🎈 Arduino Projects
Booklet for Kids

🔍 What you will make

An LED that turns ON for one second, then OFF for one second — forever! This is the classic "Hello World!" of electronics. If you can do this, you can do anything in this book.

🧠 How it works

The Arduino is a tiny computer. We tell pin D13 to push electricity out (that turns the LED ON), then tell it to stop (LED OFF). The delay(1000) line makes it wait 1000 milliseconds = 1 second between changes.

The resistor protects the LED — too much electricity would make it pop! 💥

🔌 Wire it up

🔎 See the table below for the exact connections.

💻 The Code

// PROJECT 1: Blinking LED
// The LED turns ON and OFF, again and again... like a twinkling star!

int ledPin = 13;            // The LED is connected to pin 13

void setup() {
  pinMode(ledPin, OUTPUT);  // Tell the pin: "You send electricity out!"
}

void loop() {
  digitalWrite(ledPin, HIGH);  // Turn the LED ON
  delay(1000);                 // Wait 1 second
  digitalWrite(ledPin, LOW);   // Turn the LED OFF
  delay(1000);                 // Wait 1 second
}

💡 Copy everything from int ledPin to the last }.

🚀 Test it!

1. Connect everything exactly like the picture (power OFF!).
2. Plug in the USB cable and upload the code.
3. Watch the LED blink — ON, OFF, ON, OFF… ⭐
4. If it doesn't light up, turn the LED legs around!

🔍 What you will make

Three LEDs that change exactly like a real traffic light: Red → Yellow → Green → Yellow → Red… Cars would be very impressed. 🚗

🧠 How it works

Each LED has its own pin (D2, D3, D4). We turn one ON and the others OFF, wait, then switch. The delay() decides how long each light stays.

🔌 Wire it up

🔎 Tip: on a breadboard you can join all the short legs to the long black (−) rail.

💻 The Code

// PROJECT 2: Traffic Light
// Red, yellow, green... just like a real crossing!

int red = 2;       // Red LED on pin 2
int yellow = 3;    // Yellow LED on pin 3
int green = 4;     // Green LED on pin 4

void setup() {
  pinMode(red, OUTPUT);
  pinMode(yellow, OUTPUT);
  pinMode(green, OUTPUT);
}

void loop() {
  digitalWrite(red, HIGH);     // Red ON
  digitalWrite(yellow, LOW);
  digitalWrite(green, LOW);
  delay(3000);                 // Cars stop for 3 seconds

  digitalWrite(red, LOW);
  digitalWrite(yellow, HIGH);  // Yellow ON
  delay(1000);

  digitalWrite(yellow, LOW);
  digitalWrite(green, HIGH);   // Green ON - GO!
  delay(3000);

  digitalWrite(green, LOW);
  digitalWrite(yellow, HIGH);  // Yellow again
  delay(1000);
  digitalWrite(yellow, LOW);
}

🚀 Test it!

1. Wire the 3 LEDs exactly like the picture.
2. Upload the code and watch the lights change.
3. Count the seconds — is it like a real traffic light?

🔍 What you will make

Press the button and the buzzer plays "Twinkle Twinkle Little Star"! ✨ The Arduino becomes a little music box.

🧠 How it works

Every note is just a frequency — how fast the buzzer wiggles. The tone() command plays a note, and noTone() stops it. We store the song as two lists: the notes and how long each one lasts.

The button uses INPUT_PULLUP: when pressed, the pin reads LOW (0).

🔌 Wire it up

🔎 Buzzer legs: one long (+), one short (−). Button: use any two opposite legs.

💻 The Code

// PROJECT 3: Melody Maker
// Press the button and the buzzer plays Twinkle Twinkle Little Star!

int buttonPin = 7;   // Button on pin 7
int buzzerPin = 8;   // Buzzer on pin 8

// The notes of the song (frequency in Hertz)
int melody[] = { 262, 262, 392, 392, 440, 440, 392,
                 349, 349, 330, 330, 294, 294, 262 };

// How long each note plays (milliseconds)
int noteTime[] = { 400, 400, 400, 400, 400, 400, 800,
                   400, 400, 400, 400, 400, 400, 800 };

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);  // Button with built-in helper
  pinMode(buzzerPin, OUTPUT);
}

void loop() {
  if (digitalRead(buttonPin) == LOW) {   // If the button is pressed
    for (int i = 0; i < 14; i++) {       // Play every note one by one
      tone(buzzerPin, melody[i]);        // Start the note
      delay(noteTime[i]);                // Let it play
    }
    noTone(buzzerPin);                   // Stop playing
    delay(500);
  }
}

🚀 Test it!

