Challenge 1
Clean a strip of scattered paper and leave the brush stopped at the end. Anyone should be able to see the machine has finished, because nothing on it is still turning.
What you are building 3 min
The Automatic Cleaner: a driving base with a spinning brush on the front. Two Large Motors move it across the floor, and a Medium Motor spins the brush.
Here is the problem. The brush needs to spin fast — a slow brush pushes dirt rather than sweeping it. The base needs to move slowly, because a cleaner that races across a room has not cleaned it.
Two different speeds, on the same machine, at the same moment. And this time you are going to do it in one stack.
In the real world 5 min
Where you have seen it
Robot vacuum cleaners are in ordinary houses now. Underneath, they all look the same: two driven wheels, a spinning side brush that flicks dirt out of corners and into the machine’s path, and a roller underneath.
Why it is built that way
The brush and the wheels are doing completely unrelated jobs, so nothing says they should run at the same rate. The brush’s speed is set by what it takes to throw dirt inwards — too slow and it just nudges it along. The wheels’ speed is set by how long the machine needs to spend over any patch of floor to actually pick it up.
So they get separate motors. Not because two is neater, but because one motor could only ever give both jobs the same speed.
What would go wrong without it
Look at the photo again. That machine is doing everything it was told to. The wheels are turning, the brush is turning, and it is achieving nothing, because it drove somewhere it could not get out of and nothing in it is checking whether the driving is working.
A machine that runs is not the same as a machine that is working.
The main concept — which blocks hold up the stack 6 min
Two motor blocks that look almost the same behave completely differently, and the difference is whether the block waits.
| Block | Does the stack wait for it? | When does the motor stop? |
|---|---|---|
| run for (2) rotations at (50) % | Yes. Nothing below it happens until those two rotations are done. | By itself, after 2 rotations. |
| start motor at (80) % speed | No. The very next block runs immediately. | Never — until something else tells it to. It is still going after the program ends. |
That second row is how you get two speeds out of one stack. Start the brush — the stack does not wait, so it carries straight on — and then drive the base. The brush is spinning the whole time the drive block is running, because nothing ever asked it to stop.
[A v] start motor at (80) % speed :: motors move [forward v] for (4) [rotations v] :: movement [A v] stop motor :: motors
Read the middle block again. While the base is grinding through those four rotations, the stack is stuck on that block — and that is fine, because the brush does not need the stack. It was set going and it stays going.
The trap
Leave off the last block and the brush never stops. The program will end, the screen will go back to the menu, and the brush will still be spinning, because nothing that starts a motor ever stops it. Every start motor you write is a promise to write a stop motor somewhere.
One motor, on and off
Everything an EV3 does begins with a motor turning. Before worrying about how far or how fast, there are only two things to say to a motor: start and stop. The blocks are the same whichever motor you use — only the port letter changes.
Blocks reference
| Block | What it does |
|---|---|
[A v] start motor [clockwise v] :: motors | Starts the motor and immediately carries on to the next block. The motor keeps turning on its own. |
[A v] stop motor :: motors | Stops the motor. |
wait (2) seconds | Holds the program here, which is how you control how long a motor runs. |
Start, wait, stop
Because start motor does not wait, a motor started on its own would run until the program ended. The pattern that gives a motor a length is three blocks:
when program starts :: events hat [A v] start motor [clockwise v] :: motors wait (2) seconds [A v] stop motor :: motors
Read it aloud: start it, leave it two seconds, stop it. Change the wait and you change how far the motor gets.
Watch it run
The highlight below follows the block the Brick is running, and the shaft turns only while the program is between start motor and stop motor. Notice that the count keeps climbing all the way through the wait — the wait does not pause the motor, it pauses the program.
when program starts
A start motor clockwise
wait 2 seconds
A stop motor
0.00rotations so farnoshaft turning
The program begins. Both motors are still.
The count does not pause when the program does. A wait holds up the blocks, and the motor carries on turning underneath it.
This is the whole idea behind start motor: it hands the motor its instruction and moves on, leaving the motor running behind it. Nothing stops the shaft until a block tells it to.
Which way is clockwise?
The direction dropdown says clockwise or counterclockwise — but clockwise seen from where? Always from the axle end: look straight at the shaft coming towards you, and clockwise is the way a clock’s hands go.
