4×4 Keypad with an Arduino Uno

// 4×4 Keypad on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/keypad-4x4/arduino-uno
#include <Keypad.h>

const byte ROWS = 4;
const byte COLS = 4;

char keys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};

byte rowPins[ROWS] = {2, 3, 4, 5};  // R1, R2, R3, R4
byte colPins[COLS] = {6, 7, 8, 9};  // C1, C2, C3, C4

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

// Returns the next pressed key character, or NO_KEY (== 0) if none.
// Combines the Keypad library (matrix scan on real hardware) with
char readKey() {
  char k = kpd.getKey();
  if (k) return k;
  if (Serial.available()) {
    char c = Serial.read();
    if (c >= ' ') return c;  // skip newlines / control chars
  }
  return NO_KEY;
}

void setup() {
  Serial.begin(115200);
}

void loop() {
  char key = readKey();
  if (key) {
    Serial.println(key);
  }
}