EV3 Robotics›Level 1 · Beginner›Lesson 9
Level 1 · Lesson 9 · EV3-L01-0960 minutes · Ages 9–16 · Model: EV3 Fan
The EV3 Fan: a motor driving a set of blades through a pair of gears. It genuinely moves air — you will feel it on your hand.
The Folding Fan Robot in Lesson 4 waved a fan about. This one spins one, and spinning is where a fan gets serious.
It is also the model that is going to run you into a wall, on purpose. You will want it to blow harder, you will turn the speed up to 100 %, and then there will be nowhere left to go — because 100 % is all there is. What happens next is the lesson.
Every classroom in Malaysia has one on the ceiling, and most houses have a standing fan in the corner as well. In a country this warm, a fan is not a luxury — it is how a room becomes usable in the afternoon.
A fan blade is set at an angle. As it sweeps round it meets the air edge-on and pushes it sideways — out of the way, and therefore forwards. Turn the blade faster and it shifts more air every second.
Notice what a fan does not do: it does not cool the air. The air coming out is exactly as warm as the air going in. It cools you, by carrying the heat and sweat off your skin. That is why a fan in an empty room is doing nothing useful at all.
Here is the engineering problem. A fan wants speed — lots of turns per second — and a motor has a top speed it cannot exceed, no matter what you write in a program.
So real fans do not rely on the motor alone. They are designed around the speed they need: the blade size, the angle and any gearing are all chosen before anyone switches it on. Get that wrong and no amount of fiddling with the controls will fix it.
A program can only ask for what the machine is able to do. If the machine cannot do it, the answer is a different machine.
Two meshed gears trade speed against force. You cannot have more of both — but you get to choose which one you want.
| Arrangement | What you get | Good for |
|---|---|---|
| Small gear driving a big one | Geared down: slower, stronger | Lifting, walking, a barrier arm |
| Big gear driving a small one | Geared up: faster, weaker | A saw blade. A fan. |
You met this with the Gear Shooter in Lesson 5 and again with the Timber Saw in Lesson 13. What is new today is when you reach for it.
The number of teeth is the whole story. 8 teeth driving 24 means 8 ÷ 24 of a turn out for every turn in — and the force changes by exactly the opposite amount.
Count the teeth in the demo and the trade is exact rather than vague: three turns of the driver make one turn of the follower, and the force is trebled. Reverse the pair and you get the deal the other way round.
Your program is going to say set speed to (100) %. That is the largest number the block accepts. There is no 150.
So when somebody asks for a stronger breeze, there is nothing to type. The keyboard is finished. The next move is to take the model apart and put a bigger gear on the motor — and that is a completely different kind of thinking from everything you have done in this course so far.
Before you spend an hour on a program, ask whether the machine can physically do what you are asking. Sometimes the fix is a gear, not a block.
Two meshed gears are a bargain. Whatever one of them gains in turns it loses in force, and whatever it loses in turns it gains in force. There is no arrangement of gears that gives you more of both, and no amount of building will find one.
Count the teeth on the gear you are turning (the driver) and the teeth on the one being turned (the follower). That is the whole calculation:
turns out = turns in × driver teeth ÷ follower teeth
An 8-tooth gear driving a 24 gives ⅓ of a turn out for every turn in — and three times the turning force. Turn the pair round and you get three turns out and a third of the force. Count the turns yourself below.
The number of teeth is the whole story. 8 teeth driving 24 means 8 ÷ 24 of a turn out for every turn in — and the force changes by exactly the opposite amount.
| Arrangement | Turns | Force | What it is for |
|---|---|---|---|
| Gearing down — small driving large | fewer | more | winches, lifts, robot arms, anything that has to move a weight |
| Gearing up — large driving small | more | less | fans, spinners, launchers, wheels on a light fast robot |
| 1 : 1 — same size | same | same | moving the drive to a different axle, or fixing a direction |
The names are worth getting right because they are backwards from what people expect: gearing down makes the output slower, not smaller, and it is the setting that makes a weak motor able to lift things.
Put several pairs in a row and the ratios multiply. Two 1:3 reductions in series give 1:9 — which is how a Medium Motor ends up able to raise something it could never shift directly. This is also how a gearbox with a sensible number of parts reaches a ratio that a single pair never could: a 40-tooth gear driven by an 8 is 1:5, and doing it twice is 1:25.
Only the first and last gear affect the ratio. Anything in the middle passes the motion along and changes nothing but the direction — which is a whole idea of its own, in Changing the direction of a turn.
Every mesh loses a little to friction, so a long train is less efficient than a short one. Gearing down far enough to lift a heavy load also makes the mechanism slow, and slow is not always acceptable. And a gear train that is geared down hard is very hard to turn backwards by hand, which is either a useful brake or a nuisance depending on what you are building.
A bicycle is the same idea with a chain instead of teeth in mesh: the low gear that gets you up a hill is turning the back wheel slowly and pushing hard, and the high gear you use going downhill does the opposite. Cars, drills, cranes and clocks are all making the same trade.
