Robot Hardware, Scratch Blocks and Variables – Robot Logic in Scratch (Senior 3 ICT) | Ganzaa

Robot Logic in Scratch

Lessons

Senior 3 · ICT

Robot Logic in Scratch

5 lessons

Lesson 2 of 5

Robot Hardware, Scratch Blocks and Variables

50 min

Same program, two worlds

A Scratch program can drive a sprite across the stage, or, through an extension, drive real motors on a desk. The logic is the same: read a sensor, decide, act. This lesson looks at the hardware that schools use to program real robots with Scratch, the Scratch block categories that control a robot, and the variables that store what a robot knows. If your school has no robot kit, every activity in this unit also works as a simulation on the stage.

Definition

Microcontroller board: a small, cheap computer on one board that reads sensors and controls outputs, such as the BBC micro:bit.

Extension (in Scratch): an extra category of blocks added with the Add Extension button, such as Pen, micro:bit or LEGO MINDSTORMS EV3.

Scratch Link: a free program from the Scratch Team that lets Scratch 3 talk to hardware over Bluetooth on Windows and macOS.

Simulation: a program that imitates a real system, so its behaviour can be tested safely and cheaply on screen.

Hardware for robot programming

Item	Why it is needed
Computer or tablet with Scratch 3	To write and run the program: online at scratch.mit.edu or with the Scratch app.
Bluetooth and Scratch Link	Scratch's hardware extensions connect by Bluetooth. On Windows and macOS, Scratch Link must be installed and running, and the computer needs a Bluetooth receiver.
A programmable board or kit	The robot's controller. Scratch 3 has extensions for the BBC micro:bit, LEGO MINDSTORMS EV3, LEGO Education WeDo 2.0, LEGO BOOST and a Vernier force-and-acceleration sensor.
Sensors	Built into the board (the micro:bit has buttons, a tilt sensor and more) or added (EV3 distance, touch and colour sensors).
Actuators	Motors, lights, buzzers. A micro:bit drives motors through an add-on board; an EV3 brick has four motor ports.
Power	Batteries for a robot that moves away from the computer.
Cables and a body	USB cable to set up the board; a chassis with wheels to build on.

To use a micro:bit with Scratch, it is set up once with a special program (the Scratch micro:bit HEX file, from Scratch's micro:bit page). Then in Scratch: Add Extension → micro:bit, and connect it in the window that opens.

Scratch block categories for controlling a robot

Category	Robot job	Examples
Events (yellow)	Start the robot, or react to a message	when green flag clicked; when [space] key pressed; broadcast; when I receive
Control (orange)	Repeat the sense–think–act loop and make decisions	forever; repeat until; if … then; if … then … else; wait
Motion (blue)	Move and turn a simulated robot	move (3) steps; turn right (90) degrees; point in direction
Sensing (light blue)	Simulated sensors	touching [Wall]?; color [ ] is touching [ ]?; distance to [ ]; loudness; timer
Operators (green)	Compare readings and combine conditions	( ) < ( ); ( ) > ( ); and; or; not; pick random
Variables (dark orange)	Store readings, speeds, counts and states	set [speed] to (2); change [turns] by (1)
My Blocks (pink)	Name a behaviour you use often	define turn left (angle)
Extensions	Real hardware, or drawing the robot's path	micro:bit, LEGO MINDSTORMS EV3, Pen

Variables: what the robot remembers

A robot must remember its speed, its last sensor reading, how far it has gone and what it is doing. Each is a variable: a named store whose value can change while the program runs.

Make: Variables → Make a Variable; give a clear name (speed, not x1); choose For all sprites or For this sprite only.
Set a starting value: set [speed] to (2), always at the green flag, so every run starts the same way.
Change it: change [turns] by (1) adds 1; a negative number subtracts.
Use it: drop the round (speed) block into any input, such as move (speed) steps.
Show it: tick its checkbox, so its monitor appears on the stage. Right-click a monitor → slider to change it by hand while the program runs: a simple way to imitate a sensor reading.

Here is a simulated robot. Draw a top view of a small robot as a sprite called Robot and add the Pen extension so that it draws its path:

A My Block turns a common action into one named block:

And with a micro:bit connected, its buttons can start and stop the simulated robot:

Block diagrams drawn by Ganzaa. Teal: extension blocks (Pen, micro:bit); pink: My Blocks.

Worked example 1: speed, distance and time

The simulated robot moves speed = 2 steps per frame, and Scratch runs about 30 frames per second, so it covers $2 \times 30 = 60$ steps (pixels) per second. The stage is 480 pixels wide; starting at x = −180, the right edge (x = 240) is $240 - (- 180) = 420$ pixels away.

$t = \frac{420}{60} = 7 \text{ s}$

After 5 seconds, the distance travelled monitor shows about $60 \times 5 = 300$ . Setting speed to 3 makes the trip $\frac{420}{90} \approx 4.7$ s. One variable changed the whole behaviour, without editing the loop.

