Line Follower II — Scratch L2 Lesson 18

Learnt28 min · two dots, colour is touching colour, the steering rule
Basic24 min · paint the dots, build the follower
Challenge 1–327 min · say it, a new track, a line with an end
Extrabonus · two robots, one line

WHAT IT SHOULD LOOK LIKE

Today nobody drives. Two coloured dots on the robot’s nose watch the line, and the robot steers itself all the way round.

green flag

round and round, by itself

panels run left to right

Review — where we got to 5 min

<touching color [#23262D]?> asks if any part of the robot is on that colour.
Inside a forever loop, the question is asked every frame.
Green pads, red holes and the finish all worked the same way.

Quick-fire

Last lesson’s autopilot drove straight past the first bend. Why?

Reveal the answer

It only knew “on the line” or “off the line”. It never knew which side the line went. To steer, you need to know left from right.

Today’s Topic 3 min

Why one sensor cannot steer
Two sensor dots, painted on the robot
<color [#FF00D4] is touching [#23262D]?>
The steering rule, written as if / else

Learning outcome

By the end of this lesson you will be able to:

Ask whether one part of a sprite touches a colour.
Put an if / else inside another if / else.
Make a robot follow a line with no keys pressed.

Learnt — a robot that steers 28 min

1 · One sensor is not enough

Look at these two robots. Both are touching the line. Touching colour says “yes” to both.

WHAT IT SHOULD LOOK LIKE

Both robots are touching the line, so touching colour says “yes” to both. But one must turn left and the other must turn right.

the line bends right

the line bends left

panels run left to right

But one must turn right and one must turn left. “Yes” does not say which. We need to know where the line is.

2 · Two dots on the nose

Real line robots have two small light sensors at the front. Ours are two painted dots.

The pink dot is on the robot’s left.
The sky-blue dot is on the robot’s right.
The line should fit between them, with a little gap each side.

The nose points right, so the robot’s left is the top of the drawing. Pink goes at the top. Sky blue goes at the bottom.

3 · A question about one part

Sensing has a block with two colour swatches:


<color [#FF00D4] is touching [#23262D]?>

First swatch: a colour ON the robot. Second swatch: a colour on the floor.

It asks: is my pink part touching the line? The rest of the robot does not count. Only the pink dot.

Drag the block into the script area.
Click the first swatch. Use the eyedropper to click the pink dot on your robot.
Click the second swatch. Use the eyedropper to click the line.
Click the block. It shows true or false.

This demo only reports what the dots see. You still drive.


when flag clicked
go to x: (-17) y: (118)
point in direction (90)
forever
  if <key [left arrow v] pressed?> then
    turn ccw (6) degrees
  end
  if <key [right arrow v] pressed?> then
    turn cw (6) degrees
  end
  if <key [up arrow v] pressed?> then
    move (3) steps
  end
end

Driving, from last lesson.


when flag clicked
forever
  if <color [#FF00D4] is touching [#23262D]?> then
    say [Left sees the line]
  else
    if <color [#00E0FF] is touching [#23262D]?> then
      say [Right sees the line]
    else
      say []
    end
  end
end

The two dots, reporting. An empty say clears the bubble.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, then click the stage once. Hold ↑ to drive the robot, and ← or → to turn it.

Turn a little left or right on the line. The bubble tells you which dot is on it.

4 · The steering rule

What the dots see	What the robot does
The pink dot is on the line	`turn ccw (5) degrees` — left, towards the line
The sky-blue dot is on the line	`turn cw (5) degrees` — right, towards the line
Neither dot is on the line	`move (3) steps` — the line is between them

WHAT IT SHOULD LOOK LIKE

The line sits between the two dots. When the pink dot touches it, turn left. When the sky-blue dot touches it, turn right. When neither does, drive on.

pink dot on the line → turn left

sky-blue dot on the line → turn right

panels run left to right

5 · The rule, in blocks

Three answers need two questions. An if / else goes inside the else of another one.


when flag clicked
go to x: (-17) y: (118)
point in direction (90)
forever
  if <color [#FF00D4] is touching [#23262D]?> then
    turn ccw (5) degrees
  else
    if <color [#00E0FF] is touching [#23262D]?> then
      turn cw (5) degrees
    else
      move (3) steps
    end
  end
end

Nine blocks. Read it out loud: pink? turn left. Otherwise, sky blue? turn right. Otherwise, go.

