What is a finite state machine in Arduino?

A way to organise a program as a set of states, only one active at a time. A variable holds the current state, a switch statement runs that state's code on each pass of loop(), and each state checks its own conditions to decide when to move to another.

Why use a state machine instead of delay()?

A state that waits checks a timer on each pass instead of stopping the program, so loop() keeps running and every sensor is read all the time. The robot can react to a bump or a lost target in the middle of any step.

How do I draw a state machine?

Draw a circle for each state and an arrow for each way out, labelled with the condition that takes it: a reading crossing a line, a timer running out, a button press. Then each circle becomes a case, and each arrow an if inside it.

Why not write the grab as stop, delay(500), then drive home?

For half a second nothing else is read: not RANGE, not the eye, nothing

A case forgets its break. After its own lines, the program…

Runs straight on into the next case's lines too

Why keep the mode as a number instead of separate true/false flags?

One number can only be one mode at a time, so modes can't clash

Arduino state machines with switch case, for robots

A state machine organises a robot's program into states, like SEARCH, APPROACH and GRAB, with only one active at a time. A variable holds the current state; on every pass of loop(), a switch runs that state's code, and each state checks its own conditions to decide when to move to another. Because nothing ever waits, every sensor is read on every pass.

States and arrows

Draw it before coding it. Each state is a circle: what the robot does while it's there. Each way out is an arrow, labelled with what triggers it. A fetch robot:

State	Does	Leaves when	Goes to
SEARCH	spins on the spot	range under 600	APPROACH
APPROACH	drives forward	range under 150	GRAB
APPROACH	drives forward	range over 800	SEARCH
GRAB	stops, closes the gripper	500 ms passed	CARRY
CARRY	drives home	eye sees the tape	DONE
DONE	lets go

Arrows can point backwards. If the target slips away during APPROACH, the range jumps over 800 and the robot goes back to searching. Leave that arrow out and it drives on at nothing.

In code

enum State { SEARCH, APPROACH, GRAB, CARRY, DONE };
State state = SEARCH;
unsigned long since = 0;

void loop() {
  int range = analogRead(A0);   // read everything first
  switch (state) {
    case SEARCH:
      spin();
      if (range < 600) state = APPROACH;
      break;
    case APPROACH:
      forward();
      if (range < 150) { state = GRAB; since = millis(); }
      else if (range > 800) state = SEARCH;
      break;
    case GRAB:
      stopAndGrip();
      if (millis() - since >= 500) state = CARRY;
      break;
    // CARRY and DONE follow the same pattern
  }
}

Live experiment

A fetch robot in five states

SEARCH spins until something is ahead, APPROACH drives up to it, GRAB holds on for 500 ms, CARRY drives home to the tape and DONE lets go. In the second scene the target slips away: only an APPROACH with an arrow back to SEARCH recovers.

Try it yourself: free lesson

“A brain in states” is lesson 6 of 6 in Waypoint Wharf, Robotics World 3. Free with an account. Lessons unlock in order, so you’ll do the 19 before it first.

Timers without waiting

GRAB needs half a second. delay(500) would make the robot blind for that half second: no range, no eye, nothing. Instead, note the time once, on the way in, and check it on each pass. Set since inside the GRAB case and it's always "now", so the state never ends.

Why one number

You could track the robot's behaviour with a handful of true or false flags, searching, approaching, gripping. But then two can be true at once, and the code has to untangle them. A single state variable can only ever hold one state, so the robot is always doing exactly one thing. Traffic lights, vending machines, washing machines and games are all built the same way, and so are robots that have to do more than one thing in order.

How to program a robot as a state machine

States and arrows

In code

A fetch robot in five states

Timers without waiting

Why one number

Now build it yourself

Practice questions

Questions people ask

What is a finite state machine in Arduino?

Why use a state machine instead of delay()?

How do I draw a state machine?

States and arrows

In code

A fetch robot in five states

Timers without waiting

Why one number

Now build it yourself

Practice questions

Questions people ask

What is a finite state machine in Arduino?

Why use a state machine instead of delay()?

How do I draw a state machine?

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