Taming Complexity: A Guide to Implementing Finite State Machines on Arduino Uno 

 June 22, 2023

By  Peter

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Ever find yourself wrangling with code, trapped in a spaghetti mess of if-else statements? Yeah, it’s a rough spot. But hey, don’t sweat it. There’s a neat trick to keep your code clean, structured, and easy to debug. It’s called a Finite State Machine, and today, we’ll take a deep dive into how to implement one in your Arduino projects. Buckle up!

So, What’s a Finite State Machine Anyway?

You can think of a Finite State Machine (FSM) as a way to manage the different situations or “states” your Arduino might find itself in. Picture this: your Arduino is a superhero, with each power representing a different state. Our hero can’t use all their powers at once, right? They switch between them based on the situation, just like an FSM.

An FSM consists of states, transitions, and actions. ‘States’ are the situations or conditions your Arduino might be in. ‘Transitions’ are the triggers that move your Arduino from one state to another. Lastly, ‘actions’ are the tasks or operations executed in each state.

Why Finite State Machines are a Game Changer

Imagine a traffic light. It’s simple: green means go, red means stop, and yellow means… well, depending on where you are, it could mean speed up or slow down! In coding terms, each light colour is a state, and time is the transition between these states.

You could write an Arduino program for a traffic light using a lot of if-else statements. But as the complexity of your project increases, the code can become harder to manage and debug. That’s where FSMs come in.

By using FSMs, you’re making your code more structured, modular, and maintainable. It’s easier to add, remove, or change states. Plus, debugging is a breeze since you can isolate issues to specific states.

Example uses of Finite State Machines

In the industry, FSMs are like Swiss Army knives, handy for many tasks. They’re used across fields, from telecommunications to software development and from control systems to game design. Let’s get into some real-world examples.

Vending Machines

You’ve probably used a vending machine to grab snacks or soda. Under the hood, these machines use an FSM to manage the sequence of operations. Each state represents a step in the process, like waiting for the input, validating the payment, dispensing the product, or giving change. The transitions are triggered by user actions or the completion of tasks. This makes the design of the machine straightforward, and troubleshooting becomes simpler.

Traffic Light Control Systems

Traffic lights are a classic example of FSMs in action. Each light colour (red, yellow, green) represents a state, and the timer represents the transition. The system stays in each state for a set amount of time before transitioning to the next. If the system is linked to a sensor, the transition could also be triggered by the presence of vehicles or pedestrians. FSMs make it easier to manage and coordinate traffic light systems.

Telecommunication Systems

Ever wondered how a phone call works? Well, FSMs play a key role here. A telecommunication system uses FSMs to manage call processes like dialling, ringing, talking, and hanging up. Each of these is a state in the FSM. For example, when you dial a number, the system moves from the “idle” state to the “dialling” state. Once the other person picks up, it transitions to the “talking” state. FSMs make these systems more efficient and reliable.

Video Game Development

If you’re into video games, you’ve enjoyed the magic of FSMs without knowing it. Game characters often use FSMs to manage their behaviours. Each state might represent an action like idle, attack, defend, or retreat. Transitions are triggered by in-game conditions, like the player’s actions or the character’s health level. FSMs help to create dynamic and engaging gameplay.

Industrial Automation Systems

In industrial automation, FSMs are used to control the sequence of production processes. For example, in an assembly line, each state could represent a stage in the manufacturing process, like fitting parts, welding, or painting. The transition to the next state could be triggered by completing a task or by sensors. FSMs make it easier to design, control, and troubleshoot these systems.

In a nutshell, wherever a system with distinct states and conditions triggers a transition from one state to another, you will likely find a Finite State Machine at work. And with their knack for taming complex processes, FSMs aren’t going anywhere soon.

Breaking Down a Finite State Machine on Arduino Uno

Let’s create a simple example to grasp the concept better. We’ll build a small system to control an LED connected to an Arduino Uno. This system will have two states: ON and OFF.

In the ON state, the LED will light up. In the OFF state, it’ll be turned off. We’ll use a button to switch between these states. When the button is pressed, the system will move from OFF to ON, and vice versa.

Here’s a simple code snippet to implement this FSM:

enum State {OFF, ON};  // Define the states

State state = OFF;  // Start in the OFF state

void setup() {
  pinMode(2, INPUT);

void loop() {
  int buttonState = digitalRead(2);

  switch (state) {
    case OFF:
      digitalWrite(LED_BUILTIN, LOW);
      if (buttonState == HIGH) {
        state = ON;
    case ON:
      digitalWrite(LED_BUILTIN, HIGH);
      if (buttonState == HIGH) {
        state = OFF;

In this example, the enum keyword is used to define the states. The switch statement then controls what happens in each state. The system moves to the next state when the button is pressed (a transition).

FSM Libraries to Boost Your Arduino Coding

Implementing an FSM from scratch is simple, as shown in our example. But FSM libraries are available for more complex systems, saving you a lot of time. One such library is the Arduino Finite State Machine Library by Jon Black.

This library simplifies the process of creating and managing FSMs. It provides an easy way to define states, transitions, and actions. You can also add entry and exit actions for each state performed when entering or leaving a state.

Finite State Machines are a powerful tool for structuring and managing your Arduino code. They provide a clear framework for defining states and transitions, making your code readable, maintainable, and debuggable. So, next time you find yourself tangled in a web of if-else statements, remember FSMs. They might just be the superhero you need.


Arduino programming, finite state machines

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