Challenge 1
Make the forklift refuse. Run it with no pallet on the forks and leave it for thirty seconds — the mast must not move at all. Then push a pallet on and watch it start. Both halves of that are the challenge.
What you are building 3 min
The Forklift: a driving base with a mast on the front, a Medium Motor to raise and lower the forks, and a Touch Sensor that knows when a pallet is actually on them.
Raising the forks is easy. Knowing when it is safe to raise them is the job — and it is not one question.
Is there a pallet on the forks? And is the mast still below its limit? Both have to be true. Either one on its own gives you a forklift that lifts thin air, or one that keeps winding when it has already run out of mast.
In the real world 5 min
Where you have seen it
Forklifts are everywhere goods are moved — warehouses, ports, building sites, the back of every supermarket. A counterbalance forklift can pick up a tonne and a half on two steel forks and carry it at walking pace.
Why it is built that way
A forklift is a lever with a very heavy load on the short end, and it only stays upright because of a counterweight at the back. That balance has limits, so real forklifts are covered in interlocks — rules the machine enforces regardless of what the driver asks for.
The interesting ones are nearly always about two things at once. The mast will not tilt forward unless the load is low and the handbrake is off. The machine will not drive unless the operator is in the seat and the seatbelt is fastened. One condition is a preference. Two conditions is a rule.
What would go wrong without it
Check only whether a pallet is present and the machine will happily keep lifting after the mast has reached the top, straining the motor against a mechanical stop. Check only the height and it will lift nothing at all, wearing out the mechanism for no reason. Neither check is wrong; each is just incomplete.
Most real safety rules are two conditions joined by the word “and”.
The main concept — one question made of two 6 min
The green and block is a hexagon with two hexagonal holes in it. Put a condition in each hole and the whole thing is true only when both of them are.
<<[1 v] is pressed?> and <([D v] degrees counted) < (720)>> :: operators
Note the shapes again. Both halves are hexagons, and the result is a hexagon — which is why it drops into a Switch exactly like a simple condition. Nothing about the Switch changes. You have just made the question more precise.
| Pallet on forks? | Below the limit? | and gives | Forklift does |
|---|---|---|---|
| yes | yes | true | Lifts. The only safe case. |
| yes | no | false | Waits. The mast is already up. |
| no | yes | false | Waits. Nothing to lift. |
| no | no | false | Waits. |
Four rows, one true. That is what and is for: it is strict, and being strict is the point of an interlock.
Its two relatives
| Block | True when | Reach for it when |
|---|---|---|
| <<> and <>> | Both halves are true. | Everything must be right before something happens. Safety rules. |
| <<> or <>> | Either half is true — or both. | Any one of several reasons is enough. Stop buttons, alarms. |
| <not <>> | The one inside is false. | Saying “while there is no pallet” without writing a backwards comparison. |
Stop rules are usually or, not and. A machine should stop if the emergency button is pressed OR the load is too heavy OR the door is open — any one is enough. Notice that safety uses both words, for opposite jobs: and to permit, or to forbid.
Comparing and combining
A sensor that reports a number cannot be used to make a decision on its own — 23 is neither true nor false. An operator turns that number into an answer by comparing it with something.
Blocks reference
| Block | What it does |
|---|---|
<(x) > (50)> | True when the left value is bigger than the right. |
<(x) < (50)> | True when it is smaller. |
<<> and <>> | True only when both conditions are true. |
<<> or <>> | True when at least one of them is. |
Try it: which way round does it go?
Forget the symbols for a moment. A comparison is a question about position on a number line: is x to the left of the other number, or to the right? Left is smaller, right is bigger — and that is the whole of it.
Drag the orange x and the black marker, and change the comparison. The green stretch is every position of x that would make the answer true — so you can see where the answer flips before you get there. Turn not on and watch the green jump to the other side.
Drag either marker, or use the arrow keys.
-3 < 4true
is x to the LEFT of it?
- < is true while x sits on the left. Slide x past the marker and it flips.
- > is the same question the other way round — so exactly one of the two is true, unless the markers are on the same spot.
- = is true for one single position out of twenty-one. Try landing on it. That is why a sensor is almost never compared with =: a reading passes straight through the exact number without ever being measured there.
- not flips the answer, whatever it was. not (x < 4) covers everything x < 4 does not — including landing exactly on 4.
Try it: which numbers make it true?
