Challenge 1
Ten blows, and a screen that agrees with your ears. Count the blows out loud as they happen, then check the final number is about ten times 360. If they disagree, the machine is right and you are wrong — find out where.
What you are building 3 min
The Hammering Machine: a Large Motor driving a hammer that rises and falls, over and over, with the Brick screen and speaker reporting each blow.
You built a hammer in Lesson 7 and told it to strike ten times. But how did you know it struck ten times? You counted, out loud, watching it. The machine had no idea.
Today the machine keeps its own tally — and not by counting the loop. By reading how far its own shaft has actually turned.
In the real world 5 min
Where you have seen it
Before car parts were pressed, they were forged: a heavy weight lifted by steam and dropped onto hot metal, again and again, in a shop loud enough to be heard streets away. Drop hammers built the twentieth century.
Why it is built that way
Forging is not one big hit. It is a known number of measured blows — the metal moves a little with each one, and the smith knows roughly how many it takes. Too few and the part is the wrong shape. Too many and the metal work-hardens and cracks.
So a forge counts. A modern press counts every stroke it has ever made, because the count is how you know when a tool needs replacing — dies wear out after a known number of hits, not after a length of time.
What would go wrong without it
A machine that cannot count its own work cannot be trusted to report it. If the hammer jams half way through a run and you were not watching, a machine that only counts instructions given will cheerfully tell you it did all ten. A machine that counts shaft actually turned will not.
There is a difference between what a machine was told to do and what it did.
The main concept — the motor has been counting all along 6 min
Inside every EV3 motor is a counter that ticks round as the shaft turns. It has been running since the moment you switched the Brick on. Nobody has asked it anything yet.
([A v] degrees counted) :: motors
Reporters are a different shape for a reason. You do not put this in a stack on its own; you drop it into a slot in another block, where a number would otherwise go:
write ([A v] degrees counted) at line (2) :: display
| Block | What it does |
|---|---|
| ([A v] degrees counted) | Reports total degrees turned since the last reset. 360 is one full turn, so ten turns reads 3600. |
| [A v] reset degrees counted | Sets that number back to zero. Without it you are counting from wherever the last run left off. |
Why this beats counting the loop
You could count strikes by counting how many times the loop went round. But that only tells you how many times you gave the order. If the hammer jams on strike six, a loop count still reaches ten.
The encoder cannot lie about it. If the shaft did not turn, the number does not go up. That is why the reading on the screen is evidence and a loop count is only a claim.
Reading a motor's position
Every EV3 motor contains a sensor that counts how far it has actually turned. That means a motor is not only an output — you can ask it where it is, and the answer describes what really happened rather than what you asked for.
Blocks reference
| Block | What it does |
|---|---|
([A v] degrees counted :: sensors) | Reports how far this motor has actually turned since the count was last reset, in degrees. |
[A v] reset degrees counted :: motors | Sets the count back to zero, making right here the new reference point. |
([A v] speed :: sensors) | Reports how fast the motor is turning right now, as a percentage. A motor that is being driven but reads zero is a motor that is stuck. |
Asked for, versus achieved
Those two are not always the same. Here are two identical motors, running the same program, with only their mechanisms different.
when program starts
A reset degrees counted
A run clockwise for 2 rotations
write A degrees counted at line 1
the same program, on two identical motors
720degrees asked for0counted · free0counted · jammed
Free: asked 720, counted 720Jammed: asked 720, counted 0
Both motors are told to turn two rotations — 720 degrees. Both counters start at zero.
Nothing on the Brick announces a stall. The only evidence is that the counter stopped changing while the motor was still being told to turn.
Nothing on the Brick announces the jam. The motor is still being driven, the program is still sitting on the same block, and the only trace of the problem anywhere is a counter that has stopped climbing. Comparing what you asked for with what was counted is how a robot notices — which is the whole of stall detection.
Zero is wherever you say it is
When the Brick powers on, the count is simply whatever it happens to be. It is not a position on the machine — it becomes one only when you tie it to something physical.
That is what reset degrees counted is for, and it is not just a tidy-up block for the top of a program. Where you put it decides what zero means, so putting it part-way through — after the mechanism has been driven somewhere known — is the normal way to use it, not an abuse of it.
