State machine design for Arduino

Introduction

A state machine helps your Arduino program handle different situations clearly and in order. It makes your code easier to read and control.

When your Arduino needs to do different things based on button presses.

When you want to control a device with multiple steps, like a traffic light.

When you want to manage modes, like sleep mode and active mode.

When you want to avoid messy if-else code and keep things organized.

Syntax

Arduino

enum State {
  STATE1,
  STATE2,
  STATE3
};

State currentState = STATE1;

void loop() {
  switch (currentState) {
    case STATE1:
      // do something
      if (condition) currentState = STATE2;
      break;
    case STATE2:
      // do something else
      if (condition) currentState = STATE3;
      break;
    case STATE3:
      // do another thing
      if (condition) currentState = STATE1;
      break;
  }
}

Use enum to name your states clearly.

Use a switch statement inside loop() to handle each state.

Examples

Simple two-state machine to toggle an LED on and off with a button.

Arduino

enum State { OFF, ON };
State currentState = OFF;

void loop() {
  switch (currentState) {
    case OFF:
      // turn LED off
      if (buttonPressed) currentState = ON;
      break;
    case ON:
      // turn LED on
      if (buttonPressed) currentState = OFF;
      break;
  }
}

Traffic light example cycling through states with timing.

Arduino

enum State { RED, GREEN, YELLOW };
State currentState = RED;

unsigned long lastChange = 0;

void loop() {
  unsigned long now = millis();
  switch (currentState) {
    case RED:
      if (now - lastChange > 5000) {
        currentState = GREEN;
        lastChange = now;
      }
      break;
    case GREEN:
      if (now - lastChange > 5000) {
        currentState = YELLOW;
        lastChange = now;
      }
      break;
    case YELLOW:
      if (now - lastChange > 2000) {
        currentState = RED;
        lastChange = now;
      }
      break;
  }
}

Sample Program

This program uses a state machine to blink the built-in LED on pin 13. It starts in IDLE state. When you send the letter 'b' over the serial monitor, it starts blinking the LED on and off every 500 milliseconds. The serial prints show the current state.

Arduino

#include <Arduino.h>

enum State {
  IDLE,
  BLINK_ON,
  BLINK_OFF
};

State currentState = IDLE;
const int ledPin = 13;
unsigned long lastChange = 0;
const unsigned long interval = 500;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  unsigned long now = millis();
  switch (currentState) {
    case IDLE:
      digitalWrite(ledPin, LOW);
      Serial.println("State: IDLE");
      if (Serial.available()) {
        char c = Serial.read();
        if (c == 'b') {
          currentState = BLINK_ON;
          lastChange = now;
        }
      }
      break;

    case BLINK_ON:
      digitalWrite(ledPin, HIGH);
      Serial.println("State: BLINK_ON");
      if (now - lastChange >= interval) {
        currentState = BLINK_OFF;
        lastChange = now;
      }
      break;

    case BLINK_OFF:
      digitalWrite(ledPin, LOW);
      Serial.println("State: BLINK_OFF");
      if (now - lastChange >= interval) {
        currentState = BLINK_ON;
        lastChange = now;
      }
      break;
  }
}

Important Notes

Use millis() for timing instead of delay() to keep the program responsive.

Keep states simple and focused on one task each.

Use descriptive names for states to make your code easier to understand.

Summary

A state machine helps organize your Arduino program into clear steps.

Use enum and switch to manage states easily.

State machines make your code easier to read, debug, and extend.

Practice

1. What is the main purpose of using a state machine in an Arduino program?

easy

A. To organize the program into clear, manageable steps

B. To increase the speed of the Arduino processor

C. To reduce the power consumption automatically

D. To connect multiple Arduino boards together

Solution

1. Step 1: Understand the concept of state machines

   A state machine breaks a program into distinct states or steps to manage behavior clearly.

2. Step 2: Identify the main benefit in Arduino context

   This organization helps make the code easier to read, debug, and extend.

3. Final Answer:

   To organize the program into clear, manageable steps -> Option A

4. Quick Check:

   State machine purpose = organize steps [OK]

Hint: State machines organize code into clear steps [OK]

Common Mistakes:

• Thinking state machines speed up the processor
• Confusing power saving with state management
• Assuming state machines connect multiple boards

2. Which of the following is the correct way to declare states in an Arduino state machine using enum?

easy

A. enum State {START, RUNNING, STOPPED};

B. enum State = {START, RUNNING, STOPPED};

C. enum State {START; RUNNING; STOPPED};

D. enum State (START, RUNNING, STOPPED);

Solution

1. Step 1: Recall enum syntax in Arduino (C++)

   The correct syntax uses curly braces and commas to list states without assignment or semicolons.

