AI Smart Scale

#include <Arduino.h>
#include <Wire.h>
#include <WiFi.h>
#include <WebServer.h>
#include <Preferences.h>
#include <HX711.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define HX711_DT_PIN 32
#define HX711_SCK_PIN 33
#define TARE_BUTTON_PIN 4
#define OLED_SDA_PIN 21
#define OLED_SCL_PIN 22

constexpr uint8_t SCREEN_WIDTH = 128;
constexpr uint8_t SCREEN_HEIGHT = 64;
constexpr uint8_t OLED_ADDRESS = 0x3C;
constexpr char AP_NAME[] = "SmartScale";
constexpr char AP_PASSWORD[] = "scale-setup";

HX711 scale;
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);
WebServer server(80);
Preferences preferences;

float calibrationFactor = 0.0f;
float shownGrams = NAN;
bool calibrationMode = true;
unsigned long lastSampleMs = 0;
unsigned long lastButtonMs = 0;

void drawScreen(float grams, bool force) {
  if (!force && ((isnan(grams) && isnan(shownGrams)) || (!isnan(grams) && !isnan(shownGrams) && fabsf(grams - shownGrams) < 1.0f))) {
    return;
  }
  shownGrams = grams;
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.print("AI SMART SCALE");
  display.drawLine(0, 11, 127, 11, SSD1306_WHITE);
  display.setTextSize(3);
  display.setCursor(0, 22);
  if (calibrationMode) {
    display.print("CAL");
  } else {
    display.print(grams, 0);
    display.setTextSize(1);
    display.print(" g");
  }
  display.setTextSize(1);
  display.setCursor(0, 54);
  if (calibrationMode) {
    display.print("Open 192.168.4.1 to calibrate");
  } else {
    display.print("Button: tare");
  }
  display.display();
}

String pageHtml() {
  String body = "<!doctype html><html><head><meta name='viewport' content='width=device-width,initial-scale=1'>";
  body += "<style>body{font-family:sans-serif;max-width:34rem;margin:2rem auto;padding:1rem}input,button{font-size:1.1rem;padding:.5rem;margin:.3rem 0;width:100%}</style></head><body>";
  body += "<h1>Smart Scale</h1>";
  if (calibrationMode) {
    body += "<p>The scale needs calibration. Put a known weight on the platform, enter its mass in grams, then press Calibrate.</p>";
  } else {
    body += "<p>Live weight: <b>" + String(scale.get_units(5), 0) + " g</b></p>";
  }
  body += "<form action='/calibrate'><label>Known weight in grams</label><input name='g' type='number' min='1' required><button>Calibrate</button></form>";
  body += "<p>Remove everything first, then use the physical button to tare at any time.</p></body></html>";
  return body;
}

void handleHome() {
  server.send(200, "text/html", pageHtml());
}

void handleCalibrate() {
  if (!server.hasArg("g")) {
    server.send(400, "text/plain", "Missing g parameter");
    return;
  }
  const float knownGrams = server.arg("g").toFloat();
  if (knownGrams <= 0.0f) {
    server.send(400, "text/plain", "Enter a mass greater than zero");
    return;
  }
  const long rawValue = scale.get_value(20);
  if (rawValue == 0) {
    server.send(409, "text/plain", "No usable sensor reading. Check the load-cell wires.");
    return;
  }
  calibrationFactor = static_cast<float>(rawValue) / knownGrams;
  scale.set_scale(calibrationFactor);
  preferences.putFloat("factor", calibrationFactor);
  calibrationMode = false;
  shownGrams = NAN;
  drawScreen(scale.get_units(10), true);
  server.sendHeader("Location", "/");
  server.send(303);
}

void tareScale() {
  scale.tare(20);
  shownGrams = NAN;
  drawScreen(calibrationMode ? NAN : scale.get_units(5), true);
}

void setup() {
  Serial.begin(115200);
  pinMode(TARE_BUTTON_PIN, INPUT_PULLUP);
  Wire.begin(OLED_SDA_PIN, OLED_SCL_PIN);
  display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDRESS);
  display.clearDisplay();
  display.display();

  scale.begin(HX711_DT_PIN, HX711_SCK_PIN);
  scale.set_scale(1.0f);
  delay(300);
  tareScale();

  preferences.begin("smartscale", false);
  calibrationFactor = preferences.getFloat("factor", 0.0f);
  if (calibrationFactor != 0.0f) {
    scale.set_scale(calibrationFactor);
    calibrationMode = false;
  }

  WiFi.mode(WIFI_AP);
  WiFi.softAP(AP_NAME, AP_PASSWORD);
  server.on("/", HTTP_GET, handleHome);
  server.on("/calibrate", HTTP_GET, handleCalibrate);
  server.begin();
  drawScreen(calibrationMode ? NAN : scale.get_units(10), true);
}

void loop() {
  server.handleClient();
  const unsigned long now = millis();

  if (digitalRead(TARE_BUTTON_PIN) == LOW && now - lastButtonMs > 350) {
    lastButtonMs = now;
    tareScale();
  }

  if (now - lastSampleMs >= 250) {
    lastSampleMs = now;
    if (!calibrationMode && scale.is_ready()) {
      drawScreen(scale.get_units(5), false);
    }
  }
  delay(10);
}