Echo Sounding and Sonar

/*
  Echo sounder — time-of-flight depth measurement
  Youblob blueprint: Echo Sounding and Sonar

  Drives a 40 kHz piezo transducer, listens for the return, converts the delay
  into a distance using a sound speed you can set for the medium.

  Hardware:
    Transducer drive  -> D9 (through a driver transistor / H-bridge)
    Receiver envelope -> A0 (piezo -> amp -> rectifier -> RC envelope)
    Blanking gate     -> D8 (holds the receiver off while the transducer rings)

  NOTE the blanking window. The transducer keeps ringing after the drive stops
  and would swamp its own receiver — the acoustic twin of the radar TR cell,
  and it sets the MINIMUM range exactly the same way.
*/

const int PIN_DRIVE = 9;
const int PIN_ECHO  = A0;
const int PIN_BLANK = 8;

float SOUND_SPEED = 1500.0;      // m/s  (water; use 343.0 to test in air)
const int  BLANK_US     = 900;   // ring-down blanking -> sets minimum range
const int  BURST_CYCLES = 8;     // cycles at 40 kHz
const int  THRESHOLD    = 120;   // envelope ADC counts

void setup() {
  Serial.begin(115200);
  pinMode(PIN_DRIVE, OUTPUT);
  pinMode(PIN_BLANK, OUTPUT);
  Serial.println(F("t_us,range_m"));
  Serial.print(F("# minimum range from blanking: "));
  Serial.print(SOUND_SPEED * (BLANK_US / 1e6) / 2.0, 3);
  Serial.println(F(" m"));
}

unsigned long pingOnce() {
  digitalWrite(PIN_BLANK, HIGH);                  // deafen the receiver
  for (int i = 0; i < BURST_CYCLES; i++) {        // 40 kHz burst = 12.5 us half-period
    digitalWrite(PIN_DRIVE, HIGH); delayMicroseconds(12);
    digitalWrite(PIN_DRIVE, LOW);  delayMicroseconds(12);
  }
  unsigned long t0 = micros();
  delayMicroseconds(BLANK_US);                    // wait out the ring-down
  digitalWrite(PIN_BLANK, LOW);                   // receiver live

  while (micros() - t0 < 60000UL) {               // 60 ms listen window
    if (analogRead(PIN_ECHO) > THRESHOLD) return micros() - t0;
  }
  return 0;                                       // nothing heard
}

void loop() {
  // Median of three pings rejects a single spurious return without slowing much.
  unsigned long a = pingOnce(); delay(60);
  unsigned long b = pingOnce(); delay(60);
  unsigned long c = pingOnce();
  unsigned long v[3] = {a, b, c};
  for (int i = 0; i < 2; i++)
    for (int j = i + 1; j < 3; j++)
      if (v[j] < v[i]) { unsigned long s = v[i]; v[i] = v[j]; v[j] = s; }
  unsigned long t = v[1];

  if (t == 0) { Serial.println(F("0,no-echo")); }
  else {
    float range = SOUND_SPEED * (t / 1e6) / 2.0;  // out and back -> halve it
    Serial.print(t); Serial.print(','); Serial.println(range, 3);
  }
  delay(250);
}