1. Wire the buzzer and button like the picture.
2. Upload the code.
3. Press and hold the button… and sing along! 🎤

🔍 What you will make

Press the button and your Arduino sends SOS — the world-famous "help me!" signal: 3 short, 3 long, 3 short (… --- …) with flashing light and beeping sound!

🧠 How it works

We write two helper functions: dot() (quick blink + beep) and dash() (long blink + beep). Then sending SOS is as easy as saying: dot, dot, dot, dash, dash, dash, dot, dot, dot!

🔌 Wire it up

💻 The Code

// PROJECT 4: SOS Signal
// Press the button: ... --- ... (SOS) with light AND sound!

int ledPin = 13;
int buzzerPin = 8;
int buttonPin = 7;

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

void loop() {
  if (digitalRead(buttonPin) == LOW) {  // Button pressed?
    dot();  dot();  dot();   // S = short short short
    dash(); dash(); dash();  // O = long long long
    dot();  dot();  dot();   // S = short short short
    delay(1500);             // Rest before the next SOS
  }
}

// A short blink + beep
void dot() {
  digitalWrite(ledPin, HIGH);
  tone(buzzerPin, 1000);
  delay(200);
  noTone(buzzerPin);
  digitalWrite(ledPin, LOW);
  delay(200);
}

// A long blink + beep
void dash() {
  digitalWrite(ledPin, HIGH);
  tone(buzzerPin, 1000);
  delay(600);
  noTone(buzzerPin);
  digitalWrite(ledPin, LOW);
  delay(200);
}

🚀 Test it!

1. Wire everything like the picture.
2. Upload the code and press the button.
3. Count the beeps: 3 short, 3 long, 3 short — that's SOS! 🆘

🔍 What you will make

Three buttons control a servo (a motor that turns to exact angles): left button turns it left, right button turns it right, and home button pops it back to the middle. It's like a tiny robot arm controller! 🤖

🧠 How it works

The Servo library is a ready-made helper. We just say myServo.write(angle) and the servo turns to that angle (0° to 180°). Each button press changes the angle a little. The servo has 3 wires: orange = signal (tells it where to go), red = power (5V), brown = ground (GND).

🔌 Wire it up

💻 The Code

// PROJECT 5: Servo with Buttons
// LEFT = turn left, RIGHT = turn right, HOME = back to the middle!

#include <Servo.h>

Servo myServo;              // Make a servo object

int leftButton = 2;         // Left button
int rightButton = 3;        // Right button
int homeButton = 4;         // Home button

int angle = 90;             // Servo starts at 90 degrees

void setup() {
  myServo.attach(9);        // Signal wire on pin 9
  pinMode(leftButton, INPUT_PULLUP);
  pinMode(rightButton, INPUT_PULLUP);
  pinMode(homeButton, INPUT_PULLUP);
  myServo.write(angle);     // Start in the middle
  delay(300);
}

void loop() {
  if (digitalRead(leftButton) == LOW) {
    angle = angle - 5;      // Turn a little to the left
  }
  if (digitalRead(rightButton) == LOW) {
    angle = angle + 5;      // Turn a little to the right
  }
  if (digitalRead(homeButton) == LOW) {
    angle = 90;             // Jump back to the middle
  }

  if (angle < 0)    angle = 0;      // Don't go past 0°
  if (angle > 180)  angle = 180;    // Don't go past 180°

  myServo.write(angle);     // Move the servo!
  delay(20);                // A tiny pause so it feels smooth
}

🚀 Test it!

1. Wire the servo and buttons like the picture (servo red to 5V!).
2. Upload the code.
3. Hold LEFT or RIGHT — watch the arm swing. Press HOME — snap back!

