The Mood Machine

BRIEF Build a robot with moods

Machines follow rules. But what if the rule is a roll of the dice? Today you build a robot with three moods, calm, curious, alarmed, that drifts between them unpredictably, yet with unmistakable personality. One robot might be chilled and hard to startle; another might be a nervous wreck. The difference is nine numbers you choose. Those nine numbers are called a Markov chain, and by the end of this mission you'll understand a real piece of the mathematics behind weather forecasts, Google's original search ranking, and the AI chatbots everyone's talking about.

Time: 50 minutes · Difficulty: 2/5 · You need to know: how to wire an LED and upload a sketch (covered in Beginner Kit Lessons 01–02, but the wiring table below is enough to follow along).

KIT What you need

• Arduino-compatible R3 board with USB cable
• Breadboard and jumper wires
• 3 LEDs + 3 resistors (220–330Ω)
• Everything above is in the GAMERIFT Beginner Kit, or available individually at GAMERIFT

BUILD Wire it

USB unplugged while you wire. Three LEDs, each with its resistor bodyguard, sharing one ground rail:

From	To	Meaning
R3 GND	Blue (−) rail	Shared ground for all three
Pin 11	Resistor → LED long leg	CALM (green if you have it)
Pin 12	Resistor → LED long leg	CURIOUS (yellow)
Pin 13	Resistor → LED long leg	ALARMED (red)
Each LED short leg	Blue (−) rail	Three loops complete

CODE Program it

// GAMERIFT free mission, The Mood Machine
// A three-state Markov chain with an attitude.

const int calmLED = 11, curiousLED = 12, alarmedLED = 13;

// The personality: each row is a mood, each number a % chance.
// Row order: CALM, CURIOUS, ALARMED. Rows must add up to 100.
int personality[3][3] = {
  { 70, 25,  5 },   // from CALM:    mostly stays calm
  { 40, 40, 20 },   // from CURIOUS: could go either way
  { 10, 10, 80 }    // from ALARMED: hard to calm down
};

int mood = 0;   // 0 = calm, 1 = curious, 2 = alarmed, start calm

void setup() {
  pinMode(calmLED, OUTPUT);
  pinMode(curiousLED, OUTPUT);
  pinMode(alarmedLED, OUTPUT);
  randomSeed(analogRead(A0));   // real randomness: electrical noise from an empty pin
}

void loop() {
  // 1. show the current mood (only the matching LED burns)
  digitalWrite(calmLED,    mood == 0 ? HIGH : LOW);
  digitalWrite(curiousLED, mood == 1 ? HIGH : LOW);
  digitalWrite(alarmedLED, mood == 2 ? HIGH : LOW);
  delay(1000);   // hold each mood for a second

  // 2. roll the dice (0–99) and let the current row decide what's next
  int roll = random(100);
  if      (roll < personality[mood][0])                          mood = 0;
  else if (roll < personality[mood][0] + personality[mood][1])   mood = 1;
  else                                                           mood = 2;
}

(The ? : lines are shorthand for if/else: "if the mood matches, HIGH, otherwise LOW.") Upload it and just… watch for a minute. Long calm stretches. A flicker of curiosity. And occasionally, a panic that takes ages to settle, because row three makes alarm sticky. Nobody scripted that behaviour. It emerged from nine numbers and a dice roll. That feeling of watching something decide is why this maths conquered the world.

BREAK IT Now break it on purpose

1. Make every number 33 (near enough, rows of 34, 33, 33). Upload. The personality vanishes: pure coin-flip chaos, no character at all. Personality lives in the imbalance.
2. Make ALARMED inescapable: set its row to 0, 0, 100. Sooner or later the robot panics, and never, ever recovers. Congratulations: you've built what mathematicians genuinely call an absorbing state. (Also known, in this house, as a robot sulk.)
3. Make a mood unreachable: set the middle column to 0 in every row. Curious never lights again, a state no path leads to is as good as deleted. Real network engineers hunt exactly this kind of dead node.

REMIX Make it yours

• Design a character: tune the nine numbers until the robot is unmistakably brave, nervous, or sleepy. Then the real test: can your family guess the personality just by watching it?
• Speed as mood: make alarmed moods hold for only 300ms and calm ones for 2 seconds, suddenly the rhythm carries personality too.
• Four moods: add a fourth LED and grow the table to 4×4 (sixteen numbers, rows still summing to 100). More states, richer character, you're scaling the exact axis that separates your robot from a chatbot.