Measuring a Person: The Stop Watch

What you are building 3 min

The Stop Watch: a Touch Sensor with a lever over it, so a whole hand can slap it rather than a fingertip. Press once to start the clock, press again to stop it.

No motor. Eighteen pages, one sensor, one cable — the shortest build in the course, and the second model with nothing that moves under power, after the Fencing sword.

You already met the Brick’s clock on the Stone Grinder in Lesson 20, where it measured how long the machine ran. But the machine chose how long to run, so the clock was really just checking the program’s own homework.

Today the clock measures something the program knows nothing about and cannot control: a person.

In the real world 5 min

Where you have seen it

Round the neck of every PE teacher, and at the end of every swimming lane. A stopwatch does one thing, and it has done it since long before anything was electronic.

Why it is built that way

Notice what a stopwatch does not have: a way of knowing when the race started. It has a button, and a human presses it.

That is the whole design. The instrument is very good at the one thing a person is bad at — counting hundredths of a second — and it leaves the one thing a person is good at, deciding when the race began, to the person.

Serious athletics has since taken the human out of the loop: a starting pistol wired to the clock, and pressure pads in the blocks. They did that because of what you are about to measure.

What would go wrong without it

A person cannot react instantly. Between seeing something and your finger moving, roughly two tenths of a second goes by while the signal travels to your brain and back out to your hand.

In a hundred-metre sprint decided by hundredths, a hand-timed result is hopeless — the timekeeper’s reaction is bigger than the gap between first and second. That is not carelessness. It is the speed of a human being.

Every measurement made by a person carries that person inside it. The first job of an instrument is to find out how much.

The main concept — the clock as an instrument 6 min

Same two blocks as Lesson 20. Completely different job.

The blocks are unchanged: reset timer to start counting, and timer to read the seconds since. What has changed is that the number is now information — nobody in the room knows what it will say until it says it.

And notice the shape of the program. It is: wait for a person → start counting → wait for a person → read it. The two waits are the two ends of the measurement, and the robot does nothing at all in between except count.

Bumped, not pressed. A stopwatch must not start and stop from one slap of the lever. Use bumped — down and up — so each complete click is one event, exactly as on the Timber Saw in Lesson 16.

A clock that measures the program is a check. A clock that measures the world is an instrument.

▶The Timer, and the Touch SensorThe full reference for the Brick's clock and for the switch. Open either if it is not clear yet.Show meHide

ComponentControl4 min

The Timer

A wait pauses for a length of time. The timer is different: it runs in the background and can be read at any moment, so the robot can know how long something has taken while it is still happening.

Blocks reference

Block	What it does
(timer)	Reports the seconds since the timer was last reset.
reset timer	Sets it back to zero, so the next reading counts from here.

The timeout — a safety net

The most valuable use of a timer is escaping a wait that might never end. A robot told to drive until it sees a wall will drive for ever if the wall is not there. Combined with a timer, it can give up:

Repeat until the wall is close or five seconds have passed. That one change turns a program that can hang into one that always finishes. Both robots below are looking for a wall that is not there.

no way out

repeat until distance < 15

  start moving straight: 0

with a timeout

reset timer

repeat until distance < 15 or timer > 5

  start moving straight: 0

write GAVE UP at line 1

1.2stimer212cm · distance

Both robots are told to drive until something is within 15 cm. The room ahead is empty.Three seconds. No wall. Both are still driving — and the right-hand program is also watching its timer.The timer passes 5. The right-hand robot gives up, stops, and says so.The left robot is still going. Its condition can never become true, so that block will hold the program for ever.The right-hand program finished. The left one has not, and there is nothing to say why.

▶ Play⏭ Step↺ Restarttimer 1.2 s

The sensor is not faulty and the program is not wrong. There is simply no wall, and only one of these two programs has a way of noticing that.

The left-hand robot is not broken, and neither is its sensor. Its condition is simply one that will never come true, so the program sits on that block for ever — with nothing on the Brick to say so. The right-hand program asks the same question with an escape route bolted on, and finishes every time.

