs. If it is false, a different block can run instead.

Everyday idea

If it is raining, bring an umbrella. Else, leave the umbrella at home. Arduino code works the same way: it checks a condition and then decides which action to take.

if (condition) {
  // code to run if the condition is true
} else {
  // code to run if the condition is false
}

Section 2: Comparison Operators

A condition usually compares two values. These comparison operators help Arduino decide if something is true or false.

Useful operators

== means equal to
!= means not equal to
< means less than
> means greater than
<= means less than or equal to
>= means greater than or equal to

Important detail

= assigns a value. == compares two values. Beginners often mix these up, so read conditions carefully.

Section 3: A First `if` Statement

Start with a simple value in a variable. Then let the program decide what message to print.

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

  int score = 85;

  if (score >= 70) {
    Serial.println("You passed.");
  }
}

void loop() {
}

What happens?

The program stores `85` in `score`
It checks `score >= 70`
That condition is true, so the message prints

Section 4: Using `if` and `else`

An `if` statement can also include an `else` block. That gives the program a different action when the condition is false.

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

  int temperature = 62;

  if (temperature > 75) {
    Serial.println("It is warm.");
  } else {
    Serial.println("It is not warm.");
  }
}

void loop() {
}

Only one of these blocks runs each time the program checks the condition.

Section 5: More Than Two Choices with `else if`

Sometimes a program needs more than just yes or no. `else if` lets you check another condition if the first one is false.

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

  int score = 92;

  if (score >= 90) {
    Serial.println("Grade: A");
  } else if (score >= 80) {
    Serial.println("Grade: B");
  } else if (score >= 70) {
    Serial.println("Grade: C");
  } else {
    Serial.println("Keep practicing.");
  }
}

void loop() {
}

Order matters

Arduino checks from top to bottom. As soon as one condition is true, it runs that block and skips the rest.

Section 6: Using Conditionals with Hardware

Decision-making becomes even more useful when the program reacts to a real input. A button can tell the Arduino when to turn an LED on or off.

Hardware setup: wire the button and LED first

LED wiring

Connect a jumper wire from Arduino pin 8 to a 220Ω resistor
Connect the resistor to the long leg of the LED
Connect the short leg of the LED to GND

Button wiring

Place the pushbutton across the center gap of the breadboard
Connect one side of the button to Arduino pin 2
Connect the other side of the button to GND
Because the code uses INPUT_PULLUP, you do not need an extra resistor for the button

When the button is not pressed, pin 2 reads HIGH. When the button is pressed, it connects to GND, so pin 2 reads LOW.

This example uses a button on pin `2` and an LED on pin `8`. The button uses `INPUT_PULLUP`, so pressed usually reads as `LOW`.

const int ledPin = 8;
const int buttonPin = 2;

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

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

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

This is a real Arduino decision: if the button is pressed, the LED turns on. Else, it stays off.

Section 7: Common Beginner Mistakes

Mistake 1

Using = instead of == in a condition.

Mistake 2

Forgetting that `INPUT_PULLUP` usually reads `LOW` when pressed.

Mistake 3

Expecting every `if` block to run even after a true condition was already found in an `else if` chain.

Mistake 4

Not checking the actual value in the Serial Monitor while debugging.

Hands-On Activity: Pass, Retry, or Excellent

Write a program that stores a score in a variable and prints one of three messages:

If the score is `90` or higher, print `Excellent work!`
Else if the score is `70` or higher, print `You passed.`
Else, print `Try again.`

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

  int score = 76;

  if (score >= 90) {
    Serial.println("Excellent work!");
  } else if (score >= 70) {
    Serial.println("You passed.");
  } else {
    Serial.println("Try again.");
  }
}

void loop() {
}

Extension: change the value of `score` and predict what will print before you run the program.

Assessment and Homework

Lesson 4 quiz topics

What `if`, `else`, and `else if` do
Comparison operators
How program flow changes based on a condition
Simple button and LED decision-making
Common conditional mistakes

Homework ideas

Write two small `if`/`else` programs using different numbers
Change conditions and predict what will happen before running the code
Practice reading button logic with `INPUT_PULLUP`
Be ready to learn counted repetition and loops in Lesson 5

Strong programmers trace the condition carefully: what value do I have, what comparison am I making, and which block will run?

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