EV3 Robotics›Level 1 · Beginner›Lesson 37
Level 1 · Lesson 37 · EV3-L01-3760 minutes · Ages 9–16 · Model: Rubber Band Gun 2
The Rubber Band Gun: a stretched rubber band held on a big gear, a motor driving that gear through a worm, and a Touch Sensor for a trigger. Press it and the gear turns just far enough to let the band go.
You have stored energy in rubber before, with the Elastic Car in Lesson 14. That one you wound up with your hand and let go immediately. This one has to hold the stretch — sitting there, loaded, with the motor switched off and nothing else stopping it.
That is the interesting problem. A stretched band pulls back hard enough to spin most gear trains straight round. This one does not budge, and today you find out why.
Every guitar has six of them. The tuning pegs on the headstock are worm drives: you turn the little key and the string tightens, and then the string — pulling with real force — cannot turn the key back.
A worm is a screw with a gear wrapped around it. As the screw turns, its thread pushes the wheel’s teeth along, one tooth per turn. So one full turn of the worm moves the wheel by a single tooth — a huge reduction from one small part.
And here is the useful bit. The thread meets the teeth at such a shallow angle that pushing on the wheel just jams the thread harder into itself. The friction wins. The wheel cannot turn the worm, no matter how hard you push.
Think about a lift, or a crane, or a car’s steering. All three hold something heavy in place. If the load could turn the motor backwards, then the moment the power went off, the lift would drop and the crane would swing.
A worm drive means the machine holds where it is, with no power, no brake and no program running at all. That is not a convenience — for some machines it is the entire safety design.
A worm drive is a one-way street. Power goes in through the worm and cannot come back out through the wheel.
This model has no step-by-step manual, and the video below is not one. It is a film of a finished Rubber Band Gun working. Watch it for the target — especially for how the worm sits against the big gear — then build from the model your teacher has, or from a photograph of a finished one. You are not missing a page; the page does not exist.
Two gears that mesh normally can be driven from either side. A worm and a wheel cannot.
The reduction is enormous, and you can count it. A LEGO worm advances the wheel by one tooth per turn. Meshed with a 24-tooth gear, that is 24 turns of the motor for one turn of the wheel — which is why your gun winds so slowly and holds so firmly.
| Gear pair | Can the output drive the input? |
|---|---|
| Two ordinary gears (Lessons 5, 13, 18) | Yes. Turn either one and the other turns. |
| A worm and a wheel | No. The worm drives the wheel; the wheel cannot drive the worm. |
Slow, very strong, and it holds by itself. Those three come together, and a worm is how you get all of them at once.
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.
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| 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. |
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.
versus two hat blocks
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:
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.
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: “The worm can turn the wheel. The wheel cannot turn the worm.”
Three parts with cables — and you have to tell us which motor it is, because this model has no written parts list.
Large Motor
Medium Motor| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | Runs the program and holds the frame steady. |
| One motor — you decide which | Turns the worm. Port A, and every program below says A. |
| Touch Sensor | The trigger. Port 1, as always. |
| The worm and the big gear (no cable) | The one-way drive. This is the part today’s lesson is about. |
Brick off, and no rubber band on it yet. This takes a minute and it is the most convincing minute in the lesson.
That is the whole lesson in your hands. Nothing in the program holds the rubber band. The mechanism does, and it does it with the power off.
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 |
|---|---|---|
| The motor (turns the worm) | A | A single working motor takes A. The programs say A rather than naming a motor type, because the port is what the Brick actually knows. |
| Touch Sensor (the trigger) | 1 | Touch is always port 1 in this course. |
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.
Load the band last. Get the Brick connected and the port tiles checked before you stretch anything. A loaded gun sitting on the desk while you fiddle with Bluetooth is a band that goes off when somebody nudges the table.
Four blocks. It is the shortest program in the course, and it is short because the mechanism is doing the hard part.
when program starts :: events hat [A v] set speed to (30) % :: motors [1 v] wait until [bumped v] :: sensors [A v] run [clockwise v] for (2) [rotations v] :: motors
Walk it in the order the Brick runs it:
What success looks like: the machine sits loaded and silent, a click fires it, and the gear stops dead the moment the motor block finishes — no coasting, no unwinding, because the worm will not let it.
If two rotations is not enough to release the band, go up in ones and find your own number. If it is far too much and the gear whips round past the release point, come down. Every build hooks the band at a slightly different tooth.
One change at a time, and predict before each run. Firing line clear on every single run.
Step 2 is the one people argue about. Turn the motor up and the band goes exactly as far as before, because the motor never pushed the band at all — it only got out of the way.
Find out what is actually doing the work before you try to make a machine better. Here it is your arm, not the motor.
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.
Find the smallest release. Work down from two rotations one at a time until the band no longer fires, then go back up one. Report the number, and then prove it is reliable: five loads and five fires in a row with no adjustment between them.
Make it reload itself. After firing, the mechanism must wind back to exactly the position where a fresh band can be hooked on, so the operator only has to fit the band and press. Show three complete cycles without anyone repositioning anything by hand.
Prove the worm holds, with evidence rather than assertion. Load the band, switch the Brick off entirely, and leave the machine for two minutes. It must still be loaded. Then explain in one sentence what is holding it, and why an ordinary pair of gears could not.
Build a machine that is safe to leave loaded. Your gun must be able to sit loaded and unattended on a table without firing, must fire only when the trigger is deliberately clicked, and must give a clear warning before it fires so nobody is surprised by it. Somebody walking past and knocking the table must not set it off. Plan on paper before you build. List every way this machine could fire when nobody meant it to — a knock, a nudge, a program restart, the Brick being switched on — and decide what stops each one. Two questions when you demonstrate it. What holds the band when there is no power at all, and how do you know? And which of your safety measures is in the program, and which is in the mechanism — and which of the two would you trust more?