Block signals on an Arduino Uno

// Block signals: two three-aspect signals following a train through two blocks.
//
//   pin 8   detector at the entrance to block A   (a beam sensor, HIGH while a car breaks it)
//   pin 7   detector at the entrance to block B
//   pin 6   detector at the exit of block B
//   pins 5, 4   signal 1 at the entrance to A: the red and green legs of a bicolor LED
//   pins 3, 2   signal 2 at the entrance to B, the same
//
// Each signal is one bicolor LED, the way many modelers build a searchlight
// signal: red, green, or both legs lit together for yellow.
//
// Automatic block signaling, the simple one-direction kind. Each signal guards
// the block in front of it:
//   red     that block is occupied: stop
//   yellow  that block is clear but the next one is not: approach, ready to stop
//   green   the next two blocks are clear
//
// Real layouts usually detect occupancy by current: a detector sees the small
// current a locomotive motor or a resistor wheelset draws anywhere in the block.
// Mokxi does not model track current, so these are spot detectors at the block
// boundaries, and the sketch remembers which block the train is in: a block
// becomes occupied when the train trips its entrance detector, and clears once
// the train has fully passed the next detector. Past the exit detector is block
// C, beyond this panel, which the sketch counts as occupied for C_CLEARS_MS.
//
// Click a detector's card into its slot to put a car in the beam, and out again.

const int DETECTOR_PIN[3] = {8, 7, 6};
const int SIGNAL_PINS[2][2] = {{5, 4}, {3, 2}};  // red leg, green leg
const unsigned long QUIET_MS = 1500;     // a detector clear this long: the train has passed
const unsigned long C_CLEARS_MS = 8000;  // how long block C stays occupied after the exit

bool occupied[3];  // blocks A, B and C
bool sawTrain[3];  // detector i has a train over it, or had one recently
unsigned long lastSeen[3];
unsigned long cEnteredAt = 0;
int shown[2] = {-1, -1};
const char *const ASPECTS[] = {"red", "yellow", "green"};
const char BLOCK_NAMES[] = "ABC";

void enter(int block) {
  if (occupied[block]) return;
  occupied[block] = true;
  Serial.print("block ");
  Serial.print(BLOCK_NAMES[block]);
  Serial.println(" occupied");
}

void leave(int block) {
  if (!occupied[block]) return;
  occupied[block] = false;
  Serial.print("block ");
  Serial.print(BLOCK_NAMES[block]);
  Serial.println(" clear");
}

void setSignal(int s, int aspect) {
  if (shown[s] == aspect) return;
  shown[s] = aspect;
  // red lights the red leg, green the green one, yellow both
  digitalWrite(SIGNAL_PINS[s][0], aspect != 2 ? HIGH : LOW);
  digitalWrite(SIGNAL_PINS[s][1], aspect != 0 ? HIGH : LOW);
  Serial.print("signal ");
  Serial.print(s + 1);
  Serial.print(": ");
  Serial.println(ASPECTS[aspect]);
}

void setup() {
  Serial.begin(9600);
  for (int i = 0; i < 3; i++) pinMode(DETECTOR_PIN[i], INPUT);
  for (int s = 0; s < 2; s++) {
    for (int i = 0; i < 2; i++) pinMode(SIGNAL_PINS[s][i], OUTPUT);
  }
  Serial.println("Block signals ready: blocks A and B, then C beyond the panel");
}

void loop() {
  unsigned long now = millis();

  for (int d = 0; d < 3; d++) {
    if (digitalRead(DETECTOR_PIN[d]) == HIGH) {
      if (!sawTrain[d]) enter(d);  // the train is at the entrance to block d
      sawTrain[d] = true;
      lastSeen[d] = now;
      if (d == 2) cEnteredAt = now;
    } else if (sawTrain[d] && now - lastSeen[d] >= QUIET_MS) {
      // The whole train has passed detector d, so it has left the block behind it.
      sawTrain[d] = false;
      if (d > 0) leave(d - 1);
    }
  }
  if (occupied[2] && !sawTrain[2] && now - cEnteredAt >= C_CLEARS_MS) leave(2);

  // Each signal looks at the block it guards and the one after.
  for (int s = 0; s < 2; s++) {
    if (occupied[s]) setSignal(s, 0);
    else if (occupied[s + 1]) setSignal(s, 1);
    else setSignal(s, 2);
  }
  delay(10);
}