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the robot drives itself. Use ↺ Reset to run it again.

The robot laps the Loop on its own, for as long as you let it.

Basic — paint the dots, build the follower 24 min

Step 1 — a loop to follow

Add a new backdrop. Fill it pale, like last lesson.
Draw a thick dark loop with the Line tool, or the Circle tool with no fill.
Keep every bend gentle and round.

Step 2 — the dots

Open your robot’s costume.
With the Circle tool, paint a pink dot at the front, on the top side.
Paint a sky-blue dot at the front, on the bottom side.
Check the line fits between them, with a gap each side.

Step 3 — the follower

Remove the arrow-key script. Keep the start blocks.
Add forever and an if / else inside it.
Put <color [#FF00D4] is touching [#23262D]?> in the hexagon. Pick both colours with the eyedropper.
In the top half, put turn ccw (5) degrees.
In the else, put a second if / else for the sky-blue dot.
Its top half turns right. Its bottom half is move (3) steps.

Step 4 — test it

Put the robot on the line and press the flag. Let it do three laps.

The robot spins round and round on the spot.

Both dots are on the line, or the turn goes the wrong way. Check pink turns ccw (left) and sky blue turns cw (right).

The robot drives straight off the line.

The dots never say true. Pick both colours again with the eyedropper — first on the dot, then on the line.

The robot wobbles left, right, left at one corner and stops.

The corner is too sharp. Make that bend rounder, or move the dots a little closer to the costume’s centre.

CHALLENGE 1 · L2-18-M1🪙 30 Adcoins8 min

Say what you are doing

Make the robot tell you what it is doing, as it does it.

It says “left” while it turns left.
It says “right” while it turns right.
It says “straight” while it drives forward.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — the robot says what it is doing, every frame.

on a straight

on a bend

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the robot drives itself.

Mostly “straight”, with quick flashes of left and right on the bends.

Done when: the bubble changes on every bend, and still says “straight” on the long straight parts.

Teacher note

One say [] in each of the three places — above the turn or move block. Pupils who put all three says in one place have found out that the order of blocks inside an if matters less than which half they are in.

CHALLENGE 2 · L2-18-M2🪙 50 Adcoins9 min

A new track

Paint a second track with a wave in it. Your robot must follow it too.

The new track has at least one wave or S-bend.
You change only the start position. The steering code stays the same.
The robot does two whole laps without getting stuck.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — a new track with a wave in it. The same nine blocks. Only the start changes.

the Wave

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the robot drives itself.

The same follower on the Wave. Only the go to and point in direction blocks changed.

Done when: your robot does two laps of your own new track, and the only blocks you changed are the two start blocks.

Teacher note — when it gets stuck

A pupil whose robot wobbles at one bend has drawn that bend too sharp. The fix is in the paint editor, not the code. Let them find it by watching where it stalls.

CHALLENGE 3 · L2-18-M3🪙 70 Adcoins10 min

A line with an end

Delivery robots do not go round in circles. They follow a route and stop at the end.

Paint a line with two ends. Put a yellow pad over the far end.
The robot follows the line by itself.
On the yellow pad, it stops and says “Arrived!”.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — a line with two ends. The robot follows it and parks on the yellow pad.

set off

arrived

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — watch it through, then use ↺ Reset to run it again.

The robot follows the route and parks on the yellow pad.

Done when: the robot follows the whole route alone, stops on the yellow pad and says Arrived! — and does not move after that.

Teacher reveal — forever becomes repeat until


when flag clicked
go to x: (-114) y: (-20)
point in direction (55)
repeat until <touching color [#FACC15]?>
  if <color [#FF00D4] is touching [#23262D]?> then
    turn ccw (5) degrees
  else
    if <color [#00E0FF] is touching [#23262D]?> then
      turn cw (5) degrees
    else
      move (3) steps
    end
  end
end
say [Arrived!]