Worked example 2: from screen to hardware

The same decision can be written for a sprite or for a real robot. Only the sensing and acting blocks change:

Job	Simulation on the stage	LEGO MINDSTORMS EV3 extension
Start	when green flag clicked	when green flag clicked, or when button [1] pressed
Sense distance	(distance to [Box])	(distance)
Sense touch	<touching [Wall]?>	<button [1] pressed?> (a touch sensor in port 1)
Move	move (3) steps	motor [A] turn this way for (1) seconds
Signal	say [Stop!]	beep note (60) for (0.5) secs

The if, forever and comparison blocks are identical in both. That is why programmers test robot logic in a simulation first: it is quick, free, and nothing gets broken.

Warning

Working with real robots

Test a moving robot on the floor, away from table edges, stairs and water.
Keep fingers, hair and loose clothing away from wheels and gears while the motors run.
Use the right batteries, never mix old and new ones, and take them out when the robot is stored.
Unplug and switch off before changing wires.

Design&Tech: Scratch 3 and the BBC micro:bit: connecting a micro:bit to Scratch and using its extension blocks.

✦

Hands-on: a simulated robot with variables

Start a new Scratch project. Paint a Robot sprite as seen from above (about 40 × 30 pixels), facing right. Paint an Arena backdrop with a light floor.
Add the Pen extension. Make the variables speed and distance travelled, and build the drive script.
Show both monitors. Run it and press space before the robot reaches the edge. Check the monitor against Worked example 1.
Change speed's monitor to a slider (range 0 to 5). Change it while the robot drives.
Make the My Block turn left (angle) and a variable turns. Use it to make the robot drive a square: repeat 4 × (move 100 steps, turn left 90). What does turns show at the end?
If your school has a micro:bit or an EV3: connect it with Scratch Link and make a button start and stop the robot.
Save as Robot_Sim_YourName. You will add sensors next lesson.

Practice

Define a microcontroller board, a Scratch extension and a simulation.
List five items of hardware needed to program a real robot with Scratch, and say why each is needed.
Name the Scratch block category you would use to: (a) repeat the robot's loop; (b) compare a reading with 60; (c) store the number of turns; (d) start the program; (e) detect a wall in a simulation.
Explain why variables should be set to a starting value when the green flag is clicked.
A simulated robot moves 4 steps per frame at 30 frames per second. How long does it take to cross 360 pixels?
Give two advantages of testing robot logic in a simulation before using hardware, and one limitation.
A pupil's robot drives a square, but turns shows 8 after one run instead of 4. Suggest two possible causes.
Your school can buy either ten micro:bits or one large robot kit for the same price. Evaluate the two options for an S3 class.

Show answersHide answers

Microcontroller board: a small computer on one board that reads sensors and controls outputs. Extension: an extra category of blocks added to Scratch. Simulation: a program that imitates a real system so it can be tested on screen.
Any five: a computer with Scratch (to write the program); Bluetooth and Scratch Link (to connect); a board or kit (the controller); sensors (to measure); motors or other actuators (to act); batteries (power); cables and a chassis (to set up and build).
(a) Control; (b) Operators; (c) Variables; (d) Events; (e) Sensing.
A variable keeps its last value between runs. Without a reset, the second run would start with the old distance, speed or count, and give wrong results.
$4 \times 30 = 120$ pixels per second; $\frac{360}{120} = 3$ s.
Advantages: no cost or broken parts; quick to change and re-run; can test dangerous situations. Limitation: real sensors are noisy and motors slip, so a program that works on screen may need tuning on a real robot.
turns was not reset to 0 at the green flag, so it counted two runs; or the program was started twice (two green-flag scripts), or the change block is used twice in each turn.
Ten micro:bits let many pupils program at once and are easy to carry and store; they need add-on boards to drive motors. One large kit can build a more impressive robot but leaves most pupils watching. For a class, the ten micro:bits are usually better, using simulations for motor work; a large kit suits a robotics club.

Sources and credits

Scratch Link, micro:bit Extension, LEGO MINDSTORMS EV3 Extension and Variable, Scratch Wiki contributors, Scratch Wiki, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Scratch is a project of the Scratch Foundation; block names and colours follow Scratch 3.
Photo: BBC micro bit v2.jpg, SimonWaldherr, Wikimedia Commons, licensed under CC BY 4.0.
Photo: Lego Mindstorms EV3 Robot.jpg, Javier Leiva, Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: Scratch 3 and The BBC Micro:bit, Design&Tech, embedded from YouTube.
The block diagrams, tables, worked examples, hands-on task and practice items were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Same program, two worlds

2Hardware for robot programming

3Scratch block categories for controlling a robot

4Variables: what the robot remembers

5Hands-on: a simulated robot with variables

Same program, two worlds

Hardware for robot programming

Scratch block categories for controlling a robot

Variables: what the robot remembers

Hands-on: a simulated robot with variables