In this tutorial I will show you how to flash a LED light with Raspberry Pi and Java. This will be a complete step by step tutorial starting with connecting the cables, trough GPIO explanation and finally writing the java code. At the end of this tutorial I will show you how to write a complete java program using the LED light connected to the Raspberry.

Introduction

One of the simplest electrical circuits that you can build is a battery connected to a light source and a switch (the resistor is there to protect the LED):

The Raspberry Pi replaces both the switch and the battery in the above diagram.

Requirements

Raspberry Pi board with Raspbian OS (learn how to install and configure Raspbian)

1 x LED light

2 x female to female jumper cable wires (you can buy 40 pcs of them online for less than 2$)

Raspberry Pi GPIO

According to the model the Raspberry Pi provides a different numbers of GPIO (general purpose input/output) pins along the edge of the board, next to the yellow video out socket.

These pins are a physical interface between the Pi and the outside world. Each pin can turn on or off,or go HIGH or LOW in computing terms. When the pin is HIGH it outputs 3.3 volts (3v3); when the pin is LOW it is off. 17 of the 26 pins (for model A and B) are GPIO pins; the others are power or ground pins. Model A+ and B+ have 40 pins and 26 of them are GPIO pins. This tutorial will work for all Raspberry Pi models.

Assembling

Look at the diagram below. We will connect the LED cathode (-) to Raspberry Pi pin #6 and the anode (+) to pin #12 using the jumper wires

First connect the cables to the LED. I connected a red cable to the anode and a brown one to the cathode. You may use whatever colors you like.

Now connect the other end of the cables to your Raspberry board pins 6 and 12

The assembly is done. Now power on the Pi.

Install Pi4J on Raspberry Pi

For this tutorial we will use Pi4J. Pi4J is a open source Java API which does the low-level communication with the device and gives us an abstract and object oriented approach to control the GPIOs of the Pi using the Java programming language.

The easiest way to install it is to run following command on your Pi. This will download all the dependencies and files you need and put them in /opt/pi4j/lib folder. You can explore other installation option here if this method does not work for you.

curl -s get.pi4j.com | sudo bash

Pi4J gives names and numbers to GPIO pins as shown below:

Writing the Code

First include the Pi4J libs into your project. If you use Maven you can add this dependency to your POM file:

<dependency>
	<groupId>com.pi4j</groupId>
	<artifactId>pi4j-core</artifactId>
	<version>${project.version}</version>
</dependency>

You can also do this the old-fashioned way:

1) Download pi4j ZIP file from the download page

2) Copy and include following JAR files (found in the lib folder in the archive you downloaded) to your project:

The JAR files are needed for the code to compile.

Now, that we have all the dependencies, lets write the code. Our program will do following:

1) turn on the LED for 2 seconds

2) turn off the LED and wait 1 second

3) turn on the LED for 1 second

import com.pi4j.io.gpio.GpioController;
import com.pi4j.io.gpio.GpioFactory;
import com.pi4j.io.gpio.GpioPinDigitalOutput;
import com.pi4j.io.gpio.PinState;
import com.pi4j.io.gpio.RaspiPin;

public class LedControl {
	
    public static void main(String[] args) throws InterruptedException {
    	
        // get a handle to the GPIO controller
    	final GpioController gpio = GpioFactory.getInstance();
        
        // creating the pin with parameter PinState.HIGH
        // will instantly power up the pin
        final GpioPinDigitalOutput pin = gpio.provisionDigitalOutputPin(RaspiPin.GPIO_01, "PinLED", PinState.HIGH);
        System.out.println("light is: ON");
        
        // wait 2 seconds
        Thread.sleep(2000);
        
        // turn off GPIO 1
        pin.low();
        System.out.println("light is: OFF");

        // wait 1 second
        Thread.sleep(1000);

        // turn on GPIO 1 for 1 second and then off
        System.out.println("light is: ON for 1 second");
        pin.pulse(1000, true);
        
        // release the GPIO controller resources
        gpio.shutdown();
    }
}

Execute the program on Raspberry like this

sudo java -classpath .:classes:/opt/pi4j/lib/'*' LedControl

You should now see the LED go on first for 2 seconds and than for 1 second.