The steering is unchanged. Only the loop around it changed: a question to stop on, and a block after it. Two sensors steer; a third question stops.

EXTRA MISSION · L2-18-MX🪙 100 Adcoinsbonus

Two robots, one line

Put a second, slower robot on the same loop. The fast one must never crash into it.

The blue robot moves 5 steps. The orange robot moves 2.
They start on opposite sides of the loop.
When blue gets close to orange, it waits and says “Waiting...”.
When orange has moved on, blue carries on.

WHAT IT SHOULD LOOK LIKE

Extra — two robots on one line. The blue one is faster, so it catches up. When it gets close, it waits.

blue catching up

too close — wait

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — watch the blue robot catch up and wait.

Blue laps faster, catches orange, waits, and sets off again when there is room.

Teacher reveal — the blue robot


when flag clicked
go to x: (-17) y: (118)
point in direction (90)
forever
  if <(distance to [Orange robot v]) < (90)> then
    say [Waiting...]
  else
    say []
    if <color [#FF00D4] is touching [#23262D]?> then
      turn ccw (5) degrees
    else
      if <color [#00E0FF] is touching [#23262D]?> then
        turn cw (5) degrees
      else
        move (5) steps
      end
    end
  end
end

The orange robot is the plain follower with move (2) steps, starting at go to x: (176) y: (-37). Neither robot is confused by the other: no robot has the line’s colour on it.

Summary 5 min

One sensor knows “on the line”. It cannot know which way the line went.
<color [#FF00D4] is touching [#23262D]?> asks about one coloured part of a sprite.
Pink sees the line: turn left. Sky blue sees it: turn right. Neither: go.
An if / else inside an else gives three choices.
Round bends are easy to follow. Sharp corners trap the robot.

Line follower: A robot that steers itself along a line using sensors.
Colour is touching colour: A question about one part of a sprite, not the whole sprite.
Nested if: An if / else inside another one. Three answers from two questions.
Costume centre: The point a sprite turns around.

Hand in

Save your follower. Next lesson its speed and turn become numbers you can tune, and it gets a lap timer.

Learnt28 min · two dots, colour is touching colour, the steering rule
Basic24 min · paint the dots, build the follower
Challenge 1–327 min · say it, a new track, a line with an end
Extrabonus · two robots, one line

WHAT IT SHOULD LOOK LIKE

Today nobody drives. Two coloured dots on the robot’s nose watch the line, and the robot steers itself all the way round.

green flag

round and round, by itself

panels run left to right

Review — where we got to 5 min

<touching color [#23262D]?> asks if any part of the robot is on that colour.
Inside a forever loop, the question is asked every frame.
Green pads, red holes and the finish all worked the same way.

Quick-fire

Last lesson’s autopilot drove straight past the first bend. Why?

Reveal the answer

It only knew “on the line” or “off the line”. It never knew which side the line went. To steer, you need to know left from right.

Today’s Topic 3 min

Why one sensor cannot steer
Two sensor dots, painted on the robot
<color [#FF00D4] is touching [#23262D]?>
The steering rule, written as if / else

Learning outcome

By the end of this lesson you will be able to:

Ask whether one part of a sprite touches a colour.
Put an if / else inside another if / else.
Make a robot follow a line with no keys pressed.

Learnt — a robot that steers 28 min

1 · One sensor is not enough

Look at these two robots. Both are touching the line. Touching colour says “yes” to both.

WHAT IT SHOULD LOOK LIKE

Both robots are touching the line, so touching colour says “yes” to both. But one must turn left and the other must turn right.

the line bends right

the line bends left

panels run left to right

But one must turn right and one must turn left. “Yes” does not say which. We need to know where the line is.

2 · Two dots on the nose

Real line robots have two small light sensors at the front. Ours are two painted dots.

The pink dot is on the robot’s left.
The sky-blue dot is on the robot’s right.
The line should fit between them, with a little gap each side.

The nose points right, so the robot’s left is the top of the drawing. Pink goes at the top. Sky blue goes at the bottom.

