Last change: 29Oct13 Version Release 3.0
Scratch Controlling the GPIO Pins on a Raspberry Pi
Part 1 of 4
This post is intended to make it as Simple as Pi to get up and running and make your Raspberry Pi control some lights and small motors and to respond to switches and sensors.
Minimum Requirements – a Raspberry Pi with Raspbian installed (a working internet connection is very handy but not required) a breadboard, some Light Emitting Diodes (LEDs), some resistors and some wire connectors. Total cost £5-£10.
Blue italics are extra information for those who like to understand things a bit more - Pink italics are for the more advanced users and can be completely ignored by normal users.
How to get a Raspberry Pi to control the GPIO Pins using Scratch
Your Raspberry Pi needs to be connected to the internet to install the software but not needed to run ScratchGPIO.
Copy the text below ( left click just before the s of sudo and drag right until all the text in the line as been selected) then right-click and select copy. Open up an LX Terminal window and select Edit and the Paste that into an LX Terminal window and run it to download the installer.
sudo wget http://goo.gl/dANpKr -O isgh.sh
Once the installer has been downloaded then just type (or copy and paste the text below as before)
sudo bash isgh.sh
(If logged in as a different user to standard pi, then type sudo bash install_scratchgpio3.sh yourusername)
This will install all the necessary extra software and some simple examples.
(If you do not have internet on your Pi then, put your SD card into a card reader and try using your browser to right-click and save the script direct to your SD card and then put it back into you Pi and run the second instruction)
Connecting Components Up
EXTREME care should be taken when connecting hardware to the GPIO pins. You can cause your Pi to die by connecting the wrong things together – only do this if your confident of your ability to follow instructions correctly
AT A MINIMUM - get a breadboard and use some female-male 0.1 leads (available from lots of on-line suppliers or your local Maplin shop)
Check out GPIO pin guides to make sure you know what pins are what.
Wire up Pin 1 (3.3V) to (at least) a 330ohm resistor – connect that resistor to the long lead of an LED and then connect other end of LED to Pin 6 (0V).
It should light up. If it doesn’t try reversing your LED.
Run the special Scratch icon (Scratch GPIO) on your desktop.
(It is actually a completely normal version of Scratch, it just runs a little Python background program as well that handles communications between Scratch and the GPIO and automatically enables Scratch’s Remote Sensor Connections(RSC))
To test out control from Scratch, click on File then Open and then click on the My Projects button and select blink11 and click on OK. Once the project opens, just click on the OK to enable Remote Sensor Connections. To run the script just click on the Green Flag.
What more can I do with Scratch and the GPIO
As it comes, you can control six pins as outputs (Pins 11,12,13,15,16 and 18) and treat all the rest as simple inputs (22,7,3,5,24,26,19,21,23,8 and 10)
(GPIO pin numbers/ordering do not follow anything that makes sense to most people so you just have to go with the seemingly random numbering arrangement)
As you can see in the blink11 script , you can simply use a broadcast message telling Pins to go on or off (Up to 3.3V and down to 0V)
The valid messages are
along with the corresponding pin off messages.
And you can replace the word on with high and replace off with low if you want to talk in pure logic levels.
To check an input, you should go into the Sensing block and click on the word “slider” at the bottom and you’ll notice that you have pins 22,7,3,5,24,26,19,21,23,8 and 10.
If you connect a switch to one of these pins (through a resistor don’t forget) to OV, then you can detect when the switch is open or closed. The inputs will normally read 1 and go to 0 when they are connected (through a resistor) to ground. Click on the checkbox next to pin7 and try it out.
Using variables instead of broadcasts
For more advanced Scratchers, you can use variables instead (or as well as broadcast messages) .
For example: create a global variable called pin11
To make pin11 go on or off use
On can be replaced with high or 1 and off can be replaced with low or 0 so that you can use whatever logic scheme you’d like.
(this will set Pin 11 , Pin 13, Pin 16 and Pin 18 on and Pins 12 and 15 off)
Note – currently there is an unfortunate “bug” in Scratch in that it remembers variable states and only sends changes out. Even when you press the Green Flag, it will not send the state of all the variables out, it will only send them when a variable changes. I recommend (nay insist even!) setting any gpio variables to an invalid value – say a full-stop and then to their initial state in a Green Flag start-up script.
Need more Pins as outputs?
If you need more than 6 pins to be outputs then, for example, you can use
to change pin22 from being an input to being an output. All the pins can be re-configured this way.
I for some reason, you need more input pins you can use
to change pin 11 from an output to an input
Part 2 – using Motors and varying brightness of LEDs
To test if the software necessary to control the GPIO is correctly installed open a LXTerminal session and type
sudo python blink11.py
If this doesn’t give an error but doesn’t make a LED on Pin 11 blink then we have real problems Houston
Try connecting the lead going to Pin 11 back to Pin 1 to make sure the LED lights up then just in case you have a loose connection.