Physical Computing – Scratch 2.0 for Raspberry Pi

Pin Numbering Important to mention that we will use Broadcom (BCM) pin numbering for the GPIO pins, as opposed to physical (BOARD) numbering. Any pin marked “GPIO” in the above diagram can be used as a pin number. For example, if a LED was attached to “GPIO13” (physical pin 33) you would specify the pin number as 13 and not “33”.

Step 4: Blinking a LED

t's time for the “Hello World” of Physical Computing, the “blink of a LED”!

Let's start connecting a LED to GPIO 13 of our Raspberry Pi (RPi, for short).

For starting, drag 2 new “set gpio” blocks to the SCRIPTS AREA,

set gpiot <13> to [output high]
set gpio <13> to [output low]

Clicking on those blocks you will turn ON and OFF the LED

Now, let's create a real blink code using Scratch 2.

• Start your program when the “Green Flag” is clicked, for that you will need the block:

• When “green flag” clicked (it is the first block in the “Event menu” – colored in brown)

• Drag the loop block “Forever” (you will find it on “Control menu” – colored in yellow)

• Inside the loop Forever, put the blocks that you used on the before.

• if you run your code, you will see that nothing happens, or better, the LED will be ON and OFF so quickly that you will not realize it. So, you must add some delay, let's say 1 second after each block.

Step 5: Using Custom Blocks – Multiples LEDs Blinking

Now, that we know how to control one LED, let's install 2 new ones, with different colors:

• RED

• YELLOW

• GREEN

Repeat what you did at last step, connecting the LEDs as shown above:

• RED ==> GPIO 13

• YELLOW ==> GPIO 19

• GREEN ==> GPIO 26

Suppose that we want now the 3 LEDs blinking independently at different frequencies. The first thing that you can do is to copy the set of block developed on the last step, changing the GPIO number and the delays for each one of the blocks, let's say:

• Blink Block for GPIO 13 ==> delay: 1 second

• Blink Block for GPIO 19 ==> delay: 2 seconds

• Blink Block for GPIO 26 ==> delay: 4 seconds

It would work, but let's use the opportunity here and introduce a new feature of Scratch 2, the “Custom Blocks”.

First, let's create a custom block naming it “setup”, that will be used to turn OFF all LEDs

• Go to “More Blocks” menu

• Select “Make a Block”

• Enter the name “setup” and click OK

You will see that a new block will appear “define [setup]“, below it, we can add 3 blocks setting the GPIOs to LOW as shown at above print screen. Now, this custom block “setup” (that it is, in fact, a “function”), can be called anytime in the program. Let's call it at starting, under the green flag, so anytime time that the program run, we will guarantee that the LEDs will be OFF.

Now, we will create a more elaborate custom block. We will create a type that can receive parameters allowing further generalization and reuse of code blocks. The idea is to create a generic “Blink block”, where the parameters will be the GPIO number and the time. So, our final block will be like:

blink <GPIO> <time>

So, let's do the following:

• Go to “More Blocks” menu

• Select “Make a Block”

• Enter the name “Blink” and click at sub-menu “Options”

• Click on the first option: “Add number input” and type “GPIO”

• Click again on same option and type “time”

• Enter OK

Now, a new block will appear:

define [Blink] [GPIO] [time]

Under this block we will add the blocks that we used for the blink, but, instead of specific numbers, we will add the “blue” and generic parameters: “GPIO” and “time”

Now if we want to blink the LEDs independently it is enough to use blocks like those:

Blink <13> <1>
Blink <16> <2>
Blink <26> <4>

The code for this example is here: blink_V2.sb2

Step 6: Introducing a Push-Button and Variables

if <20> is high?> then
 
else
 

I used the internal editor to create the simple costumes. We will want that this sprite will always appear at Stage, but with its costume “button OFF” if the push-button is released (variable button = OFF) and of course, with the costume “button ON” if the push-button is pressed. One simple block that should run all the time.

Now we must create the Traffic Light that will show the LED status on the Stage. We can do it or creating a new Sprite, or creating new backdrops. We will do the second on, so you can see 2 different examples of animation creation here.

With the paint editor, I created 3 empty circles, that I called “OFF”. Coping this backdrop 3 times, I filled one of the circles with one of the LEDs color (Red, Yellow, and Green) in this sequence. I name them (SURPRISE!): RED, YELLOW, and GREEN.

So, at Backdrop”s Script tab I included a piece of code that will run forever, changing the backdrop according to LED variables (see above). Very simple and straight forward.

And that's it! As you can see, the final project is a group of various simple parts, what's make it at end also very simple to understand.

NOTE: The secret here is to be ORGANIZED with the code.

Below a video with our project and animation running:

The code can be downloaded from here: Traffic Lights V2.sb2

Step 8: It's Show Time!

On the last step, we had a lot of fun creating simple animation and mixing physical and virtual things together. Let's now, do a more elaborate project.

Before we enter in the animation part of it, let's complete the HW. This project will have a pedestrian Traffic Light and so, we will include a Buzzer that will produce a sound (“Beep”), during the time allowed for pedestrians crossing a street. The buzzer will be installed at GPIO 16. AS we did before, to simplify things, let's create a new variable “beep” that will activate GPIO 16 High or Low. The logic will be included at gpioStatus, same we did for the LEDs.

Let's take our little friend, the MJRoBot and show him London, the birthplace of Raspberry Pi!

For starting, we must define the general specification regarding our project:

• Our little friend will start its journey around London, moving right to left across all stage area.

• When he touches the left Stage's border, we must change for the next London's backdrop.

• When he reaches Abbey Road, he must stop because there is a pedestrian cross (Yes! The famous one!)

• Suppose that was installed there, a traffic light to protect tourists. The robot will ask us to press the push-button for him, so he can go across the street, same as Paul, John, George, and Ringo did years ago) but in an opposite direction 😉

Let's this time create two new sprites:

• A Traffic Light (in opposite for using several backdrops) with 3 costumes (red, yellow and green)

• The little robot, the MJRoBot that has 2 costumes to help him to walk (or something that resembles walking). 😉

We will have 4 backdrops. 3 for London surrounds and the most important one: Abbey Road.

That's it!

Try to code it by yourself and only after, compare your solution with mine! There are several different ways to do that.

Enjoy it!

A quick video about the final project working:

My code solution here: MJRoBot around London V1.sb2

Step 9: Conclusion

As always, I hope this project can help others find their way in the exciting world of electronics.

Source: Physical Computing – Scratch 2.0 for Raspberry Pi