3 · A question about one part

Sensing has a block with two colour swatches:


<color [#FF00D4] is touching [#23262D]?>

First swatch: a colour ON the robot. Second swatch: a colour on the floor.

It asks: is my pink part touching the line? The rest of the robot does not count. Only the pink dot.

Drag the block into the script area.
Click the first swatch. Use the eyedropper to click the pink dot on your robot.
Click the second swatch. Use the eyedropper to click the line.
Click the block. It shows true or false.

This demo only reports what the dots see. You still drive.


when flag clicked
go to x: (-17) y: (118)
point in direction (90)
forever
  if <key [left arrow v] pressed?> then
    turn ccw (6) degrees
  end
  if <key [right arrow v] pressed?> then
    turn cw (6) degrees
  end
  if <key [up arrow v] pressed?> then
    move (3) steps
  end
end

Driving, from last lesson.


when flag clicked
forever
  if <color [#FF00D4] is touching [#23262D]?> then
    say [Left sees the line]
  else
    if <color [#00E0FF] is touching [#23262D]?> then
      say [Right sees the line]
    else
      say []
    end
  end
end

The two dots, reporting. An empty say clears the bubble.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, then click the stage once. Hold ↑ to drive the robot, and ← or → to turn it.

Turn a little left or right on the line. The bubble tells you which dot is on it.

4 · The steering rule

What the dots see	What the robot does
The pink dot is on the line	`turn ccw (5) degrees` — left, towards the line
The sky-blue dot is on the line	`turn cw (5) degrees` — right, towards the line
Neither dot is on the line	`move (3) steps` — the line is between them

WHAT IT SHOULD LOOK LIKE

The line sits between the two dots. When the pink dot touches it, turn left. When the sky-blue dot touches it, turn right. When neither does, drive on.

pink dot on the line → turn left

sky-blue dot on the line → turn right

panels run left to right

5 · The rule, in blocks

Three answers need two questions. An if / else goes inside the else of another one.


when flag clicked
go to x: (-17) y: (118)
point in direction (90)
forever
  if <color [#FF00D4] is touching [#23262D]?> then
    turn ccw (5) degrees
  else
    if <color [#00E0FF] is touching [#23262D]?> then
      turn cw (5) degrees
    else
      move (3) steps
    end
  end
end

Nine blocks. Read it out loud: pink? turn left. Otherwise, sky blue? turn right. Otherwise, go.

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the robot drives itself. Use ↺ Reset to run it again.

The robot laps the Loop on its own, for as long as you let it.

Basic — paint the dots, build the follower 24 min

Step 1 — a loop to follow

Add a new backdrop. Fill it pale, like last lesson.
Draw a thick dark loop with the Line tool, or the Circle tool with no fill.
Keep every bend gentle and round.

Step 2 — the dots

Open your robot’s costume.
With the Circle tool, paint a pink dot at the front, on the top side.
Paint a sky-blue dot at the front, on the bottom side.
Check the line fits between them, with a gap each side.

Step 3 — the follower

Remove the arrow-key script. Keep the start blocks.
Add forever and an if / else inside it.
Put <color [#FF00D4] is touching [#23262D]?> in the hexagon. Pick both colours with the eyedropper.
In the top half, put turn ccw (5) degrees.
In the else, put a second if / else for the sky-blue dot.
Its top half turns right. Its bottom half is move (3) steps.

Step 4 — test it

Put the robot on the line and press the flag. Let it do three laps.

The robot spins round and round on the spot.

Both dots are on the line, or the turn goes the wrong way. Check pink turns ccw (left) and sky blue turns cw (right).

The robot drives straight off the line.

The dots never say true. Pick both colours again with the eyedropper — first on the dot, then on the line.

The robot wobbles left, right, left at one corner and stops.

The corner is too sharp. Make that bend rounder, or move the dots a little closer to the costume’s centre.

CHALLENGE 1 · L2-18-M1🪙 30 Adcoins8 min

Say what you are doing

Make the robot tell you what it is doing, as it does it.

It says “left” while it turns left.
It says “right” while it turns right.
It says “straight” while it drives forward.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — the robot says what it is doing, every frame.

on a straight

on a bend

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the robot drives itself.

