A Note, Not a Recording: The Fencing Sword

What you are building 3 min

The Fencing model: a long, light sword with a Touch Sensor at its tip. Touch something with the point and the Brick knows about it instantly.

There is no motor in this build. Not one. The Brick and a single Touch Sensor is the entire electronic parts list, and nothing on the model moves under its own power.

Lesson 14’s Elastic Car was all mechanism and no electronics. This is the exact opposite: all sensing, no movement. It is a machine whose only job is to notice, and then say so — which makes it the right place to learn a new way of saying things.

In the real world 5 min

Where you have seen it

Competitive fencing has been electric since the 1930s. Each blade carries a switch in its tip, wired back to a scoring box, and a hit registers as a light and a tone before any judge could possibly have seen it.

Why it is built that way

A fencing hit takes a few thousandths of a second. Two fencers can both land one so close together that no human being could say which came first — and in épée, if they are within a twenty-fifth of a second, both score.

That is not a judgement a person can make. So the sport gave the job to a switch: a button in the tip that closes when it is pressed against something. Not clever, not precise about where — just very, very fast.

What would go wrong without it

Before the electric box, hits were called by four judges standing round the piste, and arguments were part of the sport. The switch did not make fencing faster. It made it agreed.

And notice what the box actually does when it detects a hit: it makes a tone. Not a recorded voice, not a tune — a single clear note that carries across a noisy hall and means exactly one thing.

A machine that senses something has to be able to say so instantly, and in a way nobody can mistake.

The main concept — making a sound instead of playing one 6 min

Every sound in this course so far has been a recording picked from a dropdown. The Brick can also make a tone from a number — and a number is something you can change.

play sound [Information / Blip 1 v] until done :: sound

play beep (60) for (0.2) seconds :: sound

start playing beep (60) :: sound

The first number is the note. Bigger means higher. The exact numbers are worth knowing roughly rather than exactly:

There is no plain “beep” block and no “play note” block. A tone in EV3 Classroom is always a note number and a length, together. If you go looking for something simpler you will not find it.

Here is why this matters for a scoring machine. A recorded file says one thing. A number lets one machine say several things — a high note for a hit on target, a low one for off target, a different one again for the end of the bout — all from the same block with a different number in it.

A sound file is a thing you chose. A beep is a thing you can calculate. That is the whole difference, and it is why beeps scale and files do not.

▶Sound and beeps, and the Touch SensorThe full reference for both — the sound blocks in full, and the sensor this model is built around. Open either if it is not clear yet.Show meHide

ComponentOutput5 min

Sound and beeps

The Brick has a small speaker. It can play one of the built-in sound files, or beep a note you choose. Sound is what makes a machine feel finished — and it is also a way for the robot to tell you something without you having to look at it.

Blocks reference

Block	What it does
play sound [Communication / Hello v] until done :: sound	Plays the sound and waits for it to finish before the next block runs.
start sound [Communication / Hello v] :: sound	Starts the sound and moves straight on to the next block, so the sound plays while the robot keeps working.
play beep (60) for (0.5) seconds :: sound	Plays a single note for a set time — useful for short alerts. The number is a note, not a volume: bigger means higher.
set volume to (100) % :: sound	Sets how loud everything after it will be. Worth putting at the top of a program — the Brick remembers the last volume it was given, even from somebody else’s program.
stop all sounds :: sound	Cuts off anything that is still playing, including a long sound started earlier.

Wait for it, or not

These two blocks play the very same file. The difference is what the rest of the program does while it plays — so it cannot be heard on its own, only seen. Here are both, each lifting a barrier.

play sound until done

when program starts

play sound Communication / Hello until done

A run clockwise for 1 rotations

start sound

start sound Communication / Hello

A run clockwise for 1 rotations

0.0suntil done0.0sstart sound

until done: warn first, then movestart sound: warn while moving

Two programs. Same sound, same movement — only the sound block is different.The sound is playing on both. On the left the program is waiting for it, so the barrier has not moved. On the right the barrier is already lifting.The sound has finished. Only now does the left barrier lift — the right one finished a moment ago, sound and movement together.Both played the warning and both lifted the barrier. One took twice as long to do it.Finished. Neither program is wrong — they are answers to two different questions.

