Zombie tripwire on an Arduino Uno

// Zombie tripwire: an HC-SR04 across a corridor, a traffic light, a TM1637
// and a piezo siren, on an Uno.
//
//   HC-SR04   TRIG -> pin 3, ECHO -> pin 2
//   traffic light   green -> A0, yellow -> A1, red -> A2
//   TM1637    CLK -> A3, DIO -> A4
//   piezo     pin 10 and GND
//
// The sensor looks across the corridor. Anything closer than 120 cm is a
// walker: it is counted once, however long it stands there. The traffic light
// shows how busy the last 20 seconds were:
//
//   0 or 1 walker    green   quiet
//   2 or 3 walkers   yellow  walkers about
//   4 or more        red     HORDE, and the siren sounds
//
// The TM1637 shows the total since power-up.
//
// # Counting a thing once: edge detection with hysteresis
//
// Asking "is something there?" 15 times a second would count one walker 15
// times a second. What matters is the moment the answer changes from no to
// yes: an edge. `blocked` remembers the last answer, and the walker is counted
// only on the way from false to true.
//
// A walker at exactly 120 cm would flicker in and out of the beam, so the
// beam is two thresholds, not one: it trips under 120 cm but lets go only
// past 140 cm. That gap is hysteresis, and it is why a thermostat does not
// click on and off at the set temperature either.
//
// # The last 20 seconds
//
// Eight timestamps are kept in a ring: a new crossing overwrites the oldest.
// "How many in the last 20 seconds" is a count of the entries younger than
// that, so old walkers fade out on their own and the light drops back to
// green with nobody resetting it.

#include <TM1637Display.h>

const uint8_t TRIG = 3;
const uint8_t ECHO = 2;
const uint8_t GREEN_LED = A0;
const uint8_t YELLOW_LED = A1;
const uint8_t RED_LED = A2;
const uint8_t PIEZO = 10;

const int TRIP_CM = 120;
const int CLEAR_CM = 140;
const unsigned long WINDOW_MS = 20000UL;
const int RING = 8;

TM1637Display display(A3, A4);

unsigned long seen[RING];
int next = 0;
int total = 0;
bool blocked = false;
int threat = -1;  // 0 green, 1 yellow, 2 red; -1 so the first pass prints
unsigned long lastScan = 0;
unsigned long lastSiren = 0;
bool sirenHigh = false;

// Centimeters to the nearest thing, or 0 if no echo came back.
int measureCm() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  unsigned long us = pulseIn(ECHO, HIGH, 25000UL);
  return (int)(us / 58);
}

int recent(unsigned long now) {
  int n = 0;
  for (int i = 0; i < RING; i++) {
    if (seen[i] != 0 && now - seen[i] < WINDOW_MS) n++;
  }
  return n;
}

void setLight(int level) {
  digitalWrite(GREEN_LED, level == 0);
  digitalWrite(YELLOW_LED, level == 1);
  digitalWrite(RED_LED, level == 2);
}

void setup() {
  Serial.begin(115200);
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(GREEN_LED, OUTPUT);
  pinMode(YELLOW_LED, OUTPUT);
  pinMode(RED_LED, OUTPUT);
  pinMode(PIEZO, OUTPUT);
  display.setBrightness(0x0f);
  display.showNumberDec(0);
  Serial.println("zombie tripwire armed");
}

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

  if (now - lastScan >= 70) {
    lastScan = now;
    int cm = measureCm();
    bool inBeam = cm > 0 && cm < TRIP_CM;
    bool outOfBeam = cm == 0 || cm > CLEAR_CM;
    if (inBeam && !blocked) {
      blocked = true;
      total++;
      seen[next] = now;
      next = (next + 1) % RING;
      display.showNumberDec(total);
      Serial.print("walker #");
      Serial.print(total);
      Serial.print(" at ");
      Serial.print(cm);
      Serial.println(" cm");
    } else if (outOfBeam) {
      blocked = false;
    }
  }

  int n = recent(now);
  int level = n >= 4 ? 2 : (n >= 2 ? 1 : 0);
  if (level != threat) {
    threat = level;
    setLight(level);
    Serial.println(level == 2 ? "threat: HORDE" : (level == 1 ? "threat: walkers about" : "threat: quiet"));
    if (level != 2) noTone(PIEZO);
  }

  // the siren: two notes, swapped every 300 ms, without stopping the loop
  if (level == 2 && now - lastSiren >= 300) {
    lastSiren = now;
    sirenHigh = !sirenHigh;
    tone(PIEZO, sirenHigh ? 960 : 640);
  }
}