Python and the Raspberry Pi

Women Who Code
@WomenWhoCodeTO
PyLadies, @pyladiestoronto
Get Your Bot On!, @getyourboton
Python on the Raspberry Pi 2

© 2015 - 2016 Get Your Bot On!
Agenda
15 mins - intro to RPI and GPIO etc
15 mins - basic hardware set up, software installation etc
20 mins - try it: boot up, software etc, make something work (light)
35 mins - Intro to Python - focus on interaction with the GPIO
30 mins - try it: load sample code, make it work, change something
20 mins - Intro to Connecting a motor
30 mins - try it: connect 2nd motor, change the code

GPIO Library and Python
Let’s make something!
Photo Credits: https://openclipart.org/detail/225947/Robot

Open the Terminal

Let’s find our files
Type ls to list all files and folders
We need to find the “Documents” folder
which holds all our code
If you see Documents listed in the
terminal, type:
cd Documents
Type in ls again to list files and folders.

Linux Command Line
Folders and files are in tree structures
You have to open folders to access other folders.
Imagine you’re peeling an onion!
Photo Credits:https://openclipart.org/detail/175418/cut-onion

Linux Command Line
“cd” __name of a folder__ tells the command line to
navigate to the folder name listed
(Open a layer of the onion)
Use “ls” to check the layer for the file or folder
you’re looking for
If you go into a layer and want to get out of it,
type in “cd ..” (two dots)

Run Marley’s LED
Once you have found the Documents folder and code
files type the following into the terminal:
sudo python blink.py
Ask a mentor for help
if you can’t find the code!

Marley’s Servo Motor
Let’s make this servo motor move!
Run the following command in the terminal
window:
sudo python servo.py

Marley’s Ultrasonic
Sensor
You can read distance data from the Ultrasonic
sensor on the Raspberry Pi. It will output distance
between the sensor and the object in front of it in
centimetres.
● Open the program file called “sensor.py”
Run the following command in the terminal window:
sudo python sensor.py

Let’s Customize Marley!
We’ve tested out the
hardware, now let’s make it
even better!
Photo Credits: https://openclipart.org/detail/191072/blue-robot

Challenge: Add an LED to
the circuit
Now that you have seen how
some of the hardware pieces
work on Marley, let’s re-wire
and reprogram the hardware!
Photo Credits: https://openclipart.org/detail/227532/red-led

BUT FIRST! Mapping
GPIO to the Pins
Layout on the board vs
Pin Number reference

What is the Pi GPIO?
● General Purpose Input/Output
● Digital input and output
● Not buffered so be careful

Challenge: Reprogram
an LED
The LED has a positive and a
negative side.
You are going to match the
cathode with
the negative side (ground)
on the breadboard.
Photos Credits to https://en.wikipedia.org/wiki/LED_circuit#/media/File:%2B-_of_LED_2.svg

Breadboard: Terminal Strips
Photo Credits: http://computers.tutsplus.com/tutorials/how-to-use-a-breadboard-and-build-a-led-circuit--mac-54746

Breadboard: Power Rails
The power rails give you lots of
easy access to power wherever
you need it in your circuit.
Usually they will be labeled with a
‘+’ and a ‘-’ and have a red and
blue or black stripe, to indicate
the positive and negative side.
Pick one vertical strip for a
GROUND and another for POWER

Resistors
● A resistor implements electrical resistance to
reduce current flow, and, at the same time, act to
lower voltage levels within circuits.
● The electrical resistance of a resistor is measured
in ohms.
● We are using resistors in our LED circuit so that
the LED doesn’t blow a fuse.
● For this workshop we are using a 220K Resistor
for each LED
Photo Credits: https://openclipart.org/detail/8281/resistors

Okay Time to Wire it Up!
Photo Credits: https://openclipart.org/detail/191072/blue-robot
Ask a mentor
for help if
you’re
stuck!

Let’s get programing! But
wait, Why Python?
● It’s readable
● Quick to setup
● It’s great for beginners
● It’s ranked 5th most popular language
based on the TIOBE index
● Lots of python community support
Photo Credits: Python Foundation https://www.python.org/static/community_logos/python-logo-master-v3-TM.png

Write the new_blink.py program
Go to Menu-> Accessories->Text editor
File-> open new_blink.py
At the top of the code file below the #comment type
in:
#import the libraries <- This is a comment
import RPi.GPIO as GPIO
import time

Let’s Create a Function
● We are going to create a function that’s going
to control the GPIO pin we just rewired.
● Functions "take in" data, process it, and "return"
a result.
● Functions are useful! Once a function is
written, it can be used over and over again

Declare a Function
‘def’ keyword defines the function
new_blink is the name of your function
(pin) ‘pin’ is the value (parameters) you are
passing into the function
: to finish your function declaration
[def] [function name] [(parameters)] [:]
Type in the following to start your function:
def new_blink(pin):

Indents, Spaces & Commenting
def blink(pin):
#turn LED on
[indent] Start writing code here
#turn LED off
[indent]Writing more code here
● Indents and spacing matter in python, so
when you declare a function, indent code
after the declaration so it knows it’s part of
the function
● Everything written after the “#” symbol is
called a comment, it does not affect the code

blink function
def blink(pin):
#turn LED on
GPIO.output(pin,GPIO.HIGH)
time.sleep(1)
#turn LED off
GPIO.output(pin,GPIO.LOW)
time.sleep(1)
return
GPIO.output(pin, GPIO.HIGH) tells the pi to
light up the LED
time.sleep(1) tells it how long to light up the
LED

blink function
def blink(pin):
#turn LED on
time.sleep(1)
#turn LED off
time.sleep(1)
return
GPIO.out ( pin, GPIO.LOW) tells the pi to turn
off the LED
time.sleep(1) tells it how long to turn off the
LED

