Arduino - Keypad - Relay

In this tutorial, we are going to learn how to use a keypad, relay, and Arduino together. In detail, If a user inputs the password on the keypad correctly, Arduino turns the relay on.

The tutorial also provides the code that turns on a relay in a period of time and then turns off without using delay() function. The Arduino code also supports multiple passwords.

By connecting relay to Electromagnetic Lock, Solenoid Lock, , Linear, Heating Element, Pump, or Fan ... We can control them by keypad.

Arduino keypad relay

Hardware Required

1×Arduino UNO or Genuino UNO
1×USB 2.0 cable type A/B
1×Keypad
1×Relay
n×Jumper Wires
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About Keypad and Relay

If you do not know about keypad and Relay (pinout, how it works, how to program ...), learn about them in the following tutorials:

Wiring Diagram

Arduino keypad relay wiring diagram

Image is developed using Fritzing. Click to enlarge image

Arduino Code - turn relay on if the password is correct

The below codes turn a relay on if the password is correct

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-keypad-relay */ #include <Keypad.h> #define RELAY_PIN A0 // the Arduino pin, which connects to the IN pin of relay #define ROW_NUM 4 // four rows #define 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 ); const String password_1 = "ABC1234"; // change your password here const String password_2 = "5642B"; // change your password here const String password_3 = "9765"; // change your password here String input_password; void setup() { Serial.begin(9600); input_password.reserve(32); // maximum password size is 32, change if needed pinMode(RELAY_PIN, OUTPUT); // initialize pin as an output. digitalWrite(RELAY_PIN, LOW); } void loop() { char key = keypad.getKey(); if (key) { Serial.println(key); if (key == '*') { input_password = ""; // reset the input password } else if (key == '#') { if (input_password == password_1 || input_password == password_2 || input_password == password_3) { Serial.println("The password is correct, turning ON relay"); digitalWrite(RELAY_PIN, HIGH); } else { Serial.println("The password is incorrect, try again"); } input_password = ""; // reset the input password } else { input_password += key; // append new character to input password string } } }

Quick Steps

  • Connect Arduino to PC via USB cable
  • Open Arduino IDE, select the right board and port
  • 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
  • Search “ezOutput”, then find the ezOutput library by ArduinoGetStarted
  • Click Install button to install ezOutput library.
  • Arduino output library
  • Copy the above code and open with Arduino IDE
  • Click Upload button on Arduino IDE to upload code to Arduino
  • Arduino IDE Upload Code
  • Press 1234 keys and press #
  • Press 9765 keys and press #
  • See the result on Serial Monitor and the state of relay
  • COM6
    Send
    The password is incorrect, try again The password is correct, turning ON relay
    Autoscroll Show timestamp
    Clear output
    9600 baud  
    Newline  

Code Explanation

Authorized passwords are pre-defined in the Arduino code.

A string is used to store the password inputted by users, called input string. In keypad, two keys (* and #) are used for special purposes: clear password and terminate password. When a key on keypad is pressed:

  • If the pressed key is not two special keys, it is appended to the input string
  • If the pressed key is *, input string is clear. You can use it to start or re-start inputing the password
  • If the pressed key is #:
    • The input string is compared with the pre-defined passwords. If it matched with one of the pre-defined passwords, the relay is turned on.
    • No matter the password is correct or not, the input string is clear for the next input

Arduino Code - turn a relay on in a period of time if the password is correct

The below code turns the relay on for 5 seconds if the password is correct. After 5 seconds, the relay is turned off.

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-keypad-relay */ #include <Keypad.h> #include <ezOutput.h> #define RELAY_PIN A0 // the Arduino pin, which connects to the IN pin of relay #define ROW_NUM 4 // four rows #define COLUMN_NUM 4 // four columns #define RELAY_ON_TIME 5000 // in milliseconds 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 ); ezOutput relay(RELAY_PIN); const String password_1 = "ABC1234"; // change your password here const String password_2 = "5642B"; // change your password here const String password_3 = "9765"; // change your password here String input_password; void setup() { Serial.begin(9600); input_password.reserve(32); // maximum password size is 32, change if needed relay.low(); } void loop() { relay.loop(); // MUST call the loop() function first char key = keypad.getKey(); if (key) { Serial.println(key); if (key == '*') { input_password = ""; // reset the input password } else if (key == '#') { if (input_password == password_1 || input_password == password_2 || input_password == password_3) { Serial.println("The password is correct, turning ON relay"); relay.low(); // set low before making a high pulse relay.pulse(RELAY_ON_TIME); // turn on relay in RELAY_ON_TIME duration and then turn off } else { Serial.println("The password is incorrect, try again"); } input_password = ""; // reset the input password } else { input_password += key; // append new character to input password string } } }

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