Why it matters

Real systems time themselves out constantly — a lift that cannot close its doors eventually gives up and beeps rather than trying for ever. A robot with no timeout simply stops responding, and there is nothing on screen to say why.

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: “Reset the timer when it starts, read the timer when it stops.”

What’s in this build 4 min

One part with a cable, and that is the whole electronic parts list.

Make the lever good, because it is part of the measurement

1. Slap the lever with the flat of your hand. Does it press the sensor every time, or does it sometimes miss?
2. Does it spring back on its own, ready for the next press? A lever that stays down means the second click never registers.
3. Is there slack — a bit of travel before the sensor actually feels it? Slack is delay, and delay goes straight into your readings.
4. Try it ten times fast. Ten presses should give ten clicks. If you get nine, fix the lever before you write a program, because no program can recover a press the sensor never felt.

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.

The simplest port table in the whole course, and worth a look for that reason alone.

Check your own build now:

1. Sensor cable in port 1 — a number, not a letter.
2. Can you see the screen while your hand is on the lever? You are about to read a number the instant you press, so both have to be in view at once.
3. Is the model steady on the table? A stopwatch that slides away when you slap it will cost you a tenth of a second every time.
4. Volume up — you will add a sound later.

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.

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⤢ Tap to enlarge

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⤢ 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.

Either is fine — the model does not go anywhere. Just make sure the cable does not stop the whole thing sliding about when somebody hits the lever hard.

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. Slap the lever and watch the tile flip. Do it fast, ten times, and count how many register. All ten, or the lever needs work.
4. Rest your hand on the lever and hold it. The tile stays pressed — which is exactly why the program needs bumped rather than pressed.

Make it move 10 min

Seven blocks, and nothing moves — as with the Fencing sword, this is “make it work”. Click to start, click to stop, and the elapsed time appears.

when program starts :: events hat
clear display :: display
write [PRESS TO START] at line (1) :: display
[1 v] wait until [bumped v] :: sensors
reset timer
[1 v] wait until [bumped v] :: sensors
write (timer) at line (3) :: display

Walk it in the order the Brick runs it:

1. clear display — start blank so nothing from the last run is left on screen.
2. write PRESS TO START at line 1 — the instrument tells the user what to do. Line 1 is the title, and it will stay there all run, exactly as on the fire escape panel in Lesson 15.
3. wait until bumped — nothing happens until a person decides. This is the start of the race.
4. reset timer — the clock goes to zero. It must be immediately after the press, or you are timing the wait as well.
5. wait until bumped — the second click. The Brick is counting the whole time.
6. write (timer) at line 3 — the answer, on its own line below the title.

What success looks like: click, wait a few seconds, click again, and a believable number appears. Test it against something you can check: count “one elephant, two elephant” up to ten and see whether the Brick agrees with you.

If it stops the instant you start it, you have used pressed instead of bumped somewhere. Your hand is still on the lever when the program reaches the second wait, so it is satisfied immediately — the Timber Saw problem, in a new place.

Change it and test 8 min

Now use the instrument on yourselves. Write every reading down — you need several to say anything useful.

1. Try to stop the watch at exactly 5.00 seconds. Five attempts, all five written down. How close can you get, and are you consistently early or consistently late?
2. Now the fastest possible: click, then click again as fast as you physically can. Five goes. That number — a couple of tenths, probably — is roughly how long a person takes to do anything at all.
3. Add a play beep (72) for (0.2) seconds right after reset timer, so the watch says when it started. Does hearing the start make you more accurate at step 1?
4. Have somebody else press stop while you say “now”. Compare their times with your own from step 2. A measurement of somebody else is always slower than a measurement of yourself.
5. Now go back to the very first thing you measured in step 1 and ask: was the watch wrong, or were you? Both readings came from the same instrument, and only one of them had a person in it twice.

Step 2 is the number to keep. Everything a person times by hand carries that couple of tenths at each end — and now you have measured your own rather than being told it.

The clock is not the slow part. You are, and knowing by how much is what makes you able to trust a measurement.