Find home first, then reset
A conveyor has no idea where it is. Give it a touch sensor at one end and it can find out: drive it until the sensor is pressed, and it is now at a place you can name. Only then reset the count, and that end becomes 0.
when program starts :: events hat [A v] start motor [counterclockwise v] :: motors [1 v] wait until [pressed v] :: sensors [A v] stop motor :: motors [A v] reset degrees counted :: motors
The order is the whole point. Reset before the sensor is pressed and you have zeroed a random spot; reset after it, and every later reading means “how far from home”. This is called homing, and it is why a printer rattles its head to one side when you switch it on.
Drive towards home gently. The mechanism is deliberately being run into its own end stop, so a slow speed saves the gears — and the touch sensor is what stops it, which means it stops in the same place every time regardless of where it started.
Putting zero somewhere more useful
Home is often a corner, and a corner is an awkward place to measure from. Say the conveyor carries a chute that dispenses bricks, and you would rather describe its position as left and right of the middle. Then home once, drive the known distance to the middle, and reset again there:
when program starts :: events hat [A v] start motor [counterclockwise v] :: motors [1 v] wait until [pressed v] :: sensors [A v] stop motor :: motors [A v] reset degrees counted :: motors [A v] run [clockwise v] for (900) [degrees v] :: motors [A v] reset degrees counted :: motors
Now the middle is 0. Moving right counts up, moving left counts down past zero into negative numbers — the count is perfectly happy to go negative — and “go back to the middle” becomes the simplest instruction in the program: drive until the count reaches 0.
Both resets earn their place. The first one turns a meaningless number into a distance from a real, repeatable place. The second one moves zero to where the maths is easiest. A reset in the middle of a program is only a mistake when the mechanism is somewhere you cannot name.
Turning turns into distance
Because the count is in degrees, and a wheel of known size travels a known distance per turn, the reading can be converted into how far the robot has actually driven. That is how a robot reports a distance in centimetres rather than in rotations — and it is why changing the wheels changes the answer.
Why it matters
This is how a printer knows the paper jammed, how a car window stops when it meets your hand, and how a robot arm knows it has reached its limit. A machine that can only give orders is fragile; one that can check what happened can recover.
Ask the machine what it did. Do not assume it did what you asked.
Writing to the screenLines, clearing, and why line numbers matter. Open it if the display blocks need a refresher.Show meHide
The Brick display
The Brick’s screen is a small black-and-white whiteboard. You can wipe it, draw one of the built-in pictures on it, or write your own words at a spot you choose. It is how the robot tells a person what it is doing.
Blocks reference
| Block | What it does |
|---|---|
clear display :: display | Wipes the screen blank, ready for something new. |
display [Eyes / Neutral v] :: display | Draws a built-in picture chosen from a list — eyes, faces, arrows, symbols. |
display [Eyes / Neutral v] for (2) seconds :: display | Draws the picture, holds the program for that long, and then carries on. Handy when a face should be seen before anything else happens. |
write [Hello] at line (1) :: display | Writes your own text on one of the screen’s eight lines. |
write [Hello] at x: (10) y: (40) with font [normal black v] :: display | Writes text at an exact spot instead of a line, and lets you choose the size. Use it when a message has to line up with something else on the screen. |
Clear it, or write on top of what is there
The screen has no memory of whose writing is whose. It keeps every mark until something wipes it — including the marks left by the last run. Watch the same program with and without its clear display block.
with clear display
when program starts
clear display
display Eyes / Neutral
write GATE OPEN at line 7
without it
display Eyes / Neutral
write GATE OPEN at line 7
1lines · cleared1lines · not cleared1run
Cleared: one message, readableNot cleared: 1 messages on line 7
Both Bricks still show whatever was left on the screen when the last run ended.
Let it loop two or three times. The left screen still reads cleanly; the right one is the same program with one block missing.
Both Bricks are told exactly the same thing. The left one wipes the screen first, so line 7 holds one message and reads correctly. The right one never wipes, so each new message is drawn over the last and the words turn into a smudge. This is why a program that seems to display nonsense is usually displaying the truth — several times over.
