#include <LiquidCrystal_I2C.h>
#include <Keypad.h>

// LCD setup
LiquidCrystal_I2C lcd(0x27, 16, 2);

// Keypad setup
const byte ROWS = 4, COLS = 4;
char keys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte rowPins[ROWS] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};
Keypad pad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

// Button setup
const int btnPin = A0;
unsigned long lastTime = 0;
int cnt = 0;
bool lcdOn = true;
String buf = "";

void setup() {
  lcd.init();
  lcd.backlight();
  pinMode(btnPin, INPUT);
}

void loop() {
  checkBtn();
  checkPad();
}

void checkBtn() {
  static bool last = LOW;
  bool curr = digitalRead(btnPin);

  if (curr && !last) {
    unsigned long now = millis();

    cnt = (now - lastTime < 3000) ? cnt + 1 : 1;
    lastTime = now;

    lcdOn = !lcdOn;
    lcdOn ? lcd.backlight() : lcd.noBacklight();

    if (cnt >= 3) {
      buf = "";
      lcd.clear();
      cnt = 0;
    }
  }
  last = curr;
}

void checkPad() {
  char key = pad.getKey();
  if (key && lcdOn) {
    buf += key;
    if (buf.length() > 20) buf = buf.substring(buf.length() - 20);
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print(buf);
  }
}