The lab above asks one question at a time: is this x true? A robot never has just one x, though — a sensor reading slides up and down all the time, so what really matters is which stretch of the line makes the condition true. This one draws the whole answer at once.
Drag the circle to move the number you are comparing against, and change the comparison. Everything shaded green is a value of x that would make it true.
Drag the circle, or use the arrow keys. It moves in steps of 0.2.
x < 0.2x < 0.2
Every number to the left of 0.2 — but not 0.2 itself, so the circle is hollow.
Watch the circle, because it carries the part everyone gets wrong:
- Hollow ○ — the boundary is not included. x < 0.2 shades everything left of 0.2 but leaves 0.2 itself out, because 0.2 is not less than 0.2.
- Filled ● — the boundary is included. Choose = and nothing is shaded at all: one single number qualifies.
Now turn not on with x > 2 selected and watch two things happen together. The shading jumps to the other side, and the circle fills in — because “not greater than 2” means 2 or less, and 2 has to be part of it. That pairing is the whole reason a hollow circle is worth drawing.
Why a robot cares. Two conditions that look almost identical — light < 30 and not (light > 30) — differ by exactly one value, the reading of precisely 30. A robot sitting right on its threshold behaves differently under the two, and that is the sort of bug that only shows up occasionally and looks like a broken sensor.
Try it: and, or, not
These three join answers together rather than numbers. The trap is that English is looser than a program: “stop if it is close and the bumper is pressed” sounds like it covers both situations, when it covers neither on its own.
Flip the two conditions and watch the table. There are only four possible situations in total, and and and or differ on exactly two of them.
close: true and bumper: falsefalse
| close | bumper | and | or |
|---|---|---|---|
| true | true | true | true |
| true | false | false | true |
| false | true | false | true |
| false | false | false | false |
and is fussy: it wants both. Three of the four rows are false.
- and is true on one row out of four. It narrows — the robot acts less often, but more certainly.
- or is true on three rows out of four. It widens — the robot acts more readily.
- The two agree on the top and bottom rows and disagree in the middle. Whenever swapping one for the other seems to make no difference, you have only tried the rows where they agree.
Watch them decide
Two sensors are running below: an Ultrasonic reporting a number, and a Touch Sensor reporting true or false. Watch the comparison turn the number into an answer, and watch and and or disagree.
when program starts
forever
if distance < 15 and is pressed? then
stop moving
if distance < 15 or is pressed? then
play beep 60 for 0.2 seconds
Nothing is within 15 cm and the bumper is out. Both conditions are false.
and was true in one row out of four. or was true in three. That is the whole difference, and it is why one of them makes a robot look broken.
The comparison is doing one job: it takes a reading that is neither true nor false and, by holding it against a number you chose, produces something a decision can use. The moment the blue fill crosses the black marker is the moment the answer changes.
Choosing the threshold
The number you compare against is a design decision, not a fact. “Close” for a parking sensor might be 15 cm; for a robot arm it might be 3. Pick it by measuring what the sensor actually reads in the situation you care about, then leave a margin.
Combining two conditions
and narrows: both must hold, so the robot acts less often but more certainly — stop only if something is close and the bumper is pressed. or widens: either will do, so the robot acts more readily — stop if something is close or the bumper is pressed.
In the four situations above, and was true in one of them and or in three. That is the practical difference: swapping one for the other does not adjust a robot slightly, it changes how often it reacts at all.
And is strict, or is generous. Pick the one that matches the sentence you would say out loud.
The SwitchWhere the combined condition goes — unchanged from Lessons 13 to 16.Show meHide
Making a decision
Up to now a robot has been able to wait for a sensor. Deciding is different: the robot checks the sensor and does one thing or another depending on the answer — and then carries on either way.
Blocks reference
| Block | What it does |
|---|---|
if <> then end | Runs the blocks inside only when the condition is true. Otherwise skips them. |
if <> then else end | Runs one set of blocks when true and a different set when false. |
Deciding again and again
A decision made once, at the start, is almost never what you want. Here are two robots with the identical if-else, testing the identical sensor against the identical number — one inside a loop and one not.
decision inside a loop
forever
if distance < 15 then
stop moving
else · start moving
the same decision, once
if distance < 15 then
stop moving
else · start moving
247checks · in a loop1check · once only
Nothing is close, so both robots ask «is the wall within 15 cm?», both hear no, and both take the else branch and drive.