🔍 What you will make

A device that sends a sound "ping", listens for the echo, and shows "Distance: 25 cm" on the screen — just like a bat finding its way in the dark! 🦇

🧠 How it works

The sensor shoots an ultrasonic ping (too high for humans to hear) and measures how long it takes to bounce back. Sound travels about 0.034 cm per microsecond, and it goes there and back, so we halve the answer.

The LCD uses I2C — a clever "two-wire" chat system — so it only needs SDA (data) and SCL (clock) wires.

🔌 Wire it up

💻 The Code

// PROJECT 6: Distance Meter
// Measure distances and show them on the LCD screen!

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);   // Our 16x2 screen

int trigPin = 7;
int echoPin = 6;

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  lcd.init();               // Start the screen
  lcd.backlight();          // Turn on the light
  lcd.print("Distance:");   // Write a title
}

void loop() {
  // Send a little "ping"
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  // How long until the echo comes back?
  long time = pulseIn(echoPin, HIGH);

  // Turn time into centimetres
  int distance = time * 0.034 / 2;

  lcd.setCursor(0, 1);          // Go to line 2
  lcd.print(distance);
  lcd.print(" cm     ");        // Extra spaces wipe old numbers

  delay(300);
}

💡 If the screen shows nothing or boxes ▯▯▯, your LCD address might be 0x3F instead of 0x27 — just change the number!

🚀 Test it!

1. Wire the sensor and screen like the picture.
2. Install the library (see the blue box above).
3. Upload the code. Move your hand closer and farther — watch the numbers change!

🔍 What you will make

Put this near a wall and watch: far away = green light, getting close = yellow + slow beeps, too close! = red + fast beeps. It's exactly how a real car's parking sensor works! 🚗

🧠 How it works

The ultrasonic sensor measures the distance (like Project 6). Then the code decides what to do with if / else if: over 40 cm → green, 20–40 cm → yellow, under 20 cm → red and BEEEEEP. The closer, the faster the beeps!

🔌 Wire it up

💻 The Code

// PROJECT 7: Smart Parking Sensor
// Far = green, middle = yellow, close = RED + fast beeps!

int trigPin = 7;
int echoPin = 6;
int greenLed = 2;
int yellowLed = 3;
int redLed = 4;
int buzzerPin = 8;

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(greenLed, OUTPUT);
  pinMode(yellowLed, OUTPUT);
  pinMode(redLed, OUTPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop() {
  // Send the ping and measure the distance
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  long time = pulseIn(echoPin, HIGH);
  int distance = time * 0.034 / 2;

  if (distance > 40) {
    // FAR AWAY: green light, no beeping
    digitalWrite(greenLed, HIGH);
    digitalWrite(yellowLed, LOW);
    digitalWrite(redLed, LOW);
  }
  else if (distance > 20) {
    // MIDDLE: yellow light, slow beeps
    digitalWrite(greenLed, LOW);
    digitalWrite(yellowLed, HIGH);
    digitalWrite(redLed, LOW);
    tone(buzzerPin, 1000);
    delay(200);
    noTone(buzzerPin);
    delay(400);
  }
  else {
    // VERY CLOSE: red light, fast beeps!
    digitalWrite(greenLed, LOW);
    digitalWrite(yellowLed, LOW);
    digitalWrite(redLed, HIGH);
    tone(buzzerPin, 1000);
    delay(100);
    noTone(buzzerPin);
    delay(100);
  }
  delay(200);
}

🚀 Test it!

1. Wire everything like the picture.
2. Upload the code.
3. Slowly move your hand toward the sensor. Green → Yellow → RED BEEP BEEP! 🖐️

🔍 What you will make

Your first real robot! It drives forward, and when it sees something close, it stops, looks left, looks right, picks the open side and turns away. Put it on the floor and watch it explore! 🧭

🧠 How it works

The servo turns the ultrasonic sensor left and right like a neck. The L293D chip is the muscle that drives the two motors. The brain (Arduino) asks: "Is the way clear?" If yes → GO. If not → look around and turn.

🔌 Wire it up

🔎 Wires may cross in the picture — that's fine, they don't actually touch. Follow the table!

💻 The Code

// PROJECT 8: Obstacle Avoiding Robot
// Drives forward, and turns away when something is in the way!

