Raspberry Pi Using Python

RASPBERRY PI – USING PYTHON
2ND JUNE 2013
www.sf-innovations.co.uk

IDLE3 – Python Shell
Launch IDLE3 from the Raspberry GUI. This will allow us to experiment with
Python commands.
At the >>> prompt type
print („hello world‟)
You will see “hello world” on the screen.
At the >>> prompt type
20 + 5
You will see “25” on the screen.
This is a way of executing Python commands immediately. A programme is
simply a collection of commands executed consecutively.

Writing a Python program
Under the file tab in IDLE3, click on new window. You are now ready to
write a program. For now type the following exactly.
For x in range (1,10);
y=x * x
print “x=“, x, “square of x=“, y
Under the file tab, click on save and call the file “squarex”.
Launch LXTerminal from the Raspberry Pi GUI. This will bring you back to
the Linux command line prompt: pi@raspberrypi - $
Type “sudo python squarex.py” to run your program.
You will see
X = 1 square of x = 1
...
That‟s it. You‟ve written your first Python program.

Some notes on Python
You can run programs using the “run module” option under the run tab.
However when I tried this I got programming syntax errors. When I ran the
same program using the command line prompt, it worked fine.
Sudo – stands for “super user do”. With Linux, this gives you the right
privileges to run a Python program.
Comments - If you want to add comments to your program, then use # at
the start. For example, you could have started the program on the previous
page with
# program to work out squares of numbers from 1 to 9.
Using libraries – Many standard functions are available as libraries in
Python. These can be used by using the “import” command and will save
you a lot of time in programming.
For example “import time” will bring in a library which can be used for time
delays. “import random” will bring in a random number generator.

Turning an led on and off
We are going to use the GPIO port on the Raspberry Pi for this. To make the
connection easier, the Custard Pi 1 breakout board is used. This plugs
straight into the GPIO connector, provides easy screw terminal connection
and protects the Raspberry Pi from accidental damage.
The Raspberry Pi is mounted on the Custard Pi B prototyping base.

Hardware connections
We are using pin 11 of the GPIO port. This is available on connector J2 of the
Custard Pi 1 and is labelled as pin 11. This is shown on the image below.
The 0V (or Gnd) connection is the centre pin of the 3 pin power
connector J3.
J3
J2

Connecting the led
When pin 11 is True (taken high) the voltage on it will be almost 3.3V.
This needs to go to the positive side of the led. This is the longer leg of
the led. The other side of the led goes to 0V (Gnd).
Note: If you connect 3.3V across an led it will burn out. So it is important
to limit the current. This is done by using a 330 ohm resistor, in series
with the led.
From pin 11
From 0V
330 ohm resistor
Long leg of led

Python program to flash led
Type in the program steps below. The text behind the # explains what the
code does, but you do not have to enter this.
Import RPi.GPIO as GPIO # import GPIO library
Import time #import time library
GPIO.setmode(GPIO.BOARD) #use board pin numbers
GPIO.setup(11, GPIO.OUT) #setup pin 11 as output
For x in range (0,10): #repeat for x=0 to 9
GPIO.output(11, True) #set pin 11 high
time.sleep(0.2) #wait 0.2 seconds
GPIO.output(11, False) #set pin 11 low
GPIO.cleanup() #tidy up GPIO port
Import sys #exit program
Sys.exit()
Save the file as “ledonoff.py”.
Download code

Trying out the program
Open LXTerminal and type “sudo python ledonoff.py” to run the program.
The led should flash ten time at a fairly fast rate.
Try changing the time.sleep line from 0.2 seconds to 0.5 seconds. When
you run the program, it should flash ten times, but fairly slowly this time.
Tip: To rerun the program, press the upwards arrow to re-enter the last
command on the screen and then press return to run it.
Now change the x in range command from 0,10 to 0,5 to flash the led just 5
times.
Well done. You have just managed to write some Python code to flash an
led.

Reading a switch
We are going to wire a switch to pin 12 of the GPIO port only flash the led
when this is pressed.
....... (same first 3 lines as before)
GPIO.setup(11, GPIO.OUT) #setup pin 11 as output
GPIO.setup(12, GPIO.IN, pull_up_dpwn=GPIO.PUD_UP) #pull up resistor
While True: #repeat forever
input1=GPIO.input(12) #read status of pin 12 into “input1”
if input1==False: #is pin 12 false (low)
print “button pressed” #then button is pressed
For x in range (0,10): #repeat for x=0 to 9
GPIO.output(11, True) #set pin 11 high
GPIO.output(11, False) #set pin 11 low
Note: The indentation is important in Python.
Download code

Hardware setup
The picture below shows the switch set-up. One side of it is connected to pin
12 of the Custard Pi boards. The other side is connected to the 0V (GND)
connection.
The pull up option used when setting up pin 12 as an input keeps this high,
unless pulled low by the external switch.
TO 0V
Pin 12

Trying out the program
Save the file as “ledonoffsw.py” and then run from LXTerminal by typing
“sudo python ledonoffsw.py at the command line prompt.
Whenever the switch is pressed, the led should flash 10 times.
To come out of this program loop, press CTRL & C at the same time.
See if you can add another switch and modify the program to terminate
when this second switch is pressed instead of having to use CTRL & C.

Summary
Hope this presentation has been useful in getting started with Python on
the Raspberry Pi. The book “Programming the Raspberry Pi” by Simon
Monk is a useful introduction to Python.
Keep an eye on our website www.sf-innovations.co.uk for any updates to
this presentation, new Custard Pi layers or new presentations.

Raspberry Pi Using Python

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