A decision is only worth as much as the last time it was made. Inside a loop, that is a few milliseconds ago.
Nothing is wrong with the right-hand program’s decision. It asked the question, got a truthful answer, and acted on it correctly. It simply never asked again, and the world moved on. Decisions belong inside a loop, so the robot keeps re-deciding as things change.
when program starts :: events hat
forever
if <([4 v] distance in [cm v] :: sensors) < (15)> then
stop moving :: movement
else
start moving [straight: 0] :: movement
end
endFour shapes, and how to choose
Once there is more than one question, decisions can be arranged in four ways. They look nearly identical stacked up in the editor, which is exactly why they get muddled — the thing that differs is not what the blocks say, it is which routes through them exist.
One question sorts them almost completely:
| Are the questions… | Use | How many bodies can run |
|---|---|---|
| independent — any combination can be true | separate ifs | none, some, or all |
| one question, two answers | if / else | exactly one |
| the second only matters when the first is true | nested if | one, and only via the outer |
| mutually exclusive cases — exactly one should win | chained if / else | exactly one, the first that matches |
Separate ifs — independent questions
Each if is asked no matter what the others answered, so any number of them can fire on the same pass. That is the right shape when the conditions genuinely have nothing to do with each other.
forever
if <[3 v] is ambient light intensity [< v] (20) %? :: sensors> then
[A v] start motor [clockwise v] :: motors
end
if <[4 v] is distance [< v] (15) [cm v]? :: sensors> then
[D v] start motor [clockwise v] :: motors
end
endif / else — one question, two answers
Exactly one branch runs, every time. Reach for this whenever the robot must do something either way — and in preference to two ifs testing opposite conditions, which is the same idea written twice and can drift apart.
Nested if — a follow-up question
Putting one if inside another means the inner question is only ever asked when the outer one is true. Use it when the second question is meaningless otherwise: there is no point asking which side an obstacle is on when there is no obstacle.
if <[4 v] is distance [< v] (15) [cm v]? :: sensors> then
if <([2 v] angle :: sensors) < (0)> then
start moving [right: 50] :: movement
end
endWhen not to nest. If you only want “both true” and nothing happens at the outer level, an and says it in one block and reads better:
if <<[4 v] is distance [< v] (15) [cm v]? :: sensors> and <([2 v] angle :: sensors) < (0)>> then start moving [right: 50] :: movement end
Nesting earns its place when something happens at the outer level too, or when there is an else at each level and the two mean different things.
Chained if / else — one winner out of several
This is the shape for a list of cases where exactly one should win: colour bands, distance bands, speed ranges. EV3 Classroom has no else-if block, so you build a chain by putting the next if inside the else of the last one.
And here is why it matters, because this is the single commonest bug in this whole module. Three bands written as three separate ifs are each perfectly correct, and together they are wrong: a reading of 20 is under 30 and under 60 and under 90, so all three run and the last one to run is the one that sticks.
three separate ifs
if light < 30 then
stop
if light < 60 then
go slowly
if light < 90 then
go fast
chained — if / else / if
if light < 30 then
stop
else
if light < 60 then
go slowly
else
go fast
↑ the rest is inside the else — never asked
3bands matchedgo fastseparate ifs dostopchained does
Separate ifs: 3 matched, last one winsChained: first match wins, rest never asked
The sensor reads 20 — a dark surface. Both programs should stop.
Separate ifs are not wrong here so much as unguarded: nothing stops a second one matching. Chaining is what makes “the first one wins” true.
The rule to carry away: if the cases are meant to be exclusive, they must be made exclusive. Chaining does it by construction. Separate ifs only work if you are careful to write non-overlapping bands yourself — light < 30, 30 to 60, 60 and over — which is more to get right and easy to break later.
Why it matters
This is the point at which a machine stops following a script and starts responding. A thermostat, an automatic door, a robot vacuum — all of them are a decision inside a loop.
Say this back before moving on: “And means both, every time.”