▶ Play⏭ Step↺ Restartstopped

Watch the barriers, not the clock. On the left the warning finishes before anything moves; on the right it sounds while the barrier lifts.

The bars underneath are when the speaker was on and when the motor was turning. On the left they never overlap: the program is stuck at the sound block until the file has finished, and only then does the barrier lift. On the right they overlap almost completely, and the whole job is done in half the time.

• play sound until done — everything stops and waits. Use it for a warning that must be heard before something moves.
• start sound — the sound and the movement happen together. Use it when the noise belongs to the movement, like a motor whirring as an arm lifts.

Why it matters

Real machines warn before they move, not after: a lift chimes before the doors close, a reversing lorry beeps while it rolls back, a level crossing sounds before the barrier drops. Getting the order right is the difference between a warning and an apology.

ComponentSensing5 min

The Touch Sensor

The Touch Sensor is the simplest input the EV3 has: a button that is either pressed or not. That sounds trivial, but it is how a robot knows it has hit a wall, reached the end of a track, or been told to start by a person.

released

pressed

Blocks reference

Block	What it does
wait until <[1 v] is pressed? :: sensors>	Holds the program here until somebody presses the sensor.
<[1 v] is pressed? :: sensors>	Reports true or false. Drop it into a condition to make a decision rather than a wait.
[1 v] when [bumped v] :: events hat	Starts a whole stack of its own. The dropdown chooses the moment: pressed, released or bumped.

Three different events

A button is not only “pressed”. One press is three things: the moment it goes down, the time it stays down, and the moment it comes back up. Watch what a single press does to three programs at once.

when program starts

forever

  if 1 is pressed? then

    change count by 1

versus two hat blocks

1 when pressed

1 when bumped

0is pressed? in a loop0when pressed0when bumped

In a loop: 0 answers from one pressBumped: exactly one

Nobody is touching the sensor. All three programs are watching it.A finger presses the button. Watch the red button go in — a couple of millimetres is the sensor's entire movement.The finger is still down. The loop checking «is pressed?» has already run hundreds of times, and every one of them counted.The finger lifts. Only now does «bumped» count, because bumped means pressed AND released.One press. Three completely different answers.Finished. The same press, counted three ways.

▶ Play⏭ Step↺ Restartreleased

The middle counter is the one that surprises people. Nothing is wrong with it — a loop really does check that fast, and every check really is a separate answer.

Nothing there is broken. A loop really does get round hundreds of times a second, and each time it asks is pressed? the honest answer is still yes — so if that loop plays a sound or counts something, it does it hundreds of times from one finger. The two hat blocks each fire once, and they fire at different moments: pressed the instant the button goes down, bumped only when it comes back up.

The three options, and what each is for:

• Pressed — the button is down right now. Good for “hold to run”.
• Released — it is up again. Good for acting when somebody lets go.
• Bumped — pressed and released. This is what you want for “click to start”, because it will not fire repeatedly while a finger stays down.

The classic bumper

when program starts :: events hat
set movement motors to [B v] and [C v] :: movement
start moving [straight: 0] :: movement
wait until <[1 v] is pressed? :: sensors>
stop moving :: movement

The robot drives until something presses the sensor. Note that the movement is started unmeasured on purpose — the sensor decides when to stop, not a distance.

Why it matters

Touch sensors are everywhere in machines you cannot see into: a lift knows the doors are shut, a printer knows the lid is closed, a washing machine will not spin until it is latched. They are safety devices as much as inputs.

Say this back before moving on: “A beep takes a note number and a length. Bigger number, higher note.”

What’s in this build 4 min

Look at your model and find the parts with cables. There is one, and that is the shortest parts list in the whole course.

no hit

hit

Test the tip before you program

1. Press the sensor at the very tip with a finger. Does it click and spring back cleanly?
2. Now press the sword tip against a book, straight on. Does the sensor trigger? It must, or the sword scores nothing.
3. Press at an angle. Does it still trigger? Most builds are much worse at this, and it is worth knowing before you blame your program.
4. Sight down the blade. Is the cable taped or clipped along it, so it cannot catch or pull out mid-thrust?