On the Medium Motor
Here the Medium Motor faces you, so its output axle points straight out of the page. This is the view to picture when you are choosing a direction.
when program starts :: events hat [A v] start motor [clockwise v] :: motors wait (2) seconds [A v] stop motor :: motors
A Medium Motor normally lives in port A or D.
On the Large Motor
The Large Motor is shown from the side, which is the face its axle comes out of — so once again you are looking straight down the shaft.
when program starts :: events hat [B v] start motor [clockwise v] :: motors wait (2) seconds [B v] stop motor :: motors
A Large Motor normally lives in port B or C. Compare the two programs: they are the same three blocks, and only A has become B.
Same blocks, different motor
Medium · port ALarge · port BWhy it matters
A fan, a conveyor belt or a spinning ride does not need to stop at an exact position — it just needs to run while something is happening. For those, timing the motor is simpler and perfectly good enough.
“Run for” waits and stops itself. “Start” does neither.
Driving a pair — a reminder from Lesson 6The Movement blocks and why the drive base uses them. Open it if last lesson has gone.Show meHide
Driving with two motors
A driving base has two Large Motors, one per wheel. You could drive them with two separate motor blocks — but they would never start at quite the same instant, and the robot would curve away. EV3 has a separate family of Movement blocks that treat the pair as a single driving base.
port Bport CBlocks reference
| Block | What it does |
|---|---|
set movement motors to [B v] and [C v] :: movement | Tells the Movement blocks which two ports are the driving wheels. Put it at the top of the program, before any movement. |
move [forward v] for (2) [rotations v] :: movement | Drives both wheels together, so the robot travels in a straight line. |
move for (1) [rotations v] at (75) (25) % speed :: movement | Drives the two wheels at different speeds, which makes the robot curve. |
set movement speed to (50) % :: movement | Sets how fast the driving base travels from now on. |
The letters must match the sockets
set movement motors to [B] and [C] is the block that decides which two motors every Movement block after it will drive. It does not look at the robot. It drives the two ports you name, whether or not there is a motor in them — so those two letters have to be the two sockets the cables are actually in.
This is the single commonest reason a driving base does nothing, and it is invisible in the program: every block is spelled correctly and the robot still will not go.
when program starts
set movement motors to B and C
move forward for 3 rotations
the same program on both robots
2wheels driven · matched1wheel driven · mismatched
B and C: both sockets fullB and C: C is empty
Two robots, and the very same program: drive the motors in ports B and C forward.
The yellow outline marks the two ports the block is driving. A robot only goes straight when both of them have a motor in them.
Both robots run the identical program. The left one has its motors in B and C, so both named ports have a motor and the robot drives straight. The right one is plugged into A and B: the block drives B, finds nothing in C, and never mentions the motor in A at all — so one wheel turns, one wheel sits there, and the robot swings round the dead one instead of driving.
If neither named port has a motor in it — cables in A and D, say, with the block still set to B and C — the robot does not move at all. The program runs happily to the end and the Brick reports nothing wrong, because as far as it is concerned it did exactly what it was told.
So before you look for a bug in the program, look at the cables: read the port letters off the Brick, then make the block say those two. B and C are only the usual choice, not a rule — if your build has the driving motors in A and D, set the block to A and D and everything works.
Straight, curved, and on the spot
Everything a driving base can do comes from those two speed numbers. Try each button below and watch the trail the robot leaves.
when program starts
set movement motors to B and C
move for 2 rotations at 50 50 % speed
50left wheel · B50right wheel · C
Equal speeds → a straight line
Both wheels are set to the same speed.
The path is worked out from the two wheel speeds in the block, not drawn by hand — so changing the preset changes the numbers and the shape together.
- Both wheels the same speed — the robot goes straight.
- One wheel faster than the other — it curves towards the slower side. The bigger the difference, the tighter the curve.
- One wheel forward, the other backward — it spins on the spot, which is how a robot turns in a space no wider than itself.
Notice there is no steering block anywhere in that program, and no steering part anywhere on the robot. The turn is made entirely by driving the two wheels different amounts.
when program starts :: events hat set movement motors to [B v] and [C v] :: movement move [forward v] for (2) [rotations v] :: movement move for (1) [rotations v] at (50) (-50) % speed :: movement
How far is one rotation?
“Move forward 2 rotations” says nothing about the floor — it says how many times the wheels go round. How far the robot actually travels depends on how big the wheels are, and once round a standard EV3 driving wheel is about 17.5 cm.