Where the writing goes
There are two blocks for writing text, and they describe where in two completely different ways. Watch one message move around the screen under both of them.
clear display
write READY at line 1
write READY at x: – y: – with font normal black
the screen is 178 pixels across and 128 down
0x · across0y · down1line
write [READY] at line (1). Line 1 is the top of the screen, hard against the left edge.
The arrows are the two numbers. The one along the top is x; the one down the side is y, and it counts downwards from the top edge.
Writing on a line
write [EV3] at line (1) is the simple one. You give it a line number and it puts the text there, starting hard against the left edge — you do not choose how far across, only how far down.
- Line 1 is the top. The numbers count downwards, so line 2 is below line 1, not above it.
- There are eight of them, each about ten pixels lower than the last. That means all eight sit in the upper part of the screen and the line block cannot reach the bottom at all.
- One message per line. Two messages sent to the same line land on top of each other and turn into a smudge — put them on different lines, or clear the screen in between.
For most robots this is all you need: a status word on line 1, a reading on line 3, a warning on line 5. Pick your lines at the start and keep each one for one job, the way you would keep one colour of status light for one meaning.
Writing at x and y
write [EV3] at x: () y: () with font [] treats the screen as a grid of pixels — 178 across and 128 down — and lets you put the text anywhere on it.
- x is how far across, from the left edge. x: 0 is the far left; x: 177 is the far right. Bigger x moves right, which is what everybody expects.
- y is how far down, from the top edge. y: 0 is the very top; y: 127 is the bottom. Bigger y means lower — this is the one that catches people out, because on a graph at school a bigger y means higher.
- The corner is 0, 0. x: 0 y: 0 is the top-left, and it puts the text in exactly the same place as at line 1 — the two blocks are describing the same screen.
- The pair of numbers is the top-left corner of the text, not its middle. The letters go down and to the right from the point you name.
The block ends with a font dropdown, and normal black is the usual choice. A larger font makes the writing easier to read from across the room but takes more room across the screen, so fewer characters fit before the text runs off the edge.
Which one to use
- Use lines for ordinary status messages — quick to write, impossible to get wrong, and eight of them is plenty for one robot.
- Use x and y when the position itself matters: putting a label and its number side by side, lining a heading up over a reading, filling the lower part of the screen, or moving something across the screen as a value changes.
Putting a number on the screen
The text slot of either block will take a reporter, so a sensor value can be shown directly: drop (4 distance in cm) into the slot and the screen shows the reading. That is the EV3’s version of a print statement, and it is the fastest way to find out what a robot actually thinks it is seeing.
A bare number on its own is hard to read, though. To show DIST: 23 rather than 23, use the operator block join [DIST: ] () to glue the label to the value, and put the join into the write block’s text slot.
when program starts :: events hat clear display :: display forever write (join [DIST: ] ([4 v] distance in [cm v] :: sensors)) at line (1) :: display end
Note the forever: a value written once is a value from the start of the program. To watch a reading change, the write block has to be inside a loop.
Advanced — a dashboard, not a number
Once one value is on the screen the rest follows, and the useful trick throughout is join: it glues two pieces of text together and hands the result to the write block. Anything that reports a value can go in either slot — a variable, a sensor, or another join.
A variable with a label on it
Counting is the classic case. A variable counts how many times the Touch Sensor has been pressed, and a bare 5 on the screen tells nobody anything — PRESSED: 5 tells them everything:
when program starts :: events hat clear display :: display set [count v] to (0) :: variables forever wait until <[1 v] is pressed? :: sensors> change [count v] by (1) :: variables write (join [PRESSED: ] (count)) at line (1) :: display wait until <not <[1 v] is pressed? :: sensors>> end
The two wait until blocks are what make it count presses rather than counting as fast as the loop runs while your finger is down. That is the Touch Sensor’s own lesson, but it shows up here because a counter on the screen is where you first notice it going wrong.
Several sensors, one line each
Give every reading its own line and keep it. A line that changes meaning halfway through a program is unreadable at a glance, which is the only speed a screen on a moving robot gets read at.
when program starts :: events hat clear display :: display forever write (join [DIST: ] ([4 v] distance in [cm v] :: sensors)) at line (2) :: display write (join [DEG: ] ([A v] degrees counted :: sensors)) at line (5) :: display end
degrees counted is worth putting on the screen the first time you use it — it is the fastest way to find out whether a motor is turning as far as you think it is, and it is the block behind every “why did it stop early” question.