What’s in this build 4 min
| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | Sits at the back and acts as the counterweight, exactly as it does on a real forklift. Move it forward and the model tips under load. |
| Large Motor ×2 — the drive | Ports B and C, driven as a pair so the forklift approaches a pallet square rather than at an angle. |
| Medium Motor — the mast | Raises and lowers the forks. It must hold when it stops, or a lifted pallet comes straight back down. |
| Touch Sensor — the pallet detector | Mounted at the back of the forks so a pallet pushed fully on presses it. A pallet resting half on will not, which is correct behaviour rather than a fault. |
| The pallet (not electronic) | Build one you can pick up reliably. A pallet that snags is a pallet you cannot test with. |
Find the mast’s top before you program it
- Wind the mast up by hand until it will not go further. Do not force it.
- Wind it back to the bottom, counting turns. Multiply by 360 — that is the highest number your limit should ever be.
- Set your limit a little below that. A limit at the exact top means the motor strains against the stop on every run.
- Push a pallet onto the forks and check the Touch Sensor clicks. Then pull it half off and check it releases.
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 |
|---|---|---|
| Left drive (Large) | B | B is left, as the Movement blocks assume. |
| Right drive (Large) | C | C is right. |
| Mast (Medium) | D | Off the movement pair — and its port appears inside the condition as well as in the lift blocks. |
| Touch Sensor | 1 | Touch is always port 1 in this course. |
Check your own build now:
- Standing behind the forklift, left drive in B, right in C, mast in D, Touch in 1.
- Route the mast cable so it can travel the full height without pulling. A cable that goes tight at the top is a mechanical limit you did not design.
- Wind the mast to the bottom before every run, so the count starts from a known place.
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. The forklift drives to the pallet, and a cable across the floor is the one thing on the test course you did not plan for.
Confirm the connection 2 min
- Check the Brick icon: connected, or not.
- Three motor tiles and one sensor tile — Large on B and C, Medium on D, Touch on 1.
- Test both halves of your condition separately. Press the pallet sensor and watch its tile. Then wind the mast and watch D’s degrees. If either half is wrong, the
andcan never be true and the forklift will simply never lift. - Note the mast’s count at the bottom. It should be near zero after a reset.
Make it move 10 min
An interlocked mast: it lifts a little at a time, but only while both conditions hold.
when program starts :: events hat
set movement motors to [B v] and [C v] :: movement
[D v] set motor to [hold position v] at stop :: motors
[D v] reset degrees counted :: motors
clear display :: display
write [FORKLIFT] at line (1) :: display
forever
if <<[1 v] is pressed?> and <([D v] degrees counted) < (720)>> then
write [ANGKAT] at line (3) :: display
[D v] run for (90) [degrees v] at (30) % speed :: motors
else
write [TUNGGU] at line (3) :: display
end
end- hold position at stop — set once. Every one of those 90-degree lifts ends with the mast holding rather than sagging back.
- reset degrees counted — so 720 means two turns of mast from the bottom, not from wherever it was left.
- the combined condition — checked continuously. Both halves have to agree before anything moves.
- run for 90 degrees — a small bite. Lifting in small steps means the condition gets re-checked on the way up, so the limit is respected as it is approached rather than overshot.
What success looks like: with no pallet, nothing happens however long you wait — the screen says TUNGGU. Push a pallet on and the mast climbs in steps, then stops by itself at the limit and goes back to TUNGGU with the pallet still held up. Take the pallet off and it stops immediately.
If the mast never moves, check the two halves one at a time by temporarily deleting the and and using each half alone. An and that is always false usually has exactly one broken half, and testing them together tells you nothing about which.
Change it and test 8 min
Wind the mast back to the bottom before every run, and work through the four rows of the truth table rather than just the happy one.
- Delete the height half, leaving only the pallet check. Hold the pallet on and let it run — the mast reaches the top and keeps straining. Stop it, and put the half back.
- Delete the pallet half instead, leaving only the height check. Now it lifts nothing, eagerly. Neither half alone is the rule.
- Change
andtoor. Predict what happens before you run it, then find out — this is the single most useful minute of the lesson. - Put
andback and change the limit to 180. A much lower ceiling. Does it still stop cleanly? - Add a third condition with a second
and: only lift when a pallet is on, the mast is below the limit, and the mast has moved at least a little already —> (0). Say what that changes and whether it is an improvement.
Step 3 is the one to talk about. With or the forklift lifts when there is a pallet or when it is below the limit — and since it starts below the limit, it lifts immediately with nothing on the forks, and carries on to the top. The same two checks, joined by a different word, produce a machine with no interlock at all.
Test a combined condition one half at a time. Together they only ever tell you yes or no, never which half was wrong.