Read a 4×4 keypad

Keypads are everywhere: door locks, alarm panels, vending machines. The 4×4 membrane keypad has 16 keys but only 8 wires, because the keys are arranged in a grid of 4 rows and 4 columns. In this lesson you read the keys and build a secret-code lock.

What you'll learn

• How a key matrix works
• How to use the Keypad library
• How to compare a typed code with a secret code

What you need

• UNO R3 board and USB cable
• 4×4 membrane keypad
• 8 male-to-male jumper wires

Step 1: Install the library

Open Tools → Manage Libraries, search for Keypad and install the one by Mark Stanley and Alexander Brevig.

Step 2: Wiring

Hold the keypad with the keys facing you. The 8 pins on the ribbon connector are, from left to right, the 4 rows (R1 to R4) and then the 4 columns (C1 to C4). Plug the jumper wires straight into the connector.

Step 3: Print the keys

#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] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};

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

void setup() {
  Serial.begin(9600);
  Serial.println("Press a key");
}

void loop() {
  char key = keypad.getKey();
  if (key) {
    Serial.print("Key pressed: ");
    Serial.println(key);
  }
}

Open the Serial Monitor at 9600 baud and press a few keys.

Step 4: A secret-code lock

Type the code and finish with #. Press * to start again.

#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] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

const String SECRET = "1234";
String typed = "";

void setup() {
  Serial.begin(9600);
  Serial.println("Enter the code, then press #");
}

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

  if (key == '#') {
    if (typed == SECRET) {
      Serial.println("Unlocked!");
    } else {
      Serial.println("Wrong code, try again");
    }
    typed = "";
  } else if (key == '*') {
    typed = "";
    Serial.println("Cleared");
  } else {
    typed += key;
    Serial.print("*"); // hide the digits
  }
}

How it works

Each key sits where one row wire crosses one column wire. The library switches the rows on one at a time and checks which column answers. That tells it exactly which key is pressed, using only 8 pins instead of 16.

Try this next

• Combine this with lesson 3: turn the servo to 90° when the right code is entered, like a real lock.
• Add the LED from lesson 1 and blink it three times on a wrong code.

Troubleshooting

• The wrong keys appear: the connector may be reversed. Swap the order of the wires, or change the order in rowPins and colPins.
• Nothing appears: check that the Serial Monitor speed is 9600 baud and the library is installed.