State machine design for Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Arduino state machine

Consider this Arduino code snippet implementing a simple state machine with three states. What will be printed to the Serial Monitor after 5 iterations of loop()?

Arduino

enum State {IDLE, RUNNING, STOPPED};
State currentState = IDLE;

void setup() {
  Serial.begin(9600);
}

void loop() {
  switch(currentState) {
    case IDLE:
      Serial.println("Idle state");
      currentState = RUNNING;
      break;
    case RUNNING:
      Serial.println("Running state");
      currentState = STOPPED;
      break;
    case STOPPED:
      Serial.println("Stopped state");
      currentState = IDLE;
      break;
  }
  delay(10); // simulate some delay
}

A Stopped state\nIdle state\nRunning state\nStopped state\nIdle state

B Idle state\nIdle state\nIdle state\nIdle state\nIdle state

C Running state\nStopped state\nIdle state\nRunning state\nStopped state

D Idle state\nRunning state\nStopped state\nIdle state\nRunning state

💡 Hint

Look at how the currentState changes after each loop() call.

The state machine cycles through IDLE, RUNNING, and STOPPED states in order. Each loop() call prints the current state and moves to the next. After 5 iterations, the output matches option D.

🧠 Conceptual

intermediate

Understanding state transitions in Arduino

In an Arduino state machine, what is the main purpose of using a switch statement inside the loop() function?

A To select and execute code based on the current state of the machine

B To pause the program until a button is pressed

C To initialize hardware components only once

D To declare global variables for the program

💡 Hint

Think about how the program decides what to do next depending on the state.

The switch statement checks the current state and runs the code specific to that state, enabling the state machine to behave differently depending on its state.

🔧 Debug

advanced

Identify the error in this Arduino state machine code

What error will this Arduino code produce when compiled?

Arduino

enum State {START, PROCESS, END};
State currentState = START;

void setup() {
  Serial.begin(9600);
}

void loop() {
  switch(currentState) {
    case START:
      Serial.println("Start");
      currentState = PROCESS;
      break;
    case PROCESS:
      Serial.println("Process");
      currentState = END;
      break;
    case END:
      Serial.println("End");
      currentState = START;
      break;
  }
  delay(1000);
}

A SyntaxError: missing semicolon after break in case START

B TypeError: cannot assign enum to variable

C RuntimeError: infinite loop without delay

D No error, code compiles and runs correctly

💡 Hint

Check the syntax carefully after each break statement.

The break statement in the START case is missing a semicolon, causing a syntax error during compilation.

🚀 Application

advanced

Number of states in a traffic light Arduino state machine

You design a traffic light controller with states: RED, RED_YELLOW, GREEN, and YELLOW. How many states should the enum have to represent this system?

A 2

B 3

C 4

D 5

💡 Hint

Count each unique light combination as one state.

Each unique light combination is a state. Since there are four combinations, the enum should have 4 states.

❓ Predict Output

expert

Output of nested state machine with timing

What will be the output on the Serial Monitor after running this Arduino code for 3 iterations of loop()?

Arduino

enum State {WAIT, ACTION};
State currentState = WAIT;
int counter = 0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  switch(currentState) {
    case WAIT:
      Serial.print("Waiting ");
      Serial.println(counter);
      if (counter >= 2) {
        currentState = ACTION;
      } else {
        counter++;
      }
      break;
    case ACTION:
      Serial.println("Action executed");
      currentState = WAIT;
      counter = 0;
      break;
  }
  delay(10);
}

A Waiting 0\nWaiting 1\nWaiting 2\nAction executed

B Waiting 0\nWaiting 1\nWaiting 2

C Waiting 0\nWaiting 1\nAction executed

D Action executed\nWaiting 0\nWaiting 1

💡 Hint

Trace the value of counter and state changes carefully.

On the first loop, counter is 0 and state is WAIT, prints "Waiting 0" and increments counter to 1.
Second loop prints "Waiting 1" and increments counter to 2.
Third loop prints "Waiting 2" because counter is 2 or more, then sets currentState to ACTION.