int led_red = 2;
int led_yellow = 3;
int led_green = 4;
int btn_pin = 12;

const int S_RED = 0,
          S_YELLOW = 1,
          S_GREEN = 2;

int state = S_RED;    // Q: почему красный?

long int t0 = 0;

void setup()
{
  pinMode(led_red, OUTPUT);
  pinMode(led_yellow, OUTPUT);
  pinMode(led_green, OUTPUT);
}

void loop()
{
  long int t = millis();
  int button = digitalRead(btn_pin);

  if (state == S_RED && t - t0 > 2000) {
    state = S_GREEN;
    t0 = t;
  }
  
  if (state == S_GREEN && t - t0 > 2000) {
    state = S_YELLOW;
    t0 = t;
  }

  if (state == S_YELLOW && t - t0 > 700) {
    state = S_RED;
    t0 = t;
  }
  
  digitalWrite(led_green, state == S_GREEN);
  digitalWrite(led_yellow, state == S_YELLOW);
  digitalWrite(led_red, state == S_RED);
}