Turning a number into a word
Sensors report numbers, and people read words. The Colour Sensor reports 5; the person watching wants RED. A chain of if … then … else blocks does the translation, and it is worth doing once into a variable rather than in every write block:
if <([3 v] color :: sensors) = (5)> then
set [name v] to [RED] :: variables
else
if <([3 v] color :: sensors) = (4)> then
set [name v] to [YELLOW] :: variables
else
set [name v] to [OTHER] :: variables
end
end
write (join [COLOR: ] (name)) at line (4) :: displayThe same shape turns a distance into a warning. Here the screen carries the number and what the number means, which is what lets somebody across the room tell whether the robot is about to hit something:
if <([4 v] distance in [cm v] :: sensors) < (10)> then
set [status v] to [TOO CLOSE!] :: variables
else
if <([4 v] distance in [cm v] :: sensors) < (30)> then
set [status v] to [OBSTACLE FOUND] :: variables
else
set [status v] to [NOTHING FOUND] :: variables
end
end
write (status) at line (3) :: displayDrive all four of those at once below. The screen is the real thing — five write blocks, five lines, and every one of them a join.
Drive the sensors and read the screen. Every line is one write block with a join in its text slot.
24 cm
180°
Colour Sensor5 → RED
Touch Sensor3 presses
| Line | What goes in the write block | On the screen now |
|---|---|---|
| 1 | join "PRESSED: " (count) | PRESSED: 3 |
| 2 | join "DIST: " (4 distance in cm) | DIST: 24 |
| 3 | (status) — set by the if chain | OBSTACLE FOUND |
| 4 | join "COLOR: " (name) | COLOR: RED |
| 5 | join "DEG: " (A degrees counted) | DEG: 180 |
Line 4 says COLOR: RED, but the sensor only ever said 5. The if chain in between is what turns a number into a word.
Advanced — stopping the screen from flickering
The moment a reading goes inside a loop, one of two things goes wrong, and which one depends on where the clear display block went.
Almost everybody tries clear inside the loop first. It gives the right text — and a screen that blinks several times a second, because between the clear and the write there is genuinely nothing on the screen and the loop goes round faster than your eye.
So the clear gets moved to the start of the program, the blinking stops, and the words go wrong. Writing text paints only the characters it has and leaves everything past them exactly where it was. Write RED over YELLOW and the LOW is still there: the screen says REDLOW. Go the other way — yellow first, then red — and it looks fine, which is why this bug takes so long to pin down.
when program starts :: events hat clear display :: display forever write (name) at line (3) :: display end
YELLOWwhat the sensor says—what the screen says0characters left behind
never blankleftovers survive
The Colour Sensor reads a colour and the name goes on line 3, over and over, inside a forever loop.
Watch the bottom three rows rather than the Brick. They are what the screen is actually doing: old characters, new characters, and whatever survives.
The fix is to make the new text at least as long as the old one, and join already does that: pad it with spaces. RED plus three spaces is six characters — exactly enough to paint over YELLOW. Nothing is left behind, and nothing is ever blank.
write (join (name) [ ]) at line (3) :: display
Pad to the length of the longest thing that line can ever show. Six characters covers RED, BLUE and GREEN but not NOTHING FOUND — count the longest message, and add spaces to match. The same applies to numbers: DIST: 5 after DIST: 40 leaves a stray 0 on the end, so a line that shows a two-digit reading needs a trailing space.
Use clear-inside-the-loop when the whole screen changes at once and a flicker does not matter; use the padding trick whenever a value is being watched.
Advanced — a whole list on the screen
Once several values need showing, the line number becomes a counter and the values come out of a list: write item 1 at line 1, item 2 at line 2, and so on inside a repeat. That needs the list blocks rather than the display blocks, so it lives with them — Lists covers building one and reading it back item by item, and the loop that walks it is the same loop that fills the screen.