2. Step 2: Check each option for syntax errors

   enum State {START, RUNNING, STOPPED}; uses correct syntax: enum State {START, RUNNING, STOPPED}; Options B, C, and D have incorrect assignment, semicolons, or parentheses.

3. Final Answer:

   enum State {START, RUNNING, STOPPED}; -> Option A

4. Quick Check:

   Correct enum syntax = enum State {START, RUNNING, STOPPED}; [OK]

Hint: Use curly braces and commas for enum states [OK]

Common Mistakes:

• Using '=' sign after enum name
• Separating states with semicolons instead of commas
• Using parentheses instead of braces

3. What will be the output on the Serial Monitor when running this Arduino code snippet?

enum State {IDLE, PROCESS, DONE};
State currentState = IDLE;

void setup() {
  Serial.begin(9600);
  switch(currentState) {
    case IDLE:
      Serial.println("Waiting");
      break;
    case PROCESS:
      Serial.println("Working");
      break;
    case DONE:
      Serial.println("Finished");
      break;
  }
}

medium

A. Finished

B. Working

C. Waiting

D. No output

Solution

1. Step 1: Identify the initial state value

   The variable currentState is set to IDLE before setup() runs.

2. Step 2: Follow the switch-case logic

   Switch checks currentState; since it is IDLE, it prints "Waiting" to Serial Monitor.

3. Final Answer:

   Waiting -> Option C

4. Quick Check:

   State IDLE prints "Waiting" [OK]

Hint: Check initial state value to predict output [OK]

Common Mistakes:

• Assuming PROCESS or DONE prints without state change
• Thinking setup() runs multiple times
• Ignoring break statements in switch

4. Identify the error in this Arduino state machine code snippet:

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

void loop() {
  switch(currentState) {
    case START:
      Serial.println("Start");
      currentState = RUNNING
      break;
    case RUNNING:
      Serial.println("Running");
      break;
    case STOPPED:
      Serial.println("Stopped");
      break;
  }
}

medium

A. Incorrect enum declaration

B. Serial not initialized in setup()

C. Missing break in RUNNING case

D. Missing semicolon after currentState = RUNNING

Solution

1. Step 1: Check syntax line by line

   Look at the line: currentState = RUNNING without a semicolon at the end.

2. Step 2: Identify the error type

   Missing semicolon causes a compile error. Other parts are correct or not critical for this snippet.

3. Final Answer:

   Missing semicolon after currentState = RUNNING -> Option D

4. Quick Check:

   Missing semicolon = syntax error [OK]

Hint: Check each statement ends with a semicolon [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Assuming enum declaration is wrong
• Forgetting to initialize Serial in setup()

5. You want to design a state machine for an Arduino traffic light with states: RED, GREEN, YELLOW. Which approach correctly updates the state to cycle through RED -> GREEN -> YELLOW -> RED inside loop()?

hard

A. Use a while loop inside loop() to cycle states infinitely

B. Use a switch on currentState and set currentState to the next state in each case

C. Use if statements comparing currentState to integers 0,1,2 and increment currentState by 1

D. Use delay() to wait and then reset currentState to RED always

Solution

1. Step 1: Understand state machine cycling

   Using a switch-case on currentState allows clear, readable transitions between states.

2. Step 2: Evaluate each option's suitability

   Use a switch on currentState and set currentState to the next state in each case correctly updates currentState to the next state inside each case. Use if statements comparing currentState to integers 0,1,2 and increment currentState by 1 uses integers but lacks clarity and safe cycling. Use a while loop inside loop() to cycle states infinitely with while loop inside loop() blocks Arduino's main loop. Use delay() to wait and then reset currentState to RED always resets state always to RED, not cycling properly.

3. Final Answer:

   Use a switch on currentState and set currentState to the next state in each case -> Option B

4. Quick Check:

   Switch-case for cycling states = Use a switch on currentState and set currentState to the next state in each case [OK]

Hint: Use switch-case to update states clearly [OK]

Common Mistakes:

• Using while loops inside loop() causing freezes
• Resetting state without cycling
• Using integers without clear state names