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Arduino - Keypad - Electromagnetic Lock

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

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

Hardware Required

1	×	Arduino UNO or Genuino UNO
1	×	USB 2.0 cable type A/B (for USB-A PC)
1	×	USB 2.0 cable type C/B (for USB-C PC)
1	×	Keypad 3x4
1	×	Relay
1	×	Jumper Wires
1	×	Electromagnetic Lock
1	×	12V Power Adapter
1	×	(Optional) DC Power Jack
1	×	(Recommended) Screw Terminal Block Shield for Arduino Uno
1	×	(Recommended) Breadboard Shield For Arduino Uno
1	×	(Recommended) Enclosure For Arduino Uno

Or you can buy the following sensor kits:

1	×	DIYables Sensor Kit (30 sensors/displays)
1	×	DIYables Sensor Kit (18 sensors/displays)

Disclosure: Some links in this section are Amazon affiliate links. If you make a purchase through these links, we may earn a commission at no extra cost to you.
Additionally, some links direct to products from our own brand, DIYables.

About Keypad and Electromagnetic Lock

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

Arduino - Keypad tutorial
Arduino - Electromagnetic Lock tutorial

Wiring Diagram

This image is created using Fritzing. Click to enlarge image

Arduino Code - turn electromagnetic lock on if the password is correct

The below codes turn a electromagnetic lock 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-electromagnetic-lock */ #include <Keypad.h> #define RELAY_PIN A0 // the Arduino pin that controls electromagnetic lock via relay #define ROW_NUM 4 // four rows #define 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_1 = "1234"; // change your password here const String password_2 = "56789"; // change your password here const String password_3 = "901234"; // 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, HIGH); // lock the electromagnetic lock } 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 => unlock"); digitalWrite(RELAY_PIN, LOW); } 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
Navigate to the Libraries icon on the left bar of the Arduino IDE.
Search “keypad”, then find the keypad library by Mark Stanley, Alexander Brevig
Click Install button to install keypad library.

Copy the above code and open with Arduino IDE
Click Upload button on Arduino IDE to upload code to Arduino

Press 8123 keys and press #
Press 1234 keys and press #
See the result on Serial Monitor and the state of electromagnetic lock

COM6

Send

The password is incorrect, try again The password is correct => unlock

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 electromagnetic lock is turned on.
No matter the password is correct or not, the input string is clear for the next input

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

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

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-keypad-electromagnetic-lock */ #include <Keypad.h> #include <ezOutput.h> #define UNLOCK_TIME 5000 // in milliseconds #define RELAY_PIN A0 // the Arduino pin that controls electromagnetic lock via relay #define ROW_NUM 4 // four rows #define 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 ); ezOutput relay(RELAY_PIN); const String password_1 = "1234"; // change your password here const String password_2 = "56789"; // change your password here const String password_3 = "901234"; // 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.high(); // lock the electromagnetic lock } 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 => unlock"); relay.high(); // set high before making a low pulse relay.pulse(UNLOCK_TIME); // deactivate the electromagnetic lock during UNLOCK_TIME duration } 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 } } }

Please note that, the above code use ezOutput library, which makes it easy to manage time in non-blocking maner. You can refer to ezOutput Library Installization Guide

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