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Session 5 :

Physical Computing with Scratch and Python

: : :

wiwMK Academy : STEM Workshop

Compiled by: Nhamburo Ziyenge, MInstP., AMIMA

November 15, 2016. Updated: December 6, 2016

[ Use of the Raspberry Pi to Promote Community Computing Literacy ]

[ Model Community Computing Hub ]

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session 5 : physical computing with scratch and python

GPIO Pins

The objective of this session is to implement a traffic light system both in

Scratch and Python.

This requires an understanding the layout of the GPIO pins as well as the code

that allows activation of the different pins.

Considerable care is required in order to safely connect devices to the GPIO pins.

Randomly connecting devices to the GPIO can easily damage the Raspberry Pi.

There are 40 GPIO pins on both the Raspberry Pi 2 and Raspberry Pi 3 (see

illustration on the next slide)

The pins have physical layout numbers starting from 1 (top left) and 2 (top

right) to pin 40 (bottom right) – with all the odd numbered pins on the left and

the even numbered pins on the right.

In the first session we used pins 2 (or 4) (tops right) for providing 5V supply to

the touch-screen display and pin 6 (third from the top on the right)

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Raspberry Pi GPIO Pin Layout

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session 5 : physical computing with scratch and python

GPIO LED Test

To test that an LED is working, it can be connected directly to a Raspberry Pi

power supply pins such as the 5V as illustrated here – Note the long and short

legs of the LED.

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LED Test Circuit Schematic

The LED test circuit can be represented schematically as illustrated here.

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Ground PinsThere is a total of 8 ground GPIO pins – namely, pin 6, 9,

14, 20, 25, 30, 34, and 39 – all shaded in the illustration.

In the LED test slide above, pin 6 is illustrated as the

ground pin that connects to the shorter leg of the LED.

If pin 6 is already being used for connecting the 5V supply

between the Raspberry Pi and the touch-screen display,

then any of the other ground pins can be used instead.

We cannot implement the traffic light system by

connecting the LEDs directly to the power supply pins as

illustrated in the LED test slide above – because that

would mean the LEDs would be switched on all the time.

For the traffic light system, we need to use one of the

GPIO pins that can be configured to be an output or input

pin from within the Raspberry Pi.

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Configurable GPIO Pins

Any of the other pins – that are not power supply

pins or ground pins can be configured to work as

output pins or input pins.

Here we will use pins 36 (for Red), 38 (for

Amber) and 40 (for Green) as output pins

A GPIO pin configured as an input pin can also

be used to detect the closure of a switch

connected between one of the 3.3V pins and

ground – when the switch is open, the GPIO

reads a HIGH – but when the switch is closed, the

GPIO reads a LOW value.

We will use pin 12 as the input pin with a switch

connected to detect when the switch is pressed.

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Breadboard Wiring

Get your wiring

checked before

turning the

Raspberry Pi on.

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session 5 : physical computing with scratch and python

Scratch Traffic Light Code

Scratch uses broadcast blocks to

communicate with the GPIO pins.

The first broadcast you need is

gpioserveron which activates the

GPIO functionality.

After activating the GPIO

functionality, each of the traffic light

LEDs can be configured as an

output.

In our case, we are using pins 36, 38

and 40 to connect to the Red,

Amber and Green LEDs respectively

– as illustrated in the previous slide.

To configure a pin as an output pin,

use a broadcast block illustrated

below (for pin 36):

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LED On / OffTo switch ON pin 36 (for example)

broadcast a gpio36on message –

after broadcasting the config36out

signal.

To switch OFF pin 36 (for example)

broadcast a gpio36off message –

after broadcasting config36out.

The two scripts shown here illustrate

switching pin 36 ON and then OFF.

Use these two scripts to test that the

red LED does switch ON and OFF in

response to pressing the green-flag.

Repeat for pins 38 (amber LED) and

40 (green LED).

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LED On then OFF

This script switches ON the red LED (pin 36) and waits 3 seconds before

turning it OFF.

After that if stops when it reaches the “stop script” block.

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Repeated Flashing LED

Here we are switching pin 36 (red LED) ON, waiting for 2 seconds, then OFF

and waiting another 2 seconds, then ON again etc.

The flashing is repeated 10 times.

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Flashing LED : Forever

In this case the LED will keep flashing, waiting for 2 seconds between ON and

OFF.

The LED will keep flashing until the script is interrupted.

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Traffic Light Diagram

This diagram illustrates the electrical connections of the three LEDs to pins 36

(red LED), 38 (amber LED) and 40 (green LED) used for the traffic light

system.

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Traffic Light Sequence

This is the complete

traffic light sequence.

Red

Red / Amber

Green

Amber

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Access Python IDLE

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Create New Python FileThe >>> prompt provides direct

access to the Python interpreter.

In stead of typing Python commands

directly at this prompt, we want to

create a new Python script in which

to create the code to drive the LEDs.

Create a new file by clicking on File

on the main menu and save the file

as pythongpio.py

Make sure the extension is .py

otherwise the interpreter will not

recognise it as a Python file.

To configure the GPIO pins follow

the steps on the following slide.

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Python GPIO Configuration

It is important to know the location

where the file you create is located.

To configure the GPIO pins 36, 38

and 40 as output pins, copy the

Python code on the right into the

newly created file then save the file.

To run the file you need to open the

command terminal (see next slide)

and navigate to the directory where

you saved the file.

The while loop that flashes pin 36

ON and OFF.

import RPi.GPIO as GPIO

import time

GPIO.setmode(GPIO.BOARD)

GPIO.setup(36, GPIO.OUT)

GPIO.setup(38, GPIO.OUT)

GPIO.setup(40, GPIO.OUT)

while True:

GPIO.output(36, GPIO.HIGH)

time.sleep(1)

GPIO.output(36, GPIO.LOW)

time.sleep(1)

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Command Line Terminal

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Complete Traffic Light Code

import RPi.GPIO as GPIO

import time

GPIO.setmode(GPIO.BOARD)

GPIO.setup(36, GPIO.OUT)

GPIO.setup(38, GPIO.OUT)

GPIO.setup(40, GPIO.OUT)

while True:

GPIO.output(36, GPIO.HIGH) # Red LED ON

time.sleep(3)

GPIO.output(38, GPIO.HIGH) # Amber LED ON with Red ON

time.sleep(1)

GPIO.output(36, GPIO.LOW) # Red LED OFF

GPIO.output(38, GPIO.LOW) # Amber LED OFF

GPIO.output(40, GPIO.HIGH) # Green LED ON

time.sleep(3)

GPIO.output(40, GPIO.LOW) # Green LED OFF

GPIO.output(38, GPIO.HIGH) # amber LED ON

time.sleep(1)

Technical support and

guidance will be

provided to run the

code and to debug it as

necessary.

A separate session will

be devoted to

familiarisation with the

Raspbian Linux

platform.

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