#include <Servo.h>

Servo lookServo;        // The servo that looks left and right

int in1 = 4, in2 = 5;   // Left motor
int in3 = 6, in4 = 7;   // Right motor
int trigPin = 11;
int echoPin = 12;

void setup() {
  pinMode(in1, OUTPUT); pinMode(in2, OUTPUT);
  pinMode(in3, OUTPUT); pinMode(in4, OUTPUT);
  pinMode(trigPin, OUTPUT); pinMode(echoPin, INPUT);
  lookServo.attach(8);
  lookServo.write(90);        // Look straight ahead
  delay(500);
}

void loop() {
  int distance = getDistance();

  if (distance > 25) {
    driveForward();           // Road is clear - GO!
  } else {
    stopRobot();              // Oops, something is there!

    lookServo.write(30);      // Look left
    delay(500);
    int left = getDistance();

    lookServo.write(150);     // Look right
    delay(500);
    int right = getDistance();

    lookServo.write(90);      // Look straight again

    if (left > right) {
      turnLeft();             // More space on the left
    } else {
      turnRight();            // More space on the right
    }
    delay(400);
  }
}

// ---------- HELPERS ----------

int getDistance() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  long time = pulseIn(echoPin, HIGH);
  return time * 0.034 / 2;
}

void driveForward() {
  digitalWrite(in1, HIGH); digitalWrite(in2, LOW);
  digitalWrite(in3, HIGH); digitalWrite(in4, LOW);
}

void stopRobot() {
  digitalWrite(in1, LOW); digitalWrite(in2, LOW);
  digitalWrite(in3, LOW); digitalWrite(in4, LOW);
}

void turnLeft() {
  digitalWrite(in1, HIGH); digitalWrite(in2, LOW);   // Left motor forward
  digitalWrite(in3, LOW);  digitalWrite(in4, HIGH);  // Right motor backward
}

void turnRight() {
  digitalWrite(in1, LOW);  digitalWrite(in2, HIGH);  // Left motor backward
  digitalWrite(in3, HIGH); digitalWrite(in4, LOW);   // Right motor forward
}

🚀 Test it!

1. Build the chassis: wheels, motors and battery box (ask a grown-up for the hot glue!).
2. Wire the L293D, servo and sensor like the picture.
3. Plug in the battery and the USB cable. Upload the code.
4. Put the robot on the floor. Make a wall with your hands — watch it turn away! 🧱

🔍 What you will make

A robot with two "eyes" that look down at the floor. Make a black line (tape or marker), and the robot drives along it, correcting itself left and right so it never loses the track!

🧠 How it works

The two IR sensors tell us what's under them: black line or white floor. If the right eye leaves the line, the robot turns right. If the left eye leaves it, it turns left. Both on the line → go straight. It's a tiny "brain" making decisions many times per second!

🔌 Wire it up

💻 The Code

// PROJECT 9: Line Following Robot
// Two IR eyes keep the robot on the black line!

int leftSensor = 2;     // IR sensor on the LEFT
int rightSensor = 3;    // IR sensor on the RIGHT

int in1 = 4, in2 = 5;   // Left motor
int in3 = 6, in4 = 7;   // Right motor

void setup() {
  pinMode(leftSensor, INPUT);
  pinMode(rightSensor, INPUT);
  pinMode(in1, OUTPUT); pinMode(in2, OUTPUT);
  pinMode(in3, OUTPUT); pinMode(in4, OUTPUT);
}

void loop() {
  int left = digitalRead(leftSensor);    // 0 = on the black line
  int right = digitalRead(rightSensor);

  if (left == 0 && right == 0) {
    forward();      // Both on the line - GO STRAIGHT!
  }
  else if (left == 0 && right == 1) {
    turnLeft();     // Right eye lost the line - turn LEFT
  }
  else if (left == 1 && right == 0) {
    turnRight();    // Left eye lost the line - turn RIGHT
  }
  else {
    forward();      // Lost the line - keep going straight
  }
}

void forward() {
  digitalWrite(in1, HIGH); digitalWrite(in2, LOW);
  digitalWrite(in3, HIGH); digitalWrite(in4, LOW);
}

void turnLeft() {
  digitalWrite(in1, LOW);  digitalWrite(in2, HIGH);
  digitalWrite(in3, HIGH); digitalWrite(in4, LOW);
}

void turnRight() {
  digitalWrite(in1, HIGH); digitalWrite(in2, LOW);
  digitalWrite(in3, LOW);  digitalWrite(in4, HIGH);
}

💡 If the robot zig-zags the wrong way, swap the leftSensor and rightSensor pins (or just swap the two motor plugs).

