Reading the C++

Every sketch has two parts: setup() runs once when the board starts, then loop() runs again and again for ever. delay(ms) waits that many milliseconds, so the delays set the timing of a blink.

MakeCode had 'on start' and 'forever'. An Arduino sketch has the same two parts, written as C++. Which part runs when, and how many times?

MakeCode	Arduino C++
on start	`void setup() { … }`
forever	`void loop() { … }`
pause (ms)	`delay(ms);`

The lines inside { } belong to that part, and each statement ends with ;.

Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Try: Try it on the board and compare the result with your prediction.
Investigate: Work out why it happens, one step at a time.
Modify: Change the problem on purpose and see what happens.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.

WidthMore contexts, same idea.

PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.

How many times?

The blink sketch runs for 10 seconds. How many times does setup() run?Never: only loop() runsTwiceOnceEvery time loop() runs

TryTry it on the board and compare the result with your prediction.

Try: watch the counters

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Upload and run the blink. Watch the counters above the board for a few seconds.

Use the board. This ticks when you get there.

InvestigateWork out why it happens, one step at a time.

Worked example: time it

Worked exampleBefore each step, say what you think comes next.

$\text{setup(): pinMode(13, OUTPUT)}$ Runs once, at 0 ms. Pin 13 is now an output.
$\text{loop(): digitalWrite(13, HIGH)}$ LED on. Writing takes (almost) no time.
$\text{delay(500)}$ Wait 500 ms with the LED on.
$\text{digitalWrite(13, LOW); delay(500)}$ LED off for 500 ms.
$\text{end of loop()} \to \text{start of loop()}$ 1000 ms per blink, for ever.

Now fill in the times.

void setup() {
  pinMode(13, OUTPUT);
  digitalWrite(13, HIGH);
  delay(500);
  digitalWrite(13, LOW);
  delay(500);
}

void loop() {
}

Fill in the board time after each line of loop(), starting at 0 ms.

line	LED	time (ms)
digitalWrite(13, HIGH);	on	0
delay(500);	on
digitalWrite(13, LOW);	off
delay(500);	off

Sam put the blink lines in setup() and left loop() empty (below). What does the LED do?Blinks for everStays onNever lightsBlinks once, then stays off

ModifyChange the problem on purpose and see what happens.

Vary: change the timing

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Make it blink twice as fast.

Use the board. This ticks when you get there.

Now a flash: on for 100 ms, off for 900 ms.

Use the board. This ticks when you get there.

Modify TypicalChange the problem on purpose and see what happens.

Build it in Tinkercad

In Tinkercad: switch Code to Text (it keeps the blocks' C++), change the delays to 100 and 900, and start the simulation. I saw the short flash.

PractiseA few on your own, to check you have it.

Timing

loop() is: HIGH, delay(300), LOW, delay(700). How many blinks in 10 seconds?

Where does pinMode(13, OUTPUT); belong?In loop(): so it keeps workingIn setup(): it only needs doing onceBefore setup(), outside bothIt doesn't matter

MakeMake your own problem using what you have learned. Expect a few tries; that is normal.

A lighthouse

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Need a hint?

The light is on for 200 ms in every 2 seconds.

Another hint

2 s = 2000 ms: off for 2000 − 200 ms.

Another hint

delay(200) after HIGH and delay(1800) after LOW.

A lighthouse flashes for 0.2 s every 2 s. Change the delays to match.

Use the board. This ticks when you get there.

For teachers

Students watch setup() run once and loop() repeat, trace board time through a blink, see why blink code in setup() blinks only once, and change delays to set a blink's timing, including a lighthouse from its timing. About 40 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can say what setup() and loop() each do.
I can work out a blink's timing from its delays.
I can choose delays for a timing.

Before class

The board counts setup() and loop() runs and uses simulated time, so delays are exact (o and f in the checks are the last on and off times in ms).

MakeCode's on start and forever are the same two parts; say so.

Running the lesson

Predict: link to MakeCode's on start.
Worked example: ask what the LED is doing at 750 ms.
Vary: ask which delay is the on time.

The hard part (on purpose)

Students forget that writing a pin takes no time; only delay does. Trace it with a stopwatch in hand.

When students get stuck

A student changes only one delay for 'twice as fast'. Ask: How long is one blink now? Fix: Both delays set the length of a blink.

Stretch and support

Stretch: Make a heartbeat: two quick flashes, then a pause. (Four delays in loop().)

Support: A timeline strip from 0 to 2000 ms to shade on and off.

Where it leads

Next: Values and decisions (variables, analogRead and if).

Sources

Arduino language reference (link) (checked 12 October 2026)

What each step is for

how many times?: Committing to how often setup() runs tests the new move before the counters show it.
Try: watch the counters: The counters settle the prediction: setup() stays at 1 while loop() climbs, one per second.
Worked example: time it: Tracing board time line by line models how delay sets timing; Sam's blink in setup() is planted after the method.
Vary: change the timing: Changing one delay at a time shows each delay sets one part of the blink.
Build it in Tinkercad: In Tinkercad's Text view students edit the C++ directly for the first time and check the timing on a simulated board.
timing: Reading timings from delays, and where code belongs.
a lighthouse: Working from a timing back to the delays reverses the process.

Now you can

Say what setup() and loop() each do.
Work out a blink's timing from its delays.
Choose delays for a timing.

Next lesson3. Values and decisionsA variable is declared with its type, int level = 0;. analogRead(A0) gives 0 to 1023, and if (level > 500) { … } else { … } decides. == compares; a single = sets the variable, even inside an if. Every statement ends with ;, or the sketch won't upload.

Source: content/engineering/arduino/setup-loop/lesson.md