Parking assistant

/*
  07 — Parking assistant
  Board: ESP32.  Parts: HC-SR04, buzzer module, 3 x 1 kΩ from the resistor pack.

  Wiring:
    HC-SR04: VCC -> VIN (5 V), GND -> GND, TRIG -> GPIO 5
             ECHO -> 1 kΩ -> GPIO 18, then two 1 kΩ in series from GPIO 18 -> GND
    Buzzer:  VCC -> 3V3, GND -> GND, IO -> GPIO 23   (active buzzer: HIGH = sound)

  Distance becomes beep rate: slow far away, faster closing in, solid at the
  stop line. Turning a number into something a person reacts to without a screen.
*/

const int TRIG_PIN   = 5;
const int ECHO_PIN   = 18;
const int BUZZER_PIN = 23;

const float STOP_CM  = 15;    // solid tone at or below this
const float FAR_CM   = 120;   // silent beyond this

float readCm() {
  digitalWrite(TRIG_PIN, LOW);  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH); delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  unsigned long us = pulseIn(ECHO_PIN, HIGH, 30000UL);
  return us == 0 ? -1 : us / 58.0;
}

void setup() {
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
}

void loop() {
  // Average 3 readings — a single bad echo would make the beeps stutter.
  float sum = 0; int n = 0;
  for (int i = 0; i < 3; i++) { float d = readCm(); if (d > 0) { sum += d; n++; } delay(20); }
  float cm = n ? sum / n : -1;
  Serial.println(cm);

  if (cm < 0 || cm > FAR_CM) {            // nothing close: quiet
    digitalWrite(BUZZER_PIN, LOW);
    delay(100);
  } else if (cm <= STOP_CM) {             // stop line: solid
    digitalWrite(BUZZER_PIN, HIGH);
    delay(100);
  } else {                                // beep, faster as it gets closer
    int gap = map((int) cm, (int) STOP_CM, (int) FAR_CM, 60, 600);   // ms between beeps
    digitalWrite(BUZZER_PIN, HIGH); delay(40);
    digitalWrite(BUZZER_PIN, LOW);  delay(gap);
  }
}