Domains 5–8 Walkthrough

Learning Goals 5 min

Today we drill the programming-heavy domains: 5 (syntax), 6 (I/O + PWM), 7 (logic / algorithms), 8 (boards). These are 55% of the exam — the biggest single block. By the end you will:

1. Trace small sketches and predict outputs.
2. Recognise common bugs and their fixes.
3. Know which board / function / library to pick for a scenario.

Warm-Up 5 min

Same drill: pen + paper, < 2 min per question.

Sample Questions · Domains 5-8 60 min

Domain 5 · Programming Syntax

int a = 5;
int b = 3;
int c = a / b;
int d = a % b;
Serial.print(c); Serial.print(" "); Serial.println(d);

unsigned int count = -1;

for (int i = 0; i < 5; i++) {
  if (i == 3) continue;
  Serial.print(i);
}

Domain 6 · Inputs / Outputs / PWM

Domain 7 · Programming Logic & Algorithms

int f(int n) {
  int s = 0;
  for (int i = 1; i <= n; i++) s += i;
  return s;
}

Domain 8 · Frameworks & Boards

Basic 15 min

Bench sprint · Domains 5, 6 and 8

Interrupts, PWM pins and variable types all appear in today's questions. Here they are on the bench. The button is wired from D3 to GND. The LED is on D8 with a 220 Ω resistor. Each press should make the LED one step brighter. Ten presses reach full brightness, and it stays there. The count should print once per press.

const int BUTTON_PIN = 3;
const int LED_PIN = 8;

int presses = 0;

void onPress() {
  presses++;
  Serial.println(presses);
  delay(200);
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), onPress, FALLING);
}

void loop() {
  int brightness = presses * 255 / 10;
  analogWrite(LED_PIN, brightness);
}

There are four mistakes. Find them, fix them, and build the circuit to test your fix.

It works if each press brightens the LED a step, it stays full after ten, and prints one number.

Challenge 3 · Fix the traffic-light state machine 10 min

This covers Domain 7. The lights should show red for 5 s, green for 4 s, amber for 1 s, and repeat. Instead, amber never seems to light, and after about half a minute the lights race. There are three mistakes.

enum Light { RED, GREEN, AMBER };

const int RED_PIN = 10;
const int AMBER_PIN = 11;
const int GREEN_PIN = 12;
const unsigned long RED_MS = 5000;
const unsigned long GREEN_MS = 4000;
const unsigned long AMBER_MS = 1000;

Light state = RED;
int lastChange = 0;

void showLight(int pin) {
  digitalWrite(RED_PIN, LOW);
  digitalWrite(AMBER_PIN, LOW);
  digitalWrite(GREEN_PIN, LOW);
  digitalWrite(pin, HIGH);
}

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(AMBER_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
  showLight(RED_PIN);
}

void loop() {
  unsigned long now = millis();
  switch (state) {
    case RED:
      if (now - lastChange >= RED_MS) {
        state = GREEN;
        showLight(GREEN_PIN);
        lastChange = now;
      }
      break;
    case GREEN:
      if (now - lastChange >= GREEN_MS) {
        state = AMBER;
        showLight(AMBER_PIN);
      }
    case AMBER:
      if (now - lastChange >= AMBER_MS) {
        state = RED;
        showLight(RED_PIN);
        lastChange = now;
      }
      break;
  }
}

Trace one full cycle on paper to find the mistakes. Then build three LEDs (with 220 Ω resistors) on D10–D12 and test your fix.

It works if the lights run red 5 s, green 4 s, amber 1 s, correctly, for at least two minutes.

Recap 5 min

Domains 5-8 are the biggest block. Trace sketches mentally. Recall pin maps. Know which library is which. Don't get stuck — flag + move on. Tomorrow: a full mock exam.

Integer division

For ints, / rounds towards zero. Use float division for fractional results.

Modulo (%)

Remainder of integer division. Common for "every Nth iteration" tests.

continue / break

continue = skip rest of this iteration. break = exit the loop entirely.

Interrupt service routine (ISR)

Function called by hardware on an event. Must be brief; can't use delay() or Serial.print() reliably.

millis()

Milliseconds since the program started. Wraps after ~49 days. Non-blocking timing.

State machine

enum + switch pattern. The most common way to structure non-trivial Arduino sketches.

Wire library

I²C; SPI for SPI; Servo for servos; Stepper for steppers. Don't confuse names under exam pressure.

PROGMEM

AVR keyword for storing data in flash instead of RAM. Use with pgm_read_* to read.

Extra Mission 5 min

Part 1 — Design a reaction-time trainer

Domains 5 to 8 meet in one small game. On paper, design a reaction-time trainer. An LED lights after a random wait. The player presses a button as fast as they can.

Your design must include:

• A state diagram: every state, and what moves the game from one to the next.
• How you time the reaction, and which variable types you use.
• What happens if the player presses too early.
• Which pins you use, and why each one suits its job.

Part 2 — Make it

Build it with an enum + switch state machine and millis(), with no delay(). Read the button on an interrupt pin. Print each reaction time in milliseconds, and flag early presses.

Bring back next class: your uploaded sketch, your state diagram and a Serial Monitor screenshot of five rounds, including one early press.