Stoplight timer example using setInterval

stoplight_setInterval.js

/* Timer example using setInterval */

var light_state = 0;

function setup() {
  createCanvas(600, 600);
  noStroke();
  
  //change stoplight state every 1.5 seconds
  setInterval(change_state, 1500); 
}

function draw() {
  background(255);
  
  //draw back panel of stoplight
  fill(100);
  rect(width/2-50, 50, 100, 200, 5);
  
  stoplight(light_state);
}

function change_state() {
  //Increment light_state by one. Go back to 0 after 2.
  light_state = (light_state + 1) % 3;
}

function stoplight(s) {
  var y;
  switch(light_state) {
    case 0:
      fill(0, 255, 0);
      y = 100;
      break;
    case 1:
      fill(255, 255, 0);
      y = 150;
      break;
    case 2:
      fill(255, 0, 0);
      y = 200;
      break;
    default:
      break;
  }
  
  ellipse(width/2, y, 50, 50);
}