traffic_light.ino in Arduino

A junction light: enum phases, timed transitions, no delay anywhere.

// A junction traffic light: state machine, no delay in sight.

const int RED_PIN = 4;
const int AMBER_PIN = 5;
const int GREEN_PIN = 6;

enum Phase { GO, CAUTION, STOP_ALL };
Phase phase = STOP_ALL;
unsigned long phaseStart = 0;

// How long each phase holds, in milliseconds.
unsigned long phaseLength(Phase p) {
  switch (p) {
    case GO:       return 6000;
    case CAUTION:  return 1500;
    case STOP_ALL: return 5000;
  }
  return 1000;
}

Phase nextPhase(Phase p) {
  switch (p) {
    case GO:       return CAUTION;
    case CAUTION:  return STOP_ALL;
    case STOP_ALL: return GO;
  }
  return STOP_ALL;
}

void showPhase(Phase p) {
  digitalWrite(RED_PIN, p == STOP_ALL ? HIGH : LOW);
  digitalWrite(AMBER_PIN, p == CAUTION ? HIGH : LOW);
  digitalWrite(GREEN_PIN, p == GO ? HIGH : LOW);
}

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(AMBER_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
  showPhase(phase);
  phaseStart = millis();
}

void loop() {
  if (millis() - phaseStart >= phaseLength(phase)) {
    phase = nextPhase(phase);
    showPhase(phase);
    phaseStart = millis();
  }
}

How it works

1. phaseLength and nextPhase make the cycle a pair of tables.
2. showPhase maps the state onto the three pins in one place.
3. The loop is four lines: check the clock, advance, repaint.

Keywords and builtins used here

• HIGH
• LOW
• OUTPUT
• Phase
• case
• const
• digitalWrite
• enum
• if
• int
• long
• loop
• millis
• nextPhase
• phaseLength
• pinMode
• return
• setup
• showPhase
• switch
• unsigned
• void

The run, in numbers

Lines

50

Characters to type

1036

Tokens

240

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 222 seconds.

Type this snippet

Step 1 of 3 in Encore, step 25 of 27 in Language basics.

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