In this lesson we will actually begin to control the GPIO pins from the Raspberry Pi. We will start by looking at how to write a pin high or low. We will be doing this in the Python programming language. A really important thing to remember is that the default “Pi” user does not have access to the pins, so for these examples to work, you must run the programs with “sudo”. The sudo command executes as super user, and will give the program access to the GPIO pins.
To begin with, lets build a simple circuit. If you purchased the kit we showed in lesson 1, you should have all the components you need to follow along with these examples. If you have not purchased a kit yet, you can get one on amazon.com HERE. In this first lesson we will just be looking at blinking an LED. So, you can now go ahead and hook up the following circuit. For reference, we show below the pinout of the Raspberry Pi.
In this example we will be using physical pin 9 as a ground, and physical pin 11 as the control pin. You can now go ahead and hook up the following circuit. Please remember the direction you plug the LED in matters . . . the long leg needs to connect to pin 11. The resistor used should be about 330 ohms.
In order to become familiar with the commands, I like to start in the Python shell. Basically, we give the commands to python one line at a time and watch what happens. Then later, we can write and run programs.
Note that recent versions of the Raspberry Pi distribution include the RPi library, but if you have an older distribution, update your system using these commands:
$ sudo apt-get update
$ sudo apt-get upgrade
If you have not done this is a while, it can take some time to download and install the updates.
We are now ready to begin to work with the GPIO pins. To enter the python shell, open a terminal window on the Raspberry Pi, and you will want to type:
$ sudo python
Be sure and use the sudo command above, as that will give you administrative access to the GPIO pins. Now, you should get the python command shell prompt, that looks like this:
At this point, any command you type will be executed by the python interpreter. You can basically execute a python program one line at a time. Note, to exit the python shell type Ctrl-d.
OK, so lets see if we can control the LED!
First, we need to import the RPi library. Note this is case sensitive, so be careful to do capitalization exactly:
>>> import RPi.GPIO as GPIO
Now, we need to initialize the GPIO to use either the BOARD or the BCM pin numbering schemes. In the diagram at the top of this lesson, the BOARD numbering convention is shown in the center two columns. If you want to use the BCM numbering scheme, you would use the numbers indicated in the outer two columns. In these examples, I want to use the physical pin numbers, as that is easier to me to keep track of things. Hence, I will want to use the BOARD scheme. I can do that with this command:
As you can imagine, if you want to use BCM, BOARD should be replaced with BCM in the command above.
If you remember in our Arduino lessons, we had to do pinmode commands to tell the arduino whether pins are inputs or outputs. We do an analogous thing in Raspberry Pi. We need to tell the Pi whether we will be using a pin as an input or output. In the wiring diagram above, you can see that we want to power the LED from physical pin 11, so we need to set that as an output.
We are now ready to turn the LED on. We can do this by setting pin 11 to True:
Now to turn the LED off, we can do:
You can now play around with different GPIO pins, and turn the LED on and off as you like. Before leaving the Python shell, be sure to clean up the GPIO. You do this by giving the cleanup command:
This will ensure you do not get error messages if you try to work with the GPIO pins again. It is a good practice to always cleanup after you are done.