Mostly “straight”, with quick flashes of left and right on the bends.

Done when: the bubble changes on every bend, and still says “straight” on the long straight parts.

Teacher note

CHALLENGE 2 · L2-18-M2🪙 50 Adcoins9 min

A new track

Paint a second track with a wave in it. Your robot must follow it too.

The new track has at least one wave or S-bend.
You change only the start position. The steering code stays the same.
The robot does two whole laps without getting stuck.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — a new track with a wave in it. The same nine blocks. Only the start changes.

the Wave

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — the robot drives itself.

The same follower on the Wave. Only the go to and point in direction blocks changed.

Done when: your robot does two laps of your own new track, and the only blocks you changed are the two start blocks.

Teacher note — when it gets stuck

A pupil whose robot wobbles at one bend has drawn that bend too sharp. The fix is in the paint editor, not the code. Let them find it by watching where it stalls.

CHALLENGE 3 · L2-18-M3🪙 70 Adcoins10 min

A line with an end

Delivery robots do not go round in circles. They follow a route and stop at the end.

Paint a line with two ends. Put a yellow pad over the far end.
The robot follows the line by itself.
On the yellow pad, it stops and says “Arrived!”.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — a line with two ends. The robot follows it and parks on the yellow pad.

set off

arrived

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — watch it through, then use ↺ Reset to run it again.

The robot follows the route and parks on the yellow pad.

Done when: the robot follows the whole route alone, stops on the yellow pad and says Arrived! — and does not move after that.

Teacher reveal — forever becomes repeat until


when flag clicked
go to x: (-114) y: (-20)
point in direction (55)
repeat until <touching color [#FACC15]?>
  if <color [#FF00D4] is touching [#23262D]?> then
    turn ccw (5) degrees
  else
    if <color [#00E0FF] is touching [#23262D]?> then
      turn cw (5) degrees
    else
      move (3) steps
    end
  end
end
say [Arrived!]

The steering is unchanged. Only the loop around it changed: a question to stop on, and a block after it. Two sensors steer; a third question stops.

EXTRA MISSION · L2-18-MX🪙 100 Adcoinsbonus

Two robots, one line

Put a second, slower robot on the same loop. The fast one must never crash into it.

The blue robot moves 5 steps. The orange robot moves 2.
They start on opposite sides of the loop.
When blue gets close to orange, it waits and says “Waiting...”.
When orange has moved on, blue carries on.

WHAT IT SHOULD LOOK LIKE

Extra — two robots on one line. The blue one is faster, so it catches up. When it gets close, it waits.

blue catching up

too close — wait

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag to start it. Nothing else to press — watch the blue robot catch up and wait.

Blue laps faster, catches orange, waits, and sets off again when there is room.

Teacher reveal — the blue robot


when flag clicked
go to x: (-17) y: (118)
point in direction (90)
forever
  if <(distance to [Orange robot v]) < (90)> then
    say [Waiting...]
  else
    say []
    if <color [#FF00D4] is touching [#23262D]?> then
      turn ccw (5) degrees
    else
      if <color [#00E0FF] is touching [#23262D]?> then
        turn cw (5) degrees
      else
        move (5) steps
      end
    end
  end
end

The orange robot is the plain follower with move (2) steps, starting at go to x: (176) y: (-37). Neither robot is confused by the other: no robot has the line’s colour on it.

Summary 5 min

One sensor knows “on the line”. It cannot know which way the line went.
<color [#FF00D4] is touching [#23262D]?> asks about one coloured part of a sprite.
Pink sees the line: turn left. Sky blue sees it: turn right. Neither: go.
An if / else inside an else gives three choices.
Round bends are easy to follow. Sharp corners trap the robot.

Line follower: A robot that steers itself along a line using sensors.
Colour is touching colour: A question about one part of a sprite, not the whole sprite.
Nested if: An if / else inside another one. Three answers from two questions.
Costume centre: The point a sprite turns around.

Hand in

Save your follower. Next lesson its speed and turn become numbers you can tune, and it gets a lap timer.