Return Statement
def blink(pin):
#turn LED on
time.sleep(1)
#turn LED off
time.sleep(1)
return
The return statement, terminates the execution of a
function and returns control to the calling function

#Set up the GPIO pins
GPIO.setmode(GPIO.BOARD)
#Set up GPIO output
GPIO.setup(15, GPIO.OUT)
Set the GPIO pins
Tell GPIO which pin we are using (pin #15)
GPIO Pin Setup

# Make the LED blink 2 times
for i in range(0,2):
blink(15)
For Loop
We are using a FOR LOOP to program the
LED to blink 2 times (Or a number of your
choice)

For Loop
# Make the LED blink ___# of times
for i in range(0, 2):
blink(15)
for i in range( a , b ):
means
for every element in this range of numbers,
starting from a, ending in b
do something
Customize the number and change the 2! (or another
number)

For Loop & Function Call
# Make the LED blink ___# of times
for i in range(0, 2):
blink(15)
The blink function we just wrote is “called”
into the for loop.
We are telling the program that we want to turn
on/turn off an LED at pin 15 for as many times as
described in the for loop.

Exit the program
#Tell the program to exit
GPIO.cleanup()
Now you’re ready to test your code!

Run your new program!
Navigate to the terminal window
Type in “ls” to list all files
Do you see new_blink.py?
If you see new_blink.py type in:
sudo python new_blink.py
**If you need help ask a mentor! **

More challenge Ideas
Can you make the ultrasonic sensor
output different messages based on
distance?
Can you make the servo output a message
every time it turns a direction?

Challenge Idea:
Making The Sensor Talk

Let’s talk in Python
Open the new_sensor.py and scroll down to find the
following code.
Can you make python print out a message based on
these distance values?
#if distance is less than 10
if distance < 10:
elif distance < 20:
elif distance < 30:
#else print the distance is over 30 cms
else:

Printing
print “The distance is less than 10 cms”
The print function outputs values onto the screen
Type the word print followed by what you would like
to print surrounded in quotes.

If , Elif, Else
If, Elif and Else are known as conditional logic
If, Elif and Else statements are used to execute different
sets of instructions based on specific information
Think of each as a Truth test, if the statement is True
then it will run the indented piece of code underneath it
if distance < 10:
[indent]print “The distance is less than 10cms”
elif distance < 20:
[indent ]print “The distance is less than 20cms”

Else
If none of the If and Elif statements are True then the
Else will run as a default.
(Think of Else as the safety net that grabs all the cases that don’t pass
the if and elif truth tests.)
if distance < 10:
print "The distance is less than 10 cms"
elif distance < 20:
print “The distance is less than 20cms”
#else print the distance is over 30 cms
else:
print “The distance is over 30cms”

Can you make the servo
talk back now?
Modify sensor_new.py

Setting up the Ultrasonic
Sensor and Servo
Wiring and circuit details

Sensor Pins
You have 4 pins
which you will
connect
to your circuit
VCC - POWER
TRIG
ECHO
GRD - GROUND

VCC + GRD
● Connect a jumper wire to the VCC pin which
connects to PIN 2 on the Raspberry Pi
● PIN 2 on the Raspberry Pi sends a current of
5V through the sensor, which will power it
● Lastly, connect a jumper wire to GRD which
is also connected to the GROUND(negative)
power rail on the breadboard
(Diagram in next slide)

TRIG + ECHO
● Connect TRIG to PIN 12
● Connect ECHO to Terminal strip on the
breadboard, you will need two 330k resistors
and another jumper wire to wire the rest.
(Diagram in the next slide)

TRIG + ECHO
The Yellow wire is
connected to ECHO
pin and the Green
wire is connected
to GPIO PIN 22
One 330K resistors
connect the
terminal strips. The
other connects the
terminal strip with
GRD on the Power
Rail

Common Types of Motors
DC Motor: Apply voltage and it turns,
requires a DC power supply. Used in toys
and home appliances.
Stepper Motor: Rotation of the motor is
divided into equal “steps”, apply AC power
and the motor converts the pulse into steps.
Used in printers and CD Drives.
Servo Motor: DC Motor + position or speed
and direction sensors. Used in robots and
RC cars.

Types of Servos
Positional Rotation: Rotates only 180
degrees. You can control position.
Continuous Rotation: Rotates indefinitely
in either direction. You can control speed
and direction.
Linear: Rack and Pinion gear added for
linear motion.

Servo PINS
There are 3 PINS
ORANGE - Signal
RED - (VCC) POWER
BROWN - GROUND

Connecting it all
● Connect the Signal to PIN 11 on the
Raspberry Pi
● Connect the GRD to Ground (Negative) on
the BreadBoard Power Rails
● To power the Servo you need to connect a
jumper wire to PIN 4 and connect it back to
the Breadboard on the Positive Power Rail
● Then you need to connect a jumper wire to
VCC on the Servo and pin it to the same
Power Rail vertically.
(Diagram in next slide)

Servo Wiring

Python and the Raspberry Pi

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