ArduinoUnoProjex August 14, 2025

Build a Robust Arduino Calculator (LCD 16×2 + 4×4 Keypad)

A complete, beginner-friendly guide with wiring table, improved code (clear, backspace, decimals, errors), Wokwi JSON, and upgrade ideas.

Arduino calculator schematic with LCD 16x2 and 4x4 keypad
Schematic overview of the Arduino calculator project.
Note: ELEGOO boards are not official Arduino® boards, but they are compatible with the Arduino IDE and typically more affordable.

Helpful Kits & Parts (Optional)

Project Overview

In this project, you’ll build a fully functional calculator using an Arduino, a 16×2 LCD (HD44780 compatible), and a 4×4 matrix keypad. The interface is clean and responsive: you enter digits and operators (+, -, *, /), view the expression live on the LCD, and get the result with =. We’ve added quality-of-life features like clear, backspace, decimal points, error messages (e.g., division by zero), and input length protection.

This build is ideal for beginners learning hardware interfacing (LCD + keypad) and for intermediate makers who want reliable input handling and clean code structure.

Features & UX

  • Intuitive keypad: 0–9, ., +, -, *, /, =, C (clear), < (backspace).
  • LCD feedback: First line shows mode/status, second line shows the current input or result.
  • Error handling: Division by zero, invalid sequences, and overflow give readable messages.
  • Configurable: Easy to remap pins, change keypad layout, or switch to an I²C LCD.
  • Portable: Works on Arduino Uno, Nano, Mega, and compatible boards.

Components & Alternatives

  • Arduino Uno (or Nano/Mega/compatible)
  • LCD 16×2 (HD44780) + 220 Ω resistor for backlight (or a potentiometer for contrast)
  • 4×4 matrix keypad
  • Breadboard + jumper wires
  • USB cable + computer with Arduino IDE

Alternative: Use an LCD with an I²C backpack to reduce wiring. Swap the LiquidCrystal library for LiquidCrystal_I2C and change a few lines.

Wiring Table (Pins)

Arduino Pin LCD Pin Keypad Notes
5VVDD—LCD power
GNDVSS, RW, K—Common ground
D12RS—Register select
D11E—Enable
D10D4—4-bit mode
D9D5—4-bit mode
D8D6—4-bit mode
D7D7—4-bit mode
5V → 220 Ω →A—Backlight via resistor
D5, D4, D3, D2—R1–R4Keypad rows
A3, A2, A1, A0—C1–C4Keypad columns

Build Steps

  1. Place the LCD and keypad on the breadboard (or mount the LCD separately).
  2. Wire power and ground to the LCD (VDD to 5 V, VSS to GND). Tie RW to GND.
  3. Connect LCD RS=D12, E=D11, D4..D7=D10..D7. Add a 220 Ω from 5 V to A.
  4. Connect keypad rows to D5, D4, D3, D2 and columns to A3, A2, A1, A0.
  5. Upload the sketch (below) from the Arduino IDE, then test each key.

Improved Arduino Code

Enhancements over the basic version: Backspace (<), clear (C), decimal support, input guard, and clearer error messages. The calculator evaluates simple two-operand expressions like 12.5*3. (You can extend it to multiple operators later.) 

// Arduino LCD & Keypad Calculator (Improved)
// Libraries: LiquidCrystal by Arduino, Keypad by Mark Stanley/Alexander Brevig

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

// ----- LCD Setup -----
LiquidCrystal lcd(12, 11, 10, 9, 8, 7);

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

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

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

// Optional: map a "backspace" on long-press of 'C' or repurpose a key if your keypad has one.
// For demonstration, we'll treat a quick double-press of 'C' within a window as backspace.
unsigned long lastCPress = 0;
const unsigned long doublePressGap = 350; // ms

String inputBuffer = "";
const uint8_t LCD_COLS = 16;
const uint8_t LCD_ROWS = 2;

