Preparing a MicroSD Card with NOOBS

When the Raspberry Pi was first launched, you had to use special image writing software to set up an SD card. However, NOOBS changed all that—it’s as easy to install as copying files onto the SD card and no special formatting is required.

You will find the latest, up-to-date instructions on using NOOBS to set up your microSD card here: https://www.raspberrypi.org/help/noobs-setup/.

When your Raspberry Pi boots up into NOOBS it will offer you a choice of operating system (Figure 2-3). Make sure that you select the recommended option Raspbian. 

After a lot of file copying and a reboot, your Raspberry Pi will be ready to use. At this point, if you have a USB WiFi adapter, you can configure it and join your wireless network using the WiFi Config utility that you will find under the Preferences section of the desktop menu.

Figure 3-3. NOOBS offering a selection of operating systems to install

Setting Up SSH

For many of the projects and experiments in this book, having a keyboard, mouse, and monitor attached to your Raspberry Pi is more of a hindrance than a help. SSH gives you command-line access to your Raspberry Pi over your network from a second computer. 

This means that, once you have finished setting it up, the only things that need to be plugged into your Raspberry Pi are a power lead and either a network cable or USB WiFi adapter. 

Click the LXTerminal icon at the top of your Raspberry Pi desktop (Figure 3-4). In the LXTerminal window that opens, type the following command (don’t include the $—that’s the command-line prompt):

$ sudo raspi-config

Figure 3-4. Starting LXTerminal

This will open the rasp-config tool for setting up your Raspberry Pi. Use the arrow keys to move down to the Advanced menu option and press Enter. Then use the arrow keys again to select SSH (Figure 3-5).

Figure 3-5. Enabling SSH using raspi-config

Click Enable, then select the Finish menu option.

Now that your Raspberry Pi is enabled for SSH remote access, it’s time to connect from your computer.

If you have a Mac or Linux computer, it will have its own terminal program that you can use to connect to the Raspberry Pi, so skip ahead to “SSH on Mac or Linux”. 

SSH on a Windows Computer

If you use Microsoft Windows, you should download and install Putty.

The installation looks a little odd, but actually, you just download putty.exe, which is the program itself. You can just save it somewhere (say on your desktop) and run it by double-clicking it. This will open the PuTTY Configuration window (Figure 3-6).

Figure 3-6. The PuTTY Configuration window

Enter the IP address of your Raspberry Pi in the “Host Name (or IP address)” field (see “Finding the IP Address of a Raspberry Pi”), and click Open. You will then be prompted to log in to your Pi (Figure 3-7). Enter a username of pi and a password of raspberry and that’s it, you are in. You can now type commands into PuTTY on your main computer, and they will be executed on your Raspberry Pi.

Figure 3-7. Remote control of your Raspberry Pi with SSH

SSH on Mac or Linux

If you use a Mac or a Linux computer, the software you need to connect to your Raspberry Pi is already on your computer. Open a terminal session and type the following command, substituting the IP address of your Raspberry Pi in place of the IP address of my Raspberry Pi  (192.168.1.23):

$ ssh 192.168.1.23 -l pi

The first time you do this, you will see the following message:

The authenticity of host '192.168.1.23 (192.168.1.23)' can't be established.
RSA key fingerprint is 48:8f:c3:07:c2:04:9e:8b:59:ed:53:2b:0b:d0:aa:e5.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.23' (RSA) to the list of known hosts.
[email protected]'s password:

You can go ahead and confirm the authenticity of the computer you are connecting to by typing yes. From now on, any commands that you type are actually happening on the Raspberry Pi rather than your other computer.

Hello, World

Traditionally, when you’re working with a new language, the  first program you learn to write just prints out the words “Hello, World” on the screen. To try this out, fire up nano using the following command:

$ nano hello.py

The .py file extension identifies the program file as being Python. Next, type the following text into the nano editor and then save the file:

print('Hello, World')

Then run the program:

$ python hello.py
Hello, World

Tabs and Indents

Programmers have a style of laying out their code to make it easier to understand. In Arduino C, blocks of code inside a function or if statement are indented so you can see which lines of code belong to which command or function. In Python, this is not a matter of personal preference—Python insists upon it.

In Python, there are no { or } to show the start and end of a block of code—you have to use indentation to show which lines belong together.

For example, take a look at this fragment of code:

 while True :      
    GPIO.output(control_pin, False)
    time.sleep(5)
    GPIO.output(control_pin, True)
    time.sleep(2)
 print("Finished")

This is a while loop (just like while in Arduino C, as described in “Loops”). In this case, the condition is True. The value True is always True, the loop will continue forever. At the end of the first line, there is a colon (:). The colon indicates that what follows will be a block of code. So the colon in Python is just like the { in Arduino C, but there is no closing marker.

The lines inside the block must be tabbed in from the line above. As long as you are consistent, it does not matter how many spaces (or tabs) you indent the code. Most programmers tend to use four space characters for each level of indentation.

At the end of the block, there is no marker, but instead you just stop indenting the code. So in the preceding example, the final print command is outside of the loop. Because the while loop continues forever, this last line will never actually get run.

Variables

Variables in Python are like Arduino C, but differ because when you first use a variable, you do not have to specify whether it’s an int or a float or something else. You just assign it a value and there is nothing (apart from good sense) to stop you assigning the same variable to be an integer value one minute and a string the next. For example, the following code is perfectly legal, but not very sensible:

a = 123.45
a = "message"

Incidentally, when using strings, you can either use double quotes (as in the preceding example) or single quotes.

if, while, etc.

Python has an if and else structure just like Arduino C, but it uses a colon and indentation to indicate the blocks of code inside the if statement:

if x > 10 :
    print("x is big!")
else:
    print("x is small")

As you work your way through the book, you will learn more about loops and other programming structures. 

The RPi.GPIO Library

Like Arduino C, Python supports libraries—you just need to import them before you can use them in a program.   

This book is going to be doing a lot with the GPIO connector on the Raspberry Pi. The most popular Python library to control GPIO pins and the one used in this book is RPi.GPIO. 

This library is preinstalled with Raspbian, so you do not need to install anything; you can just import it into your program and start using it.

Confusingly, there are two naming conventions for the pins on a Raspberry Pi’s GPIO connector: one uses the positions of the pins on the connector (1, 2, 3, 4, etc.) and the other uses the functions of the pins. In the early days of Raspberry Pi, both were used fairly frequently. These days, pretty much everyone refers to pins based on their function. The RPi.GPIO library will work with both ways of pin naming, but you have to tell it which to use, so you will see the following code near the top of every Python program in this book:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)

The first line imports the RPi.GPIO library and the second specifies the Broadcom (BMC), which is the name of the chip at the center of the Raspberry Pi.

Unlike the Arduino, the Raspberry Pi does not have analog inputs. The RPi.GPIO library does, however, support PWM analog outputs.

The GPIO Header

Figure 3-9 shows the connections available on the GPIO header. If you have an older Raspberry Pi, with only 26 pins, then the pins below the dotted line will not be present on your board. The projects in this book can be completed on all Raspberry Pi models, so only those 26 pins will be used.

This diagram is repeated at the end of the book in Appendix B as a handy reference. Unlike the Arduino, a Raspberry Pi does not have the pin names printed on the printed circuit board (PCB), making it difficult to tell which pin is which. You can make this easier by buying or making a template to fit over the pins, such as the Raspberry Leaf.

Notice how some pins have a number (say, 2) but also a second function listed next to them (say, SDA). When using the GPIO pins in your code, you always refer to them just by number.

Figure 3-9. The GPIO header

Digital Outputs

This example shows how you would set GPIO pin 18 (as indicated in Figure 3-9) to be a digital output and then set the output high:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.output(18, True)

When setting an output using GPIO.output, you can set it high using a value of True or 1, or low using False or 0.

Digital Inputs

Digital inputs on a Raspberry Pi work in much the same way as digital inputs on an Arduino:

GPIO.setup(18, GPIO.IN)
value = GPIO.input(18)
print(value)

You can also specify that the input has a pull-up resistor on it (see “Digital Inputs”) like this: 

GPIO.setup(switch_pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

Analog Outputs

Using analog outputs on a Raspberry Pi is a two-stage process. You first have to set up the pin to be an output, and then you have to define a PWM channel that uses that output.

You will find a fully explained example that uses this in “Experiment: Mixing Colors”.