Speed is set separately from movement. You tell the motor how fast it should go, and then you tell it to go — two blocks, in that order.
| Block | What it does |
|---|---|
[A v] set speed to (25) % :: motors | Sets the speed for this motor from now on. Nothing moves — it only changes what the next movement will do. |
[A v] run [clockwise v] for (2) [rotations v] :: motors | Now moves, at whatever speed was last set. |
when program starts :: events hat [A v] set speed to (25) % :: motors [A v] run [clockwise v] for (2) [rotations v] :: motors
Swap those two blocks round and the program still contains a speed of 25 % — it just never gets used. Both shafts below are asked for exactly 2 rotations; watch how long each one takes.
speed first — works
speed last — does nothing
Both shafts turn exactly 2 rotations. Only the time they take is different — and the right-hand program never gets the slow movement it was written to have.
The right-hand movement is over before the left is a third of the way round, because it ran at the default speed. Its set speed to () block does run — you can see it light up — but by then the movement it was meant to slow down has already happened. A speed block only ever affects the movements after it. This catches people out constantly.
A high speed is not a better program. Slow movements are gentler on the gears, easier to watch and debug, and look more like the real machine — a barrier that snaps up in a fraction of a second reads as broken rather than fast.
Say this back before moving on: “A big gear driving a small gear makes the small one spin faster, and push less hard.”
Two electronic parts, and a gear pair that is really the third.
| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | Runs the program and holds the whole thing down. A spinning fan pushes back, and the Brick’s weight is what stops the model walking across the table. |
| Medium Motor | Drives the gear train. The fast one of the pair, which is the right choice for something that only has to move air. |
| The gear pair (no cable) | Sets how fast the blades turn for each turn of the motor. This is the part you are going to change. |
Keep this number. Everything in the last two sections is a comparison against it, and you cannot compare against a number you never wrote down.
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 |
|---|---|---|
| Medium Motor (drives the blades) | A | Unusually, the manual itself says so — its last step is captioned “Connect Medium Motor to Port A”. Most of the manuals in this course say nothing about ports, so when one does, follow it. |
| Sensors | none | The fan has no idea whether anyone is in the room. |
Check your own build now:
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.
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.
EV3 until somebody changes it.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.
Route the cable behind the model. You are about to build a machine whose entire job is to blow loose things around, and a USB lead drifting into the blades will stop the fan and possibly the lesson.
The Brick can only count the motor. Everything the gears do to that movement is invisible to the program — which is exactly why gearing is a build decision and not a programming one.
Five blocks, and there is deliberately nothing clever in them. The program is as good as it is going to get on the very first try.
when program starts :: events hat [A v] set speed to (100) % :: motors [A v] start motor [clockwise v] :: motors wait (10) seconds [A v] stop motor :: motors
Walk it in the order the Brick runs it:
What success looks like: a breeze you can feel at about thirty centimetres, a steady note from the gears, and no wandering across the table.
If the blades turn the wrong way — you will know, because the air pulls towards the fan instead of pushing away — change clockwise to counterclockwise. A fan blade set at an angle only works in one direction; the other way it just stirs.
The first two changes are in the program. The rest are not — and noticing where that line falls is the point of the section.
Step 3 is the whole lesson in one instruction. The program did not change by a single character and the machine got better, because the limit was never in the program.
“Turn it up” runs out. “Rebuild it” does not.
Build the model before you read any further. Everything after this is about making it do something, and none of it will make much sense with nothing on the table in front of you.
Use the viewer's own controls to zoom and turn pages. Fullscreen makes it big enough to build from.
The same build on Google Drive — sometimes a video, sometimes a scan:
Use the viewer's own controls to zoom and turn pages. Fullscreen makes it big enough to build from.
Check the finished build against the picture before you switch anything on. A motor mounted the wrong way round is far easier to spot now than it is to debug later, when it looks like a program fault.
Work through the challenges in order — each is harder than the last. The mission comes after all three, and it is meant to make you plan before you build.
Measure what you have. Count the teeth on both gears, work out how many times the blades turn for one turn of the motor, and check your answer by turning the drum by hand ten times and counting. Report the predicted ratio and the measured one, and explain any difference.
Make it blow harder without touching the program. Rebuild the gear train so the blades spin faster, run the identical program, and prove the improvement with something better than an opinion — a strip of paper at a fixed distance, or how far away you can still feel it.
Find the point where gearing up stops helping. Keep increasing the ratio and testing until more gearing makes the fan worse rather than better, then say what has gone wrong. Somewhere past a certain ratio the motor can no longer get the blades up to speed at all.
Design a fan for a job, and defend the gear ratio you chose. Pick one: a fan that must blow a paper ball across a desk, or a fan that must keep going for five minutes on one battery charge. Then build the gearing that suits it. These want opposite answers, and that is the point. Plan on paper before you rebuild. Decide what you are optimising for, predict which way the gearing should go, and write that prediction down before you test it. A prediction you make afterwards is not a prediction. Two questions when you demonstrate it. Why is your ratio right for your job and wrong for the other one? And what did you have to give up to get what you wanted, in your own words?