// Forward declaration
double calculate(const String &expr, bool &ok);

void printStatus(const char* msg) {
  lcd.setCursor(0,0);
  lcd.print("                "); // clear line
  lcd.setCursor(0,0);
  lcd.print(msg);
}

void printInput() {
  // Show tail of inputBuffer if longer than LCD width
  String toShow = inputBuffer;
  if (toShow.length() > LCD_COLS) {
    toShow = toShow.substring(toShow.length() - LCD_COLS);
  }
  lcd.setCursor(0,1);
  lcd.print("                "); // clear line
  lcd.setCursor(0,1);
  lcd.print(toShow);
}

void clearAll() {
  inputBuffer = "";
  lcd.clear();
  printStatus("Enter:");
  printInput();
}

void setup() {
  lcd.begin(LCD_COLS, LCD_ROWS);
  printStatus("Calculator");
  delay(1200);
  clearAll();
}

void handleBackspace() {
  if (inputBuffer.length() > 0) {
    inputBuffer.remove(inputBuffer.length() - 1);
    printStatus("Edit:");
    printInput();
  }
}

bool isOperator(char c) {
  return (c=='+' || c=='-' || c=='*' || c=='/');
}

bool appendChar(char c) {
  // Basic guard: avoid two operators in a row, avoid multiple decimals in one number part
  if (isOperator(c)) {
    if (inputBuffer.length() == 0) return false; // no leading operator (except minus for negative, skipped here)
    char last = inputBuffer[inputBuffer.length()-1];
    if (isOperator(last) || last=='.') return false;
  }
  if (c=='.') {
    // Ensure only one dot per current operand
    // Find last operator
    int lastOp = -1;
    for (int i = inputBuffer.length()-1; i >= 0; --i) {
      if (isOperator(inputBuffer[i])) { lastOp = i; break; }
    }
    String current = (lastOp==-1) ? inputBuffer : inputBuffer.substring(lastOp+1);
    if (current.indexOf('.') != -1) return false;
  }
  // Limit total length to avoid LCD clutter / overflow
  if (inputBuffer.length() >= 32) return false;

  inputBuffer += c;
  return true;
}

void loop() {
  char k = keypad.getKey();
  if (!k) return;

  if (k == 'C') {
    unsigned long now = millis();
    if (now - lastCPress < doublePressGap) {
      // Double-press C => Backspace
      handleBackspace();
      lastCPress = 0; // reset chain
      return;
    }
    lastCPress = now;
    // Single C => Clear
    clearAll();
    return;
  }

  if (k == '=') {
    bool ok = false;
    double res = calculate(inputBuffer, ok);
    lcd.clear();
    if (ok) {
      printStatus("Result:");
      lcd.setCursor(0,1);
      lcd.print(res, 6); // up to 6 decimals
    } else {
      printStatus("Error:");
      lcd.setCursor(0,1);
      lcd.print("Invalid input");
    }
    delay(2000);
    clearAll();
    return;
  }

  if (appendChar(k)) {
    printStatus("Enter:");
    printInput();
  } else {
    printStatus("Invalid key");
    delay(600);
    printStatus("Enter:");
  }
}

// Calculate simple "A op B" where op in + - * /
double calculate(const String &expr, bool &ok) {
  ok = false;
  if (expr.length() == 0) return 0.0;

  // Find the single operator (only one allowed in this version)
  int opIndex = -1;
  char op = 0;
  for (int i = 0; i < expr.length(); ++i) {
    char c = expr[i];
    if (isOperator(c)) {
      if (opIndex != -1) return 0.0; // multiple operators => invalid
      opIndex = i; op = c;
    }
  }

  if (opIndex <= 0 || opIndex >= expr.length()-1) return 0.0; // need A op B

  String aStr = expr.substring(0, opIndex);
  String bStr = expr.substring(opIndex+1);

  // Edge: allow "-x + y" style? (Not in this simple version—keep clean)
  double A = aStr.toDouble();
  double B = bStr.toDouble();

  double out = 0.0;
  switch (op) {
    case '+': out = A + B; break;
    case '-': out = A - B; break;
    case '*': out = A * B; break;
    case '/':
      if (B == 0.0) { printStatus("Div by zero"); delay(900); return 0.0; }
      out = A / B; break;
    default: return 0.0;
  }
  ok = true;
  return out;
}
Pin remapping tips
  • Change LiquidCrystal lcd(…) to match your digital pins.
  • Update rowPins/colPins to match your keypad header.
  • Using an I²C LCD? Replace the constructor with your I²C address and use LiquidCrystal_I2C.

Wokwi JSON (Simulator Layout)

Paste this into wokwi.com (New Project → diagram.json) to simulate the wiring quickly.

{
  "version": 1,
  "author": "https://www.youtube.com/@EasyElectronics2",
  "editor": "wokwi",
  "parts": [
    { "type": "wokwi-arduino-uno", "id": "uno", "top": 200, "left": 20, "attrs": {} },
    { "type": "wokwi-membrane-keypad", "id": "keypad", "top": 140, "left": 360, "attrs": {} },
    { "type": "wokwi-lcd1602", "id": "lcd", "top": 8, "left": 20, "attrs": {} },
    { "type": "wokwi-resistor", "id": "r1", "top": 140, "left": 220, "attrs": { "value": "220" } }
  ],
  "connections": [
    [ "uno:GND.1", "lcd:VSS", "black", [ "v-51","*","h0","v18" ] ],
    [ "uno:GND.1", "lcd:K",   "black", [ "v-51","*","h0","v18" ] ],
    [ "uno:GND.1", "lcd:RW",  "black", [ "v-51","*","h0","v18" ] ],
    [ "uno:5V",    "lcd:VDD", "red",   [ "v16","h-16" ] ],
    [ "uno:5V",    "r1:2",    "red",   [ "v16","h-118","v-244","h50" ] ],
    [ "r1:1",      "lcd:A",   "pink",  [] ],
    [ "uno:12",    "lcd:RS",  "blue",  [ "v-16","*","h0","v20" ] ],
    [ "uno:11",    "lcd:E",   "purple",[ "v-20","*","h0","v20" ] ],
    [ "uno:10",    "lcd:D4",  "green", [ "v-24","*","h0","v20" ] ],
    [ "uno:9",     "lcd:D5",  "brown", [ "v-28","*","h0","v20" ] ],
    [ "uno:8",     "lcd:D6",  "gold",  [ "v-32","*","h0","v20" ] ],
    [ "uno:7",     "lcd:D7",  "gray",  [ "v-36","*","h0","v20" ] ],
    [ "uno:A3",    "keypad:C1", "brown",  [ "v116","*","h0","v0" ] ],
    [ "uno:A2",    "keypad:C2", "gray",   [ "v120","*","h0","v0" ] ],
    [ "uno:A1",    "keypad:C3", "orange", [ "v124","*","h0","v0" ] ],
    [ "uno:A0",    "keypad:C4", "pink",   [ "v128","*","h0","v0" ] ],
    [ "uno:5",     "keypad:R1", "blue",   [ "v-34","h96","*","v12" ] ],
    [ "uno:4",     "keypad:R2", "green",  [ "v-30","h80","*","v16" ] ],
    [ "uno:3",     "keypad:R3", "purple", [ "v-26","h64","*","v20" ] ],
    [ "uno:2",     "keypad:R4", "gold",   [ "v-22","h48","*","v24" ] ]
  ],
  "dependencies": {}
}

Troubleshooting

  • Blank LCD? Ensure VDD=5V, VSS=GND, and try adjusting LCD contrast (if you added a pot). Confirm RW is tied to GND.
  • Weird characters? Check 4-bit wiring order: D4→D10, D5→D9, D6→D8, D7→D7.
  • Keypad not responding? Verify row/col pin order and keypad orientation. Swap rows/cols if needed.
  • “Invalid input” often? Avoid entering two operators in a row; use C to clear or double-tap C for backspace.

Upgrades & Ideas

  • I²C LCD to free pins and simplify wiring.
  • Multiple operations with proper order of operations (implement the shunting-yard algorithm).
  • EEPROM memory to store last results or a simple history.
  • Enclosure with 3D printed faceplate and labeled keys.
  • Buzzer feedback on keypress and error tones.

FAQ

Can I use an Arduino Nano instead of Uno?

Yes. Just remap the pins to the Nano’s layout—the code stays the same.

How do I add negative numbers?

Easiest path: allow a leading - before the first number. For full support inside expressions, you’ll need a richer parser (tokenize and handle unary minus).

What about floating-point precision?

Arduino’s double on AVR is 32-bit float precision. For a basic calculator that’s fine. For higher precision, consider larger MCUs or fixed-point arithmetic.

  • Arduino Calculator Project
  • DIY Arduino Calculator
  • Arduino LCD Keypad Calculator
  • Arduino 4x4 Keypad Tutorial
  • Arduino Math Project
  • Simple Arduino Calculator
  • Arduino Microcontroller Projects
  • Arduino Educational Projects
  • Arduino Programming for Beginners
  • Arduino LCD Display Projects
  • Matrix Keypad Arduino Example
  • Arduino LCD Calculator Code
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