Before doing any sums, get a feel for it. Drag the robot along and watch both scales at once — rotations on top, centimetres underneath, the same line. Land on the half marks: there is a number between 1 and 2, and it is 1.5.
Drag the robot, or use the arrow keys — it moves in half rotations. Rotations above the line, centimetres below.
1.5 rotations26.3 cm
1.5 — a half rotation past 1. Half a turn of the wheels is 8.75 cm, so halves matter.
cm=2rotations
35 ÷ 17.5 = 2. That is a number you can type straight into the block.
- 1 rotation = 17.5 cm. Two rotations is 35, three is 52.5.
- Halves are real. 1.5 rotations is a perfectly good thing to type into the block, and it is 26.25 cm — right between the 1 and the 2 you already tried.
- To go a distance you have chosen, divide by 17.5. The calculator does it, but do a few by hand first — 35 ÷ 17.5 = 2 is worth seeing.
- Measure your own wheels before trusting the number. A base built with different wheels travels a different distance per rotation, and every calculation on this page moves with it.
set movement motors to [B v] and [C v] :: movement set movement speed to (30) % :: movement move [forward v] for (2) [rotations v] :: movement
Turning on the spot
Steering at right 100 or left 100 drives the two wheels in opposite directions, so the base stops travelling and pivots where it stands. The angle it sweeps is a simple doubling of the rotations:
Half a rotation is the one worth remembering: a square corner, 90°.
| Rotations | The robot turns |
|---|---|
| 0.25 | 45° — half a corner |
| 0.5 | 90° — a square corner |
| 1 | 180° — turn round and face back |
| 2 | 360° — all the way round |
The one to memorise is 0.5 rotations = 90°. Everything else follows from doubling or halving it.
set movement speed to (20) % :: movement move [right: 100] for (0.5) [rotations v] :: movement
Turn slowly. The wheels on a driving base are big and the robot carries a lot of weight, so at full speed it keeps going after the motors stop and lands past the angle you asked for. Somewhere between 15% and 30% speed is where turns become repeatable. A turn that overshoots is almost always a turn taken too fast, not a wrong number.
These figures belong to your robot. The distance depends on the wheels and the angle depends on how far apart they are, so a wider base needs a different number for 90°. Test it, measure what actually happened, and adjust — the same discipline as the Gyro Sensor tolerance.
Why it matters
Every wheeled vehicle steers this way — a tank, a digger, an office chair with two driven castors. Cars use a steering rack instead, but a robot that turns by driving its wheels at different speeds needs no steering mechanism at all.
Say this back before moving on: “If I start it, I have to stop it.”
What’s in this build 4 min
| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | Runs three motors at two different speeds, and rides on the machine. Its weight is what gives the wheels grip. |
| Large Motor ×2 — the drive | Ports B and C, driven as a pair by the Movement blocks so the cleaner tracks straight. |
| Medium Motor — the brush | Fast and light. The Medium Motor is the right choice precisely because a brush needs speed rather than force. |
| The brush itself (not electronic) | Must touch the floor lightly. Pressed down hard it stalls the motor; held clear it sweeps nothing. |
Set the brush height before you program
- Stand the cleaner on the floor and look at it from the side. The brush should just brush the surface, not press into it.
- Spin the brush by hand. It should turn freely for a moment after you let go. If it stops dead, it is rubbing.
- Scatter something safe and light to sweep — torn paper, a few LEGO studs. Same test material all lesson.
- Push the cleaner forward by hand across the mess and watch what the brush does to it. That is the best case; the program cannot beat it.
Ports — and the rule 4 min
Sensors go in ports 1, 2, 3, 4. Motors go in ports A, B, C, D. They are not interchangeable, and nothing will tell you politely if you swap them.
| Part | Port | Why this one |
|---|---|---|
| Brush (Medium Motor) | A | A job motor, kept off B and C so it is never accidentally swept up into the movement pair. |
| Left drive (Large) | B | B is left, and the Movement blocks assume it. |
| Right drive (Large) | C | C is right. |
Check your own build now:
- Brush motor in A. If your brush ends up on B or C, the Movement blocks will try to drive it as a wheel and the cleaner will steer itself in circles.
- Standing behind the cleaner, left drive in B, right in C.
