Arduino - Keypad

The keypad is widely used in many devices such as door lock, ATM, calculator...

In this tutorial, we will learn:

Hardware Required

1×Arduino UNO or Genuino UNO
1×USB 2.0 cable type A/B
1×Keypad 3x4
1×Keypad 4x4 (optional)
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About Keypad

Keypad

The keypad is a set of buttons arranged in rows and columns (called matrix). Each button is called key

Keypad has various types. Two popular types for DIY projects are keypad 3x4 (12 keys) and keypad 4x4 (16 keys).

Pinout

Keypad pins are divided into two groups: row and column.

Keypad 3x4 has 7 pins: 4 row-pins (R1, R2, R3, R4) and 3 column-pin (C1, C2, C3).

Keypad 4x4 has 8 pins: 4 row-pins (R1, R2, R3, R4) and 4 column-pin (C1, C2, C3, C4).

Keypad Pinout

How It Works

This section is the in-depth knowledge. DON'T worry if you don't understand. Ignore this section if it overloads you, and come back in another day. Keep reading the next sections.

The process of detecting the key pressing is called scanning keypad.

It is called “scanning” because it checks one key by one key.

Row-pins are connected to Arduino's output pins

Column pins are connected to Arduino's input pins (INPUT_PULLUP, in this state, the value of the input pin is HIGH if the key is not pressed).

For each row:

  • Sets all row-pins is HIGH.
  • Sets only the current row-pin to LOW.
  • Reads the state of each column.
    • If a column-pin is HIGH ⇒ key at (row, column) is NOT pressed.
    • If a column-pin is LOW ⇒ key at (row, column) is pressed.
  • Repeats the above process for the next row-pins.

※ NOTE THAT:

The above is one of the methods to scan keypad. We can invert all HIGH to LOW and all LOW to HIGH to scan keypad.

Why does keypad is arranged and connected as a matrix? This makes the scanning process complicated. Why do not use each key as an independent button, then the state of the key is simply determined by reading the state of a button?

⇒ As we know, an independent button requires one Arduino's pin and GND. Let's take keypad 4x4 as an example. If we each key as an independent button, it requires 16 Arduino pin for 16 keys plus GND pin. If we arranged a connected key in matrix form, we just need to use 8 Arduino's pin, so we can save Arduino's pin. In short, the answer is: to save the Arduino pins.

Wiring Diagram

Arduino Keypad Wiring Diagram

Image is developed using Fritzing. Click to enlarge image

How To Program For Keypad

Thanks to Keypad library , using keypad with Arduino is a piece of cake, no matter whether you understand how the keypad works or not.

Arduino Code

Keypad 3x4

#include <Keypad.h> const int ROW_NUM = 4; //four rows const int COLUMN_NUM = 3; //three columns char keys[ROW_NUM][COLUMN_NUM] = { {'1','2','3'}, {'4','5','6'}, {'7','8','9'}, {'*','0','#'} }; byte pin_rows[ROW_NUM] = {9, 8, 7, 6}; //connect to the row pinouts of the keypad byte pin_column[COLUMN_NUM] = {5, 4, 3}; //connect to the column pinouts of the keypad Keypad keypad = Keypad( makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM ); void setup(){ Serial.begin(9600); } void loop(){ char key = keypad.getKey(); if (key){ Serial.println(key); } }

Keypad 4x4

#include <Keypad.h> const int ROW_NUM = 4; //four rows const int COLUMN_NUM = 4; //four columns char keys[ROW_NUM][COLUMN_NUM] = { {'1','2','3', 'A'}, {'4','5','6', 'B'}, {'7','8','9', 'C'}, {'*','0','#', 'D'} }; byte pin_rows[ROW_NUM] = {9, 8, 7, 6}; //connect to the row pinouts of the keypad byte pin_column[COLUMN_NUM] = {5, 4, 3, 2}; //connect to the column pinouts of the keypad Keypad keypad = Keypad( makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM ); void setup(){ Serial.begin(9600); } void loop(){ char key = keypad.getKey(); if (key){ Serial.println(key); } }

Quick Steps

On Arduino IDE, Go to Tools Manage Libraries

Arduino add library
  • Search “keypad”, then find the keypad library by Mark Stanley, Alexander Brevig
  • Click Install button to install keypad library.
  • Arduino keypad library
  • Copy the above code and open with Arduino IDE
  • Click Upload button on Arduino IDE to upload code to Arduino
  • Open Serial Monitor
  • Press some keys on keypad
  • See the result in Serial Monitor
  • COM6
    Send
    3 6 9 4 * #
    Autoscroll Show timestamp
    Clear output
    9600 baud  
    Newline  

Keypad and Password

A popular application of keypad is the password input. In this application, we specify two special keys:

  • A key to start/re-start the password input. For example, key "*"
  • A key to terminate the password input. For example, key "#"

The password will be a string that contains the remaining keys, except for two selected special keys.

When a key is pressed.

  • If the key is NOT neither "*" nor "#", append the key to the user's input password string.
  • If the key is "#", compare the user's input password string with the password to determine the input password is correct or not, and then clear the user's input password string
  • If the key is "*", clear the user's input password string

Keypad - Password Code

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-keypad */ #include <Keypad.h> const int ROW_NUM = 4; //four rows const int COLUMN_NUM = 3; //three columns char keys[ROW_NUM][COLUMN_NUM] = { {'1','2','3'}, {'4','5','6'}, {'7','8','9'}, {'*','0','#'} }; byte pin_rows[ROW_NUM] = {9, 8, 7, 6}; //connect to the row pinouts of the keypad byte pin_column[COLUMN_NUM] = {5, 4, 3}; //connect to the column pinouts of the keypad Keypad keypad = Keypad( makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM ); const String password = "1234"; // change your password here String input_password; void setup(){ Serial.begin(9600); input_password.reserve(32); // maximum input characters is 33, change if needed } void loop(){ char key = keypad.getKey(); if (key){ Serial.println(key); if(key == '*') { input_password = ""; // clear input password } else if(key == '#') { if(password == input_password) { Serial.println("password is correct"); // DO YOUR WORK HERE } else { Serial.println("password is incorrect, try again"); } input_password = ""; // clear input password } else { input_password += key; // append new character to input password string } } }
  • Run above code
  • Open Serial Monitor
  • Press "123456" keys and press "#"
  • Press "1234" keys and press "#"
  • See the result on Serial Monitor
  • COM6
    Send
    password is incorrect, try again password is correct
    Autoscroll Show timestamp
    Clear output
    9600 baud  
    Newline  

Video Tutorial

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Challenge Yourself

  • Display the pressed key of the keypad on LCD. Hint: Refer to Arduino - LCD
  • Make a door lock with password protection using the keypad.

Alternative Keypads

The above code also works with the following keypads:

Keypad on Commercial Products

  • Casio MS-80B Standard Calculator
  • Casio fx-115ES PLUS Engineering/Scientific Calculator
  • Electronic Door Knob
  • Yale Security Assure Lock

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