🚀 Test it!

1. Build the chassis and wire everything like the picture.
2. Tune the two IR sensors (see the blue box).
3. Make a track with black tape or a thick marker on white paper.
4. Put the robot on the line, add power… and watch it follow! 🐾

🔍 What you will make

A robot you drive like a remote-control car — but the remote is your phone! Install a free "Bluetooth RC" app, connect, and press F (forward), B (back), L (left), R (right), S (stop).

🧠 How it works

The HC-05 is a tiny Bluetooth radio. Your phone sends letters through the air; the HC-05 catches them and passes them to the Arduino. The Arduino reads each letter and tells the motors what to do. We use SoftwareSerial so the radio can talk on pins D2 and D3.

🔌 Wire it up

💻 The Code

// PROJECT 10: Bluetooth Robot
// Phone sends letters: F=forward B=back L=left R=right S=stop

#include <SoftwareSerial.h>

SoftwareSerial bluetooth(2, 3);   // HC-05: TX→pin2, RX→pin3

int in1 = 4, in2 = 5;   // Left motor
int in3 = 6, in4 = 7;   // Right motor
char command;           // The letter from your phone

void setup() {
  pinMode(in1, OUTPUT); pinMode(in2, OUTPUT);
  pinMode(in3, OUTPUT); pinMode(in4, OUTPUT);
  bluetooth.begin(9600);    // Start talking to the HC-05
  stopRobot();
}

void loop() {
  if (bluetooth.available()) {      // A letter arrived?
    command = bluetooth.read();     // Read it

    if (command == 'F') forward();
    else if (command == 'B') backward();
    else if (command == 'L') turnLeft();
    else if (command == 'R') turnRight();
    else if (command == 'S') stopRobot();
  }
}

void forward() {
  digitalWrite(in1, HIGH); digitalWrite(in2, LOW);
  digitalWrite(in3, HIGH); digitalWrite(in4, LOW);
}

void backward() {
  digitalWrite(in1, LOW); digitalWrite(in2, HIGH);
  digitalWrite(in3, LOW); digitalWrite(in4, HIGH);
}

void turnLeft() {
  digitalWrite(in1, LOW);  digitalWrite(in2, HIGH);
  digitalWrite(in3, HIGH); digitalWrite(in4, LOW);
}

void turnRight() {
  digitalWrite(in1, HIGH); digitalWrite(in2, LOW);
  digitalWrite(in3, LOW);  digitalWrite(in4, HIGH);
}

void stopRobot() {
  digitalWrite(in1, LOW); digitalWrite(in2, LOW);
  digitalWrite(in3, LOW); digitalWrite(in4, LOW);
}

🚀 Test it!

1. Wire everything like the picture (HC-05 and module).
2. Upload the code (with HC-05 RX unplugged, then reconnect it).
3. On your phone: turn ON Bluetooth. Open a free "Bluetooth RC" / "Arduino Bluetooth Controller" app and connect to "HC-05" (password is usually 1234).
4. Press the buttons — your robot drives! 🏎️

🔍 What you will make

Point the PIR at your door. When someone walks in, the relay switches the lamp ON for 10 seconds, then OFF again. Real buildings use exactly this for hallway lights! (You'll also see the little LED built into the Arduino light up as a sign.)

🧠 How it works

The PIR feels the tiny bit of heat (infrared) that people give off. When something warm moves in front of it, its OUT pin goes HIGH. The relay is an electric switch: the Arduino flips it, and the relay turns the big lamp on or off.