Why it matters
Machines in the real world tell you what they are doing: a microwave counts down, a lift shows its floor, a car dashboard warns you. A robot that shows WAITING and then OPEN is far easier to understand — and far easier to debug — than one that moves silently.
Say this back before moving on: “Reset it first, or I am counting from last time.”
What’s in this build 4 min
| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | Reads the motor’s counter and puts it on screen. Its weight also keeps the machine still while the hammer works. |
| Large Motor — the hammer drive | Lifts and drops the hammer. It contains the counter this whole lesson reads from — the counter is part of the motor, not part of the Brick. |
| The Brick screen | The instrument panel. Today it shows a measurement rather than a message. |
| The Brick speaker | One beep per blow, so you can count by ear and check the screen agrees with what you heard. |
Watch the counter before you use it
- Open Port View on the Brick, or the motor tile in EV3 Classroom, and find motor A’s degrees.
- Turn the hammer shaft by hand, slowly, one full turn. Watch the number climb by about 360.
- Turn it back the other way. The number goes down — the counter tracks direction, not just movement.
- Turn it a full ten times and check you land near 3600. That is the number your program is about to print.
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 |
|---|---|---|
| Hammer motor (Large) | A | One job motor. It matters more than usual today that you know which port it is in, because you will be naming that port inside a reporter as well as inside the motor blocks. |
| Sensors | none | The counter is not a sensor — it is inside the motor, and it arrives down the motor cable. |
Check your own build now:
- Hammer motor into A. If you put it in D, every reporter in your program has to say D too — and a mismatch there gives you a screen that reads 0 for ever.
- Put something under the hammer that you do not mind marking.
- Nothing fragile, and no fingers, at any point in the run.
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.
Either works. The machine stays put. Bluetooth is slightly nicer here because you will be reading the Brick’s own screen a lot, and a cable decides where the Brick has to sit.
Confirm the connection 2 min
- Check the Brick icon: connected, or not.
- One Large Motor tile on A.
- Watch the degrees number on that tile while you turn the hammer by hand. If it does not move, the program will report nothing and you will think your loop is broken when your cable is.
- Note what the number reads right now. It will not be zero, and that is exactly why the program starts with a reset.
Make it move 10 min
Ten blows, a beep for each, and a running total climbing on the screen as it works.
when program starts :: events hat clear display :: display [A v] reset degrees counted :: motors write [Degrees:] at line (1) :: display repeat (10) [A v] run for (360) [degrees v] at (45) % speed :: motors play beep (65) for (0.1) seconds :: sound write ([A v] degrees counted) at line (2) :: display end write [Siap] at line (4) :: display
- clear display, then reset degrees counted — both wipes. One clears the screen, the other clears the counter. Skip the second and your first reading will be some number left over from turning the shaft by hand.
- write Degrees: at line 1 — a fixed label. It is written once and never touched again.
- run for 360 degrees — one full turn of the cam, one complete blow.
- beep, then write the count to line 2 — always line 2, so each reading lands on top of the last one rather than scrolling away.
What success looks like: ten blows you can hear, and a number on screen that climbs in steps of about 360 and finishes near 3600. Count the beeps yourself and check the screen agrees.
If the final number is well short of 3600, the hammer is stalling somewhere. The motor is being told to turn 360 and is not managing it — which is precisely the fault a loop count would have hidden from you.
Change it and test 8 min
Write down the final number after every run. That column of numbers is the record of what you changed, and it is more reliable than remembering.
- Take out the
reset degrees countedand run the program twice in a row without touching anything. The second run finishes near 7200. Explain why in one sentence. - Put the reset back. Change the loop to 5 and predict the final number before you run it.
- Change the strike to 180 degrees and keep the loop at 10. Predict the final number. Then say how many actual blows you expect — it is not ten.
- Move the
writeblock above the motor block inside the loop. Same blocks, different order. What does the screen show at the end now, and why is it 360 short? - While it is running, hold the hammer gently so it cannot turn — not hard enough to strain the motor. Watch the number stop climbing while the loop carries on.
Step 5 is the point of the whole lesson, in about four seconds. The loop is still counting to ten. The screen tells you the truth anyway.
A counter inside the machine is worth more than a counter inside the program.
