Ultrasonic tape measure

/*
  06 — Ultrasonic tape measure
  Board: ESP32.  Parts: HC-SR04, LCD 1602 (I2C backpack), 3 x 1 kΩ from the resistor pack.
  Library: "LiquidCrystal I2C" (Frank de Brabander).

  Wiring:
    HC-SR04: VCC -> VIN (5 V), GND -> GND, TRIG -> GPIO 5
             ECHO -> 1 kΩ -> GPIO 18, and from GPIO 18 two 1 kΩ in series -> GND
             (ECHO outputs 5 V; the divider gives the ESP32 3.3 V. Do not skip it.)
    LCD:     VCC -> VIN (5 V), GND -> GND, SDA -> GPIO 21, SCL -> GPIO 22

  Distance = (echo time in microseconds) / 58. Sound goes there and back, so
  the time is halved, and at ~343 m/s that works out to 58 us per centimetre.
*/

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int TRIG_PIN = 5;
const int ECHO_PIN = 18;
LiquidCrystal_I2C lcd(0x27, 16, 2);

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);   // 30 ms timeout ~ 5 m
  if (us == 0) return -1;                                 // nothing in range
  return us / 58.0;
}

void setup() {
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  lcd.init();
  lcd.backlight();
  lcd.print("Tape measure");
}

void loop() {
  // Average 5 readings so one bad echo does not make the number jump.
  float sum = 0; int n = 0;
  for (int i = 0; i < 5; i++) {
    float d = readCm();
    if (d > 0) { sum += d; n++; }
    delay(30);
  }
  lcd.setCursor(0, 1);
  if (n == 0) {
    lcd.print("out of range    ");
    Serial.println("out of range");
  } else {
    float cm = sum / n;
    lcd.print(cm, 1); lcd.print(" cm         ");
    Serial.print(cm, 1); Serial.println(" cm");
  }
  delay(150);
}