Arduino Calculator using 4x4 Keypad and 16x2 LCD

 1. Introduction

Building a calculator using Arduino is an excellent beginner-to-intermediate project that combines input devices (keypad) and output devices (LCD display) with logical processing.

In this project, we design a fully functional calculator using:

• A 4x4 matrix keypad for input
• A 16x2 LCD display for output
• Arduino Uno as the brain

This calculator supports:

• Addition (+)
• Subtraction (−)
• Multiplication (×)
• Division (÷)
• Decimal numbers
• Clear function (double press =)

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2. Components

• Arduino Uno
• 16x2 LCD Display
• 4x4 Matrix Keypad
• Jumper Wires
• Breadboard
• USB Cable

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3. Circuit and Connections

Pin-out Diagram 16X2 LCD

4X4 Keypad Pin-out Diagram

🔌 LCD Connections

LCD Pin	Arduino
RS	12
EN	11
D4	10
D5	9
D6	8
D7	7
VCC	5V
GND	GND

🔌 Keypad Connections

Keypad Pin	Arduino
R1	5
R2	4
R3	3
R4	2
C1	A3
C2	A2
C3	A1
C4	A0

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4. Detailed Step By Step Circuit Working

1. The keypad acts as the input device.
2. Each key press is detected using row-column scanning.
3. The Arduino reads the key and stores it in a variable.
4. The LCD displays the entered number.
5. When an operator is pressed:
   • First number is stored in memory
   • Operation is saved
6. Second number is entered.
7. On pressing =, Arduino:
   • Performs calculation
   • Displays result on LCD
8. Double pressing = clears the screen.

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5. Libraries to be Included

#include <LiquidCrystal.h>
#include <Keypad.h>

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6. Code

#include <LiquidCrystal.h>
#include <Keypad.h>

/* LCD setup */
LiquidCrystal lcd(12, 11, 10, 9, 8, 7);

/* Keypad setup */
const byte ROWS = 4;
const byte COLS = 4;

char keys[ROWS][COLS] = {
  {'1','2','3','+'},
  {'4','5','6','-'},
  {'7','8','9','*'},
  {'.','0','=','/'}
};

byte rowPins[ROWS] = {5, 4, 3, 2};
byte colPins[COLS] = {A3, A2, A1, A0};

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

/* Variables */
String current = "";
String memory = "";
char operation = 0;

/* Splash screen */
void showSplashScreen() {
  lcd.setCursor(0, 0);
  lcd.print("Arduino Calc");
  lcd.setCursor(2, 1);
  lcd.print("Loading...");
  delay(1500);
  lcd.clear();
}

/* Calculate */
double calculate(char op, double a, double b) {
  switch (op) {
    case '+': return a + b;
    case '-': return a - b;
    case '*': return a * b;
    case '/': 
      if (b == 0) {
        lcd.clear();
        lcd.print("Error: /0");
        delay(1500);
        lcd.clear();
        return 0;
      }
      return a / b;
  }
  return 0;
}

/* Handle input */
void processKey(char key) {

  static unsigned long lastPress = 0;

  // CLEAR (double press '=')
  if (key == '=' && millis() - lastPress < 500) {
    current = "";
    memory = "";
    operation = 0;
    lcd.clear();
    lcd.setCursor(0,0);
    return;
  }

  if (key == '=') lastPress = millis();

  // Operator
  if (key == '+' || key == '-' || key == '*' || key == '/') {
    if (current != "") {
      memory = current;
      current = "";
      operation = key;

      lcd.setCursor(0,1);
      lcd.print("Op: ");
      lcd.print(operation);
    }
    return;
  }

  // Equal
  if (key == '=') {
    if (memory != "" && current != "" && operation) {
      double result = calculate(operation, memory.toDouble(), current.toDouble());

      lcd.clear();
      lcd.setCursor(0,0);
      lcd.print("Result:");
      lcd.setCursor(0,1);
      lcd.print(result, 2);
      
      current = String(result);
      memory = "";
      operation = 0;
    }
    return;
  }

  // Decimal
  if (key == '.') {
    if (current.indexOf('.') == -1) {
      current += key;
      lcd.print(key);
    }
    return;
  }

  // Numbers
  if (isDigit(key)) {

    lcd.setCursor(0,0);
    lcd.print("                ");
    lcd.setCursor(0,0);

    current += key;
    lcd.print(current);
  }
}

/* Setup */
void setup() {
  lcd.begin(16, 2);
  showSplashScreen();
}

/* Loop */
void loop() {
  char key = keypad.getKey();

  if (key) {
    processKey(key);
  }
}

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7. Detailed Step By Step Code Working

• LiquidCrystal & Keypad libraries handle LCD and keypad communication
• current → stores current number input
• memory → stores first operand
• operation → stores operator

Flow:

1. User enters number → stored in current
2. Press operator → moves current → memory
3. Enter next number
4. Press = → calculation happens
5. Result displayed
6. Double = → clears everything

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8. Tips

• Use 10k potentiometer for LCD contrast
• Avoid loose keypad connections
• Use lcd.print(result, 2) for better readability
• Keep wires short to avoid noise
• Quick double click on "=" will clear LCD display

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9. Uses

• Educational projects
• Arduino learning kits
• Embedded system demonstrations
• DIY calculator
• Student lab experiments

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10. Conclusion

This Arduino calculator project is a perfect combination of hardware interfacing and logical programming. It helps students understand real-time input processing, display control, and arithmetic operations.

With further improvements like memory functions and scientific operations, this can be upgraded into a fully featured calculator system.