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.

Today the table has a sensor in it and nothing else — the first time all course. Every letter port stays empty.

Check your own build now:

1. Sensor cable in port 1 — a number, not a letter.
2. Enough slack in the cable that a full arm’s-length thrust does not yank it out of the Brick.
3. Volume up on the Brick. Today the sound is the output.
4. Agree a safe target with your group — a book, a box, a wall. Not a person, and not anybody’s face, however light the sword is.

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

1. Switch the Brick on with the dark grey centre button.
2. Cable into the Brick’s PC port — the small square socket beside the numbered ports, not one of the numbered ones.
3. 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.

1. 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 EV3 until somebody changes it.
2. Turn Bluetooth on. Settings → Bluetooth. Tick Bluetooth and Visibility. Leave iPhone/iPad/iPod unticked.
3. Connect from EV3 Classroom. Click the Brick icon at the top of the programming area, find your Brick by name, and click Connect.
4. Say yes on the Brick. It asks “Connect?” with the computer’s name — choose the tick, then accept the passkey, which is already 1234.

⤢ Tap to enlarge

⤢ Tap to enlarge

⤢ Tap to enlarge

⤢ Tap to enlarge

⤢ Tap to enlarge

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 suits a sword. This is the one model in the course that gets waved about, and a USB lead running to a laptop is something to trip over or pull off a desk.

Confirm the connection 2 min

1. Check the Brick icon: connected, or not.
2. Port 1 shows a Touch Sensor, and every other tile is empty.
3. Press the blade tip against your hand and watch the port 1 tile flip. Then let go and watch it flip back.
4. Do it once more with a fast tap rather than a slow press. The tile should still register it. If a quick tap does not show up, the tip has too much slack in it and needs tightening — nothing in the program can fix a hit the sensor never felt.

A scoring machine that misses fast hits is worse than no scoring machine, because people will trust it.

Make it move 10 min

Six blocks — and nothing moves, because nothing on this model can. “Make it work” is nearer the truth today.

when program starts :: events hat
set volume to (100) % :: sound
forever
  [1 v] wait until [pressed v] :: sensors
  play beep (72) for (0.3) seconds :: sound
  [1 v] wait until [released v] :: sensors
end

Walk it in the order the Brick runs it:

1. set volume to 100 % — a setting, like set speed in Lesson 21. It makes no sound at all; it decides how loud the next one will be.
2. forever — a bout does not stop after one hit. Note that nothing below this loop will ever run, which is the Lesson 19 rule.
3. wait until pressed — the program sits here until the tip touches something.
4. play beep 72 for 0.3 seconds — the score. High and short, so it carries and does not drag on.
5. wait until released — and this is the block people leave out. Without it, a blade resting against the target scores over and over as fast as the Brick can loop.

What success looks like: one clear tone per hit, however fast you tap, and silence while the blade is away. Rest the tip against the target and hold it there — you should hear exactly one beep, not a stream.

If it buzzes continuously, you have left out the wait until released block, or the tip is jammed pressed. Both look identical from outside the program.

Change it and test 8 min

One change at a time, and predict before each run — today, predict what it will sound like.

1. Change the note to 40. Then 90. Stand back across the room for each. Which one can you hear best over people talking?
2. Change the length to 1.5 seconds. Now try scoring two quick hits in a row. The second one is missed entirely, because play beep holds the program while it sounds.
3. Swap it for start playing beep (72). Predict what changes. The program no longer waits, so it is ready for the next hit immediately — the Lesson 15 idea, in a new place.
4. Put the 0.3-second version back and add a set status light to [red v] after the beep and a green one after the release. Now the machine scores in sound and light, like a real box.
5. Add a second, lower beep straight after the first — say 72 then 60. You have just given your machine a two-note signal, which no sound file in the dropdown could have given you.

Step 2 is the one that catches people out in a real bout. A long beep is not just annoying — it is a scoring box that cannot see the next hit, and in fencing the next hit is often a fifth of a second away.

A signal has to be over before the next thing it needs to signal happens. Short and high beats long and impressive.