- Route all three cables up and back, clear of the brush. A cable in a spinning brush wraps instantly.
- Clear a run of floor. This one drives, and it does not look where it is going.
Connect the Brick 4 min
Two routes, and either is fine. USB is the reliable one and the one to fall back on when a room’s Bluetooth is busy; Bluetooth leaves the robot free to move, which some models need.
How to connect the BrickUSB and Bluetooth, step by step, with a photograph of every screen. Open it if you have not done this before — or if pairing is not working.Show meHide
USB — the reliable one
- Switch the Brick on with the dark grey centre button.
- Cable into the Brick’s PC port — the small square socket beside the numbered ports, not one of the numbered ones.
- Other end into the computer.
Bluetooth — name it first
Do these in order. Naming the Brick after you go looking for it in the list is how groups end up driving each other’s robots.
- Name your Brick. On the Brick: Settings (the spanner) → Brick Name. Type something nobody else will pick, then press the tick. Every Brick is called
EV3until somebody changes it. - Turn Bluetooth on. Settings → Bluetooth. Tick Bluetooth and Visibility. Leave iPhone/iPad/iPod unticked.
- Connect from EV3 Classroom. Click the Brick icon at the top of the programming area, find your Brick by name, and click Connect.
- Say yes on the Brick. It asks “Connect?” with the computer’s name — choose the tick, then accept the passkey, which is already 1234.
EV3.The two failures, every class, every time. The Brick has gone to sleep while you were building — press the centre button to wake it. Or you have paired with the group at the next table, which is why the name matters.
The long version, including Port View and how to read the port tiles, is in the Brick & Bluetooth guide.
Bluetooth, definitely. This model drives across a floor with a spinning brush on the front. A USB cable will be dragged along the floor and then into the brush.
Confirm the connection 2 min
- Check the Brick icon: connected, or not.
- Three motor tiles — a Medium on A and two Large on B and C. Three different jobs, three tiles; if you only see two, one cable is loose.
- Spin the brush by hand and check tile A counts.
- Turn the left wheel and check B, then the right and C.
Make it move 10 min
One cleaning run: brush on, sweep a strip, come back, brush off — and say so on the screen at each stage.
when program starts :: events hat set movement motors to [B v] and [C v] :: movement set movement speed to (30) % :: movement clear display :: display write [Bersih...] at line (1) :: display [A v] start motor at (80) % speed :: motors move [forward v] for (4) [rotations v] :: movement move [backward v] for (4) [rotations v] :: movement [A v] stop motor :: motors write [Siap] at line (3) :: display play beep (70) for (0.4) seconds :: sound
- The two settings — the movement pair, and the speed they will run at. Nothing moves yet.
- start motor at 80% — the brush comes up to speed and the stack immediately moves on. This is the block the whole lesson is about.
- move forward, then backward — the stack sits on these for as long as they take. Meanwhile, the brush has been spinning the entire time without being mentioned again.
- stop motor — the promise being kept. Take this out and the brush runs until someone picks the machine up.
What success looks like: the brush comes up to speed first, then the machine sets off while the brush is still spinning, and the brush stops at the same moment the machine finishes. Watch the brush during the driving — that overlap is the whole result.
If the brush only spins after the driving finishes, you have used run for instead of start motor. The stack is waiting for the brush to complete a set number of rotations before it will start driving.
Change it and test 8 min
Same scattered mess each run — sweep it back into place before each test, or you are grading a different job every time.
- Change the brush block to
[A v] run for (4) [rotations v] at (80) % speed. Predict what happens before you run it, then watch the machine do the two jobs one after the other instead of together. - Put
start motorback and delete thestop motor. Run it, and notice the brush is still going when the program has ended. Then put it back. - Drop the movement speed to 15%. Slower over the floor, same brush. Does it clean better?
- Drop the brush to 30% and put the movement back to 30. Now they are equal — which is exactly what one motor would have given you. Is the result worse?
- Try brush 100%, movement 15%. The most extreme difference you can make. Watch whether the brush now throws your test material out of the machine’s path instead of into it.
Step 5 is the real engineering. Faster is not automatically better: past a certain speed the brush stops sweeping dirt inwards and starts firing it across the room. The best setting is a balance between two motors that have nothing to do with each other, and you can only find it by trying.
Separate motors mean separate decisions. Two speeds is not twice the work — it is two questions.