Raspberry Pi Workshop 2018 Become a Coder / Maker / Inventor

In this chapter you'll learn about:

• How to write a simple program in Python
• Using variables
• Declaring functions to organise our code
• Using Python to connect to the real world via GPIO

Chapter Resources:

The Python Standard Library - Useful for a syntax reference and just to explore what pre-built functions are available to you.

Raspberry Pi GPIO Library - This is what we'll use to interact with the world outside. There are handy examples.

The IDLE is Python's shell. It's useful for trying out commands real-time. We can write a Python script, which is just a sequence of Python commands.

I've tried to squeeze in as many Python features as I could into this short script. Compare the script to what you see output when it is run; There's definitely a few tricks worth knowing!

Code for this section:

Challenge: change the for loop that counts in twos to count from -5 to 25 in steps of 5. Remember that you have the Standard Python Library Documentation available, try searching for range.

GPIO or General Purpose Input and Output is the interface that allows your Raspberry Pi to connect to the world. The pins can drive a voltage, sink current, and read voltage levels. Complex digital communications can also take place on the GPIO.

Before we begin, a word about connecting to the GPIO: The pins in the GPIO header are connected directly to the Broadcom processor that is the heart of a Raspberry Pi. You should never prototype circuits while your Raspberry Pi is powered, because a short circuit - no matter how brief - might immediately fry your Pi.

Safely shutdown your Pi as described in Section 1.3 before removing power - then you're ready to prototype some circuits!

We're going to animate a nice sine wave in our shell. To do this we'll write a simple function.

Code for this section:

# Display a pretty sine wave in the Python Shell

import math # For the Sine function
import time # For sleep() delay

numCycle = 5 # Number of times we want our sinewave to cycle
pi = math.pi # Less typing required later = more readable code

# Function to simplify calling the sine function
def sin(x):
    return math.sin(x)

x = 0
while x < (2 * pi * numCycle):
    bar = int(20*sin(x))    # An integer number for the length of our bargraph
    x += 0.3                # the same as saying x = x+0.3
    print ((21+bar)*"=")    # Print the bargraph
    time.sleep(0.03)

We'll be reusing the LED-button setup from Section 2.2 for this section.

Code for this section is attached.

The RPi.GPIO documentation also has an example which brightens/dims an LED. To acheive that end the example uses some for loops. We encourage you to check that out to see how they did it. Make sure you remember to change the GPIO pin number in the code to whichever one you're using!

In this chapter you'll learn about:

• How to write a simple BASH script
• Using variables and arguments
• Scheduling tasks
• How to use the command manual so you can use new commands correctly.

Arguments are pieces of information that we pass in to a program or function. When you issue a command like cd /home/pi, here the command is cd and there is one argument,/home/pi.

In BASH, arguments are accessed in the order they were passed in with $x, where x is the argument number. This means that $1 refers to the first argument, and $4 refers to the 4th argument (if there is one). The name of the command issued is $0.

We're going to write a useful archiving script now. This script will take whatever directory or file we give it, and copy it to a predetermined archive-directory.

We're going to whip-up a very simple GUI to toggle a GPIO pin with. This will familiarise us with the Tkinter workflow:

1. Create the main GUI window
2. Create widget, required variable(s)
3. Position the widget
4. Create and attach an event-function
5. Program the function to do what you want
6. Repeat steps 2-6 as required

Here's the gpiozero documentation if you think you'll prefer using it instead of RPi.GPIO.

We've covered several topics throughout this workshop series, and we've really only scratched the surface of each. By now we've assembled a toolbox of useful skills and are aware of the kinds of tasks our Pi can handle.

Where to find inspiration:

instructables.com or hackaday.io, and on our own projects page.

Where to get help:

The Raspberry Pi Stack Exchange

The Unix and Linux Stack Exchange