Raspberry Pi By Example

The following are, at the time of writing this, the major models of Raspberry Pi:

Additionally, Raspberry Pi is also available in a more flexible form factor intended for industrial and embedded applications. It is known as Compute Module. A Compute Module prototyping kit is also made available by the foundation.

The following table compares the currently available models of Pi:

The following image shows the top view of the Raspberry Pi Model B front:

The following image shows the top view of the flip side of Raspberry Pi Model B:

The following image shows the top view of the Raspberry Pi Model B+ front:

The following image shows the top view of the flip side of Raspberry Pi Model B+:

The following image shows the top view of the Raspberry Pi 2 Model B front. The location of the connectors and important ICs (integrated circuits) on the board is not different from Pi B+:

We will be using Raspberry Pi 2 Model B throughout this book. However, all the applications and programs in this book will work on all the models of Pi.

Raspbian is an unofficial variant of Debian armhf (ARM Hard Float) compiled for hard float code that will run on Raspberry Pi computers. It is a free operating system based on Debian optimized for the Raspberry Pi hardware.

This is the original way to install an OS into a MicroSD card, and many users, including me, still prefer it. It allows the SD card to be prepared manually before it is used and it allows easier access to configuration files such as /boot/config.txt, which we might have to modify in a few cases before booting up. The default Raspbian image consists of only two partitions, BOOT and SYSTEM, which will fit into a 2 GB card. However, I recommend that you use a minimum 4 GB card to be on safe side. Choosing an 8 GB card will be adequate for most of the applications we are going to develop in this book.

The following are the instructions for Windows users:

If you have an HDMI monitor, then skip this step. This additional step is required only if you are planning to use a VGA monitor in place of an HDMI monitor.

Browse the microSD card on the computer. Locate and open config.txt. We have to edit the file in order to enable proper display on the VGA monitor.

By default, the commented options (which have # at the beginning) are disabled. We are enabling this option by uncommenting this line, that is, by removing # from the beginning of the commented line. This is what you need to do:

Let's boot up our Pi for the first time with the microSD card:

At this stage, our Pi will start booting up. You will see a green light on the Pi board blinking. This means that it's working! Now, there are few more things we need to do before we can really start using our Pi. Once it boots up, it will show the desktop as follows:

Once the desktop is visible, go to Menu | Accessories | lxterminal. Then, type sudo raspi-config. A text-based menu, such as the following, will appear:

Perform the following steps. We need to use arrow keys and the Enter key to select options in the text-based menu. Press Enter to select a menu item. Also, we can use the Tab key to directly go to the Select and Finish buttons:

This option decides how much RAM is used by the GPU (Graphics Processor Unit). The more RAM is allocated to the GPU, the more intensive graphics processing can be done. 64 MB is a good value for most graphics purposes.

Once all these options are modified, select Finish. This will prompt for a reboot of the Pi. Choose Yes and let it reboot. Once rebooted, it will automatically take us to the Raspbian Desktop again.

You can always invoke the raspi-config tool from Command Prompt with the following command and change the settings:

sudo raspi-config

In the Raspbian menu, there are options to shut down and reboot the Pi.

If we click on the following Menu button on the desktop, it will display multiple options:

The following image shows the last option:

If we click on the preceding option, the following window will appear:

Also, from Command Prompt LXTerminal, we can shut down Pi safely by issuing the following command:

sudo shutdown –h now

An alternative command for this is as follows:

sudo halt

You can reboot Pi with the following command:

sudo reboot

Now we have a working Pi running on the Raspbian OS. Let's update our Pi. Make sure you have a working wired or wireless Internet connection with reasonable speed for this activity:

Turtle graphics is one of the best ways to learn programming for beginners. Originally, it was part of the Logo programming language, which was primarily used to introduce programming in schools. Python has the turtle module, which is an implementation of the same functionality provided by the original turtle. We can write programs with this module in a procedural as well as object-oriented way.

In Python, when we need to access a module that is not part of the current code, we need to import it. Over the course of the book, we will be importing various modules as and when needed, which will provide us with specific functionalities.

Let's get started with importing the turtle module, as shown here:

import turtle

The following code creates objects for turtle and the screen classes, respectively:

t=turtle.Turtle()
disp=turtle.Screen()

We will use the t.color() function with which we can set the pen and fill color, as follows:

t.color("black","yellow")

We will call the t.begin_fill() and t.end_fill() functions to have our shape filled with a fill color:

t.begin_fill()
t.end_fill()

The code to draw an actual shape we need will be in between these two function calls, as follows:

t.begin_fill()
while 1:
    t.forward(100)
    t.left(190)
    if abs(t.pos())<1:
        break
t.end_fill()
disp.exitonclick()

In the preceding code, t.forward() is used to move the turtle forward a specified distance, and t.left() is used to rotate the turtle left by 190 degrees. t.pos() returns the current coordinates of the turtle.

Finally, we use disp.exitonclick() to close the current output window when we click on the exit button.

The output of the program will be a cursor drawing the desired shape progressively, and it helps the programmer understand how the program is actually working. The final output of the preceding program is as follows:

Next, we will learn the concept of recursion. In terms of programming, recursion means calling the same block of code within itself. For a procedural and modular style of programming, this stands for calling a function or method within itself. Usually, this is done to break a big problem into similar problems with smaller input sizes and then collect the output of all these smaller problems to derive the output of the big problem. One of the best ways to see recursion at work is to visualize it using a turtle. We will now write a program to draw a fractal tree using recursion.

First, we start by importing the required libraries, as follows:

import turtle
import random

We need the random library for the randint() function, which returns a random integer in the provided range. This is needed to make our generated tree seem different every time. Then, we will define a function to draw a part of the tree recursively:

def fractal_tree(b_len,t):
    if b_len > 5:
        temp=random.randint(1, b_len)
        temp_angle = random.randint(1, 25)
        t.forward(temp)
        t.right(temp_angle)
        fractal_tree(b_len-10,t)
        t.left(2 * temp_angle)
        fractal_tree(b_len-10,t)
        t.right(temp_angle)
        t.backward(temp)

In the preceding program, we are calling the same function twice in order to draw the further branches of the tree. If the b_len parameter is less than or equal to 5, then it will be a leaf (which means that the function will not be called again); else, the recursion will continue. We are randomizing the angle and length of the movement of the turtle while drawing the branches here; otherwise, the tree will be symmetrical, which is very unlikely in real life. The combination of t.forward(), t.backward(), t.left(), and t.right() ensures that at the end of each function call, the turtle cursor is at the same position as where it started.

Finally, we write the routine to call this recursive function:

t=turtle.Turtle()
disp=turtle.Screen()
t.left(90)
t.up()
t.backward(100)
t.down()
t.color("green")
fractal_tree(120,t)
disp.exitonclick()

The cursor does not draw the movements between the t.up() and t.down() function calls. In the preceding code, we are moving the cursor downward by 100 positions so that the tree should fit in the turtle graphics window. When we call fractal_tree() with 120 as the argument, it takes more than 30 minutes due to the high degree of recursion. The output of the preceding program is as follows: