How to Use a Keypad: a 4x4 Password Lock with Arduino

A keypad is a cheap input device, similar to the buttons on a phone, that lets you enter numbers or letters into your project. Password door locks, safes, calculators, alarm panels and menu systems are built with keypads. The most common models are thin membrane keypads with an adhesive back, in 4x4 (16 keys) and 3x4 (12 keys) layouts.

How does a keypad work? The row-column matrix

If each of the 16 keys had its own pin, you'd need 16 pins. Instead, a keypad arranges its keys in a matrix: there are 4 rows and 4 columns, each key sits where a row and a column cross, and pressing it connects that row to that column. The Arduino sets the rows LOW one at a time and reads the columns; whichever column reads LOW while a given row is LOW tells you which key is pressed. Because this scanning happens hundreds of times a second, 16 keys are read with just 8 pins.

Keypad pinout and wiring

With the keys facing you, the ribbon cable's pins are usually, from left to right, R1, R2, R3, R4, C1, C2, C3, C4: the first four are rows and the last four are columns. No extra resistors are needed; the library uses the Arduino's internal pull-up resistors for the columns.

Reading keys with the Keypad library

In the Library Manager, search for Keypad and install the library by Mark Stanley and Alexander Brevig. The code below prints every key you press to the Serial Monitor:

1#include <Keypad.h>
2
3const byte ROWS = 4, COLS = 4;
4char keys[ROWS][COLS] = {
5  {'1', '2', '3', 'A'},
6  {'4', '5', '6', 'B'},
7  {'7', '8', '9', 'C'},
8  {'*', '0', '#', 'D'}
9};
10byte rowPins[ROWS] = {9, 8, 7, 6};     // R1–R4
11byte colPins[COLS] = {5, 4, 3, 2};     // C1–C4
12Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
13
14void setup() {
15  Serial.begin(9600);
16}
17
18void loop() {
19  char key = keypad.getKey();          // tuş yoksa 0 döner / returns 0 if no key
20  if (key) {
21    Serial.println(key);
22  }
23}

getKey() doesn't block your program: if no key is pressed at that moment it returns 0 straight away, so you can do other work in loop(). The library handles key bounce itself and reports each press only once. If you want to wait until a key is pressed, use waitForKey().

Building a password lock

Now let's collect the pressed keys in a string and compare them with a stored password. The * key clears the entry and the # key confirms it. If the password is correct, a servo motor opens the lock for 3 seconds. For servo wiring, see What Is a Servo Motor?

1#include <Keypad.h>
2#include <Servo.h>
3
4const byte ROWS = 4, COLS = 4;
5char keys[ROWS][COLS] = {
6  {'1', '2', '3', 'A'},
7  {'4', '5', '6', 'B'},
8  {'7', '8', '9', 'C'},
9  {'*', '0', '#', 'D'}
10};
11byte rowPins[ROWS] = {9, 8, 7, 6};
12byte colPins[COLS] = {5, 4, 3, 2};
13Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
14
15Servo lock;
16const String PASSWORD = "1234";
17String input = "";
18
19void setup() {
20  Serial.begin(9600);
21  lock.attach(10);
22  lock.write(0);                        // kilitli / locked
23}
24
25void loop() {
26  char key = keypad.getKey();
27  if (!key) return;
28  if (key == '*') {                     // temizle / clear
29    input = "";
30  } else if (key == '#') {              // onayla / confirm
31    if (input == PASSWORD) {
32      Serial.println("Doğru, açılıyor / correct, opening");
33      lock.write(90);
34      delay(3000);
35      lock.write(0);
36    } else {
37      Serial.println("Yanlış şifre / wrong code");
38    }
39    input = "";
40  } else if (input.length() < 8) {
41    input += key;
42    Serial.print('*');                  // tuşu gizle / hide the key
43  }
44}

Using it with an LCD

A display makes it much easier to see what you're doing while entering a password. An I2C LCD only uses pins A4 and A5, so it doesn't touch the keypad's 8 digital pins. Print a star on the LCD's bottom row with every key press, and show "Open" or "Wrong" on the top row after confirming. Display wiring and commands are covered in Using an I2C LCD Display.

Fewer pins: I2C and analog keypads

A keypad alone takes 8 of the Uno's 20 pins. When you run short of pins there are two solutions: a PCF8574-based I2C keypad adapter brings the keypad down to just two wires and shares the bus with an LCD. On analog keypads, each key creates a different resistance and the whole keypad is read from a single analog pin; it needs fewer wires but can't detect two keys pressed at once.

Keypads with the ESP32 and Raspberry Pi Pico

The Keypad library also works on the ESP32. Because the row pins are used as outputs and the column pins as inputs with pull-ups, avoid GPIO 34–39, which can't output and have no pull-up resistors; for example, choose 13, 12, 14 and 27 for the rows and 26, 25, 33 and 32 for the columns. On the Pico with MicroPython, you can write the scanning yourself in a few lines:

1from machine import Pin
2import time
3
4KEYS = ["123A", "456B", "789C", "*0#D"]
5rows = [Pin(n, Pin.OUT, value=1) for n in (2, 3, 4, 5)]        # R1–R4
6cols = [Pin(n, Pin.IN, Pin.PULL_UP) for n in (6, 7, 8, 9)]     # C1–C4
7
8def scan():
9    for r, row in enumerate(rows):
10        row.value(0)                         # bu satırı etkinleştir / activate this row
11        for c, col in enumerate(cols):
12            if col.value() == 0:             # tuş basılı / key pressed
13                row.value(1)
14                return KEYS[r][c]
15        row.value(1)
16    return None
17
18last = None
19while True:
20    key = scan()
21    if key and key != last:                  # her basışı bir kez yaz / print each press once
22        print(key)
23    last = key
24    time.sleep_ms(50)

Keypad project ideas

• Password safe: a box that opens when the code is entered, with a servo motor, an LCD and a buzzer.
• Calculator: use keys A–D as the four operations and show the result on an LCD.
• Colour picker: change the colour of an RGB LED based on the number entered.
• Alarm panel: arm and disarm an alarm with a PIR sensor using a code.
• Number guessing game: find the number the Arduino picked with "higher" and "lower" hints.
• Shortcut keyboard: turn each key into a computer shortcut with an Arduino Leonardo.

Common mistakes and how to fix them

• A different key is read from the one pressed: the row and column order is reversed; reverse the order of the rowPins and colPins arrays.
• No key is read at all: the ribbon cable is plugged in one pin off, or the row and column pins differ from those in the code.
• Some keys never work: a row or column wire has a bad contact; if all keys in one row or column fail, that's the sign.
• Using pins 0 and 1: these pins are for USB serial communication; uploading code with the keypad connected can fail.
• Some columns always look pressed on the ESP32: GPIO 34–39, which have no pull-ups, are being used; move to other pins.
• Expecting repeats on a long press: getKey() reports each press once; use the library's HOLD state to detect a key being held.

A keypad is really just buttons arranged in a matrix; you'll find the logic of buttons in How to Connect a Push Button to Arduino. In the BlueGrays app you'll find 3x4 and 4x4 keypad basics, an LCD password app, a smart electronic safe, an I2C calculator and keypad RGB LED control, with circuit diagrams and code.