🔌 Wire it up

💻 The Code

// PROJECT 11: Motion-Activated Light
// When someone moves, the light turns ON for 10 seconds!

int pirPin = 2;     // PIR sensor output
int relayPin = 7;   // Relay switch
int ledPin = 13;    // Small LED built into the Arduino

void setup() {
  pinMode(pirPin, INPUT);
  pinMode(relayPin, OUTPUT);
  pinMode(ledPin, OUTPUT);
  digitalWrite(relayPin, LOW);   // Light starts OFF
  delay(2000);                   // Give the PIR 2s to wake up!
}

void loop() {
  int motion = digitalRead(pirPin);   // 1 = movement!

  if (motion == HIGH) {
    digitalWrite(relayPin, HIGH);     // Light ON
    digitalWrite(ledPin, HIGH);
    delay(10000);                     // Stay ON for 10 seconds
    digitalWrite(relayPin, LOW);      // Light OFF
    digitalWrite(ledPin, LOW);
  }
}

🚀 Test it!

1. Wire the PIR and relay like the picture.
2. Upload the code and wait 2 seconds (PIR is waking up!).
3. Wave your hand in front of the PIR — the lamp lights up! 👋
4. Stand still for 10 seconds — it turns OFF. Move again — ON!

🔍 What you will make

A lamp you switch with a clap! Clap-clap to turn it on, clap-clap to turn it off. You'll feel like a wizard. 🧙

🧠 How it works

The sound sensor has a tiny microphone. When it hears a loud sound (a clap), its DO (digital out) pin goes HIGH for a moment. The Arduino flips a switch in the code each time: on → off → on → off. That's called toggling.

🔌 Wire it up

💻 The Code

// PROJECT 12: Clap Switch
// Clap to turn the lamp ON... clap again to turn it OFF!

int soundPin = 2;   // Sound sensor digital output
int relayPin = 7;   // Relay switch
int ledPin = 13;    // Arduino's little built-in LED

bool lightOn = false;   // The light starts OFF

void setup() {
  pinMode(soundPin, INPUT);
  pinMode(relayPin, OUTPUT);
  pinMode(ledPin, OUTPUT);
  digitalWrite(relayPin, LOW);
}

void loop() {
  int sound = digitalRead(soundPin);   // 1 = loud sound (a clap!)

  if (sound == HIGH) {
    lightOn = !lightOn;                // Flip the switch!
    digitalWrite(relayPin, lightOn);
    digitalWrite(ledPin, lightOn);
    delay(500);                        // One clap = one switch (not five!)
  }
}

🚀 Test it!

1. Wire the sound sensor and relay like the picture.
2. Upload the code.
3. Clap near the sensor… the lamp turns ON. 👏✨
4. Clap again… OFF!

🔍 What you will make

A screen that shows the temperature (°C) and humidity (how wet the air is, in %) right now. Breathe on the sensor and watch the numbers jump!

🧠 How it works

The DHT11 is a little weather scientist. It measures temperature with a tiny thermometer and humidity with a tiny moisture sensor, then sends both to the Arduino on one wire. The library DHT does the tricky reading for us.

🔌 Wire it up

💻 The Code

// PROJECT 13: Temperature & Humidity Monitor
// Shows temperature and humidity on the LCD!

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include <DHT.h>

#define DHTPIN 2          // DHT11 data pin
#define DHTTYPE DHT11     // Type of sensor

DHT dht(DHTPIN, DHTTYPE);
LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  dht.begin();            // Start the weather scientist
  lcd.init();
  lcd.backlight();
}

void loop() {
  float temp = dht.readTemperature();   // Read temperature in °C
  float hum  = dht.readHumidity();      // Read humidity in %

  if (isnan(temp) || isnan(hum)) {      // If the reading failed
    lcd.setCursor(0, 0);
    lcd.print("Sensor error!  ");
  } else {
    lcd.setCursor(0, 0);
    lcd.print("Temp: ");
    lcd.print(temp);
    lcd.print(" C   ");

    lcd.setCursor(0, 1);
    lcd.print("Hum: ");
    lcd.print(hum);
    lcd.print(" %   ");
  }

  delay(2000);    // DHT11 is slow - wait 2 seconds
}

💡 Screen showing boxes ▯▯▯? Try 0x3F instead of 0x27 in lcd(0x27, 16, 2).

🚀 Test it!

1. Wire the DHT11 and LCD like the picture.
2. Install both libraries and upload the code.
3. Watch the screen. Now breathe on the DHT11 — humidity shoots up! 🌬️
4. Put it in the fridge for 1 minute — temperature drops!

