Water tank level monitor

Install first: Adafruit SSD1306, Adafruit GFX Library

#include <WiFi.h>
#include <WebServer.h>
#include <Wire.h>
#include <Adafruit_SSD1306.h>

const char* WIFI_NAME = "your-wifi";
const char* WIFI_PASS = "your-password";

// ---- Your tank ----
const float EMPTY_CM = 120;   // lid to tank floor
const float FULL_CM  = 15;    // lid to overflow level
const int   LITRES   = 1000;
const int   LOW_PCT = 15, HIGH_PCT = 95;

const int BUZZER_PIN = 26, BTN_PIN = 27;
HardwareSerial sonar(2);
Adafruit_SSD1306 oled(128, 64, &Wire, -1);
WebServer server(80);
int percent = -1; bool muted = false; int lastAlarmState = 0;

float readDistanceCm() {                       // A02YYUW frame: 0xFF H L SUM
  while (sonar.available() >= 4) {
    if (sonar.read() != 0xFF) continue;
    uint8_t h = sonar.read(), l = sonar.read(), sum = sonar.read();
    if (((0xFF + h + l) & 0xFF) == sum) return (h * 256 + l) / 10.0;
  }
  return -1;
}

void draw() {
  oled.clearDisplay(); oled.setTextColor(SSD1306_WHITE);
  oled.setTextSize(1); oled.setCursor(0, 0); oled.print("Water tank");
  oled.setTextSize(3); oled.setCursor(0, 14); oled.print(percent); oled.print("%");
  oled.drawRect(100, 10, 24, 50, SSD1306_WHITE);
  int h = 48 * percent / 100; oled.fillRect(102, 59 - h, 20, h, SSD1306_WHITE);
  oled.setTextSize(1); oled.setCursor(0, 50); oled.printf("%d litres", LITRES * percent / 100);
  oled.display();
}

void setup() {
  pinMode(BUZZER_PIN, OUTPUT); pinMode(BTN_PIN, INPUT_PULLUP);
  sonar.begin(9600, SERIAL_8N1, 16, 17);
  Wire.begin(21, 22);
  oled.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  WiFi.begin(WIFI_NAME, WIFI_PASS);
  server.on("/", [] { server.send(200, "text/plain", String(percent) + "% (" + String(LITRES * percent / 100) + " L)"); });
  server.begin();
}

void loop() {
  server.handleClient();
  float d = readDistanceCm();
  if (d > 0) {
    percent = constrain((int)((EMPTY_CM - d) * 100 / (EMPTY_CM - FULL_CM)), 0, 100);
    draw();
  }
  int alarm = percent >= 0 && percent < LOW_PCT ? 1 : percent > HIGH_PCT ? 2 : 0;
  if (alarm != lastAlarmState) { muted = false; lastAlarmState = alarm; }
  if (digitalRead(BTN_PIN) == LOW) muted = true;
  digitalWrite(BUZZER_PIN, alarm && !muted && (millis() / 500) % 2);
  delay(100);
}