🔍 What you will make

A device that smells alcohol and other gases in the air. When the smell is too strong, it shows "ALCOHOL!" on the screen and the buzzer warns: BEEP BEEP! (Real police breathalyzers work on this idea!)

🧠 How it works

The MQ3 has a special plate inside that changes its electrical resistance when it smells alcohol. The analogRead() turns that into a number from 0–1023. Bigger number = stronger smell. We compare it to a limit: over the limit → alarm!

🔌 Wire it up

🔎 *DO is the digital output — we don't use it in this project. The LCD is optional: without it the project still works (the buzzer still alarms!).

💻 The Code

// PROJECT 14: Alcohol Detector
// Smells alcohol in the air and warns with the buzzer!

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

int mq3Pin = A0;        // MQ3 analog output
int buzzerPin = 8;
int limit = 300;        // Above this number = ALARM!

void setup() {
  pinMode(buzzerPin, OUTPUT);
  lcd.init();
  lcd.backlight();
  lcd.print("Warming up...");
  delay(30000);         // Let the sensor warm up (30s)
}

void loop() {
  int value = analogRead(mq3Pin);   // Read the smell level (0-1023)

  lcd.setCursor(0, 0);
  lcd.print("Gas: ");
  lcd.print(value);
  lcd.print("    ");

  if (value > limit) {
    // STRONG smell - alarm!
    lcd.setCursor(0, 1);
    lcd.print("ALCOHOL! BEEP!  ");
    tone(buzzerPin, 2000);     // Beep! (tone works with any buzzer)
    delay(200);
    noTone(buzzerPin);
    delay(200);
  } else {
    // Air is fine
    lcd.setCursor(0, 1);
    lcd.print("All clear       ");
    delay(500);
  }
}

🚀 Test it!

1. Wire the MQ3, buzzer and LCD like the picture.
2. Upload the code and wait 30 seconds for warm-up.
3. Wave your hand near the sensor — the number stays low.
4. Now wave it near a bottle of hand sanitizer (it contains alcohol!) — the number jumps and the buzzer BEEPS! 🚨

🔍 What you will make

A mini smart home! Button 1 turns the lamp (relay) on/off. Button 2 switches the fan (a blue LED). Button 3 switches the night light (a small LED). Press once → ON, press again → OFF.

🧠 How it works

Each button toggles (flips) its own thing using bool variables that remember "on" or "off". The relay controls the real lamp; the LEDs show what the fan and night light would be doing. This is how real home automation starts!

🔌 Wire it up

💻 The Code

// PROJECT 15: Home Automation Switchboard
// 3 buttons control a lamp (relay), a fan LED and a night LED!

int button1 = 2;   // Lamp button
int button2 = 3;   // Fan button
int button3 = 4;   // Night light button

int relayPin = 7;   // Controls the real lamp
int fanLed = 8;     // Stands for the fan
int nightLed = 9;   // Night light

bool lampOn = false;
bool fanOn = false;
bool nightOn = false;

void setup() {
  pinMode(button1, INPUT_PULLUP);
  pinMode(button2, INPUT_PULLUP);
  pinMode(button3, INPUT_PULLUP);
  pinMode(relayPin, OUTPUT);
  pinMode(fanLed, OUTPUT);
  pinMode(nightLed, OUTPUT);
}

void loop() {
  if (digitalRead(button1) == LOW) {   // Lamp button pressed?
    lampOn = !lampOn;                  // Flip it!
    digitalWrite(relayPin, lampOn);
    delay(300);                        // So one press = one flip
  }

  if (digitalRead(button2) == LOW) {   // Fan button pressed?
    fanOn = !fanOn;
    digitalWrite(fanLed, fanOn);
    delay(300);
  }

  if (digitalRead(button3) == LOW) {   // Night light button pressed?
    nightOn = !nightOn;
    digitalWrite(nightLed, nightOn);
    delay(300);
  }
}

🚀 Test it!

1. Wire the buttons, relay and LEDs like the picture.
2. Upload the code.
3. Press the LAMP button — the relay clicks and the lamp turns on!
4. Press FAN and NIGHT — their LEDs light up. Press again — everything off!