CO2 Monitor Display

#include <Arduino.h>
#include <Wire.h>
#include <SensirionI2CScd4x.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>


// Forward declarations
const char* airQualityLabel(uint16_t co2ppm);
void drawStatus(const char* title, const char* detail);
void drawMeasurement(uint16_t co2ppm, float temperatureC, float humidityRh);

constexpr int I2C_SDA = 21;
constexpr int I2C_SCL = 22;
constexpr uint8_t OLED_ADDRESS = 0x3C;
constexpr int SCREEN_WIDTH = 128;
constexpr int SCREEN_HEIGHT = 64;

Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);
SensirionI2CScd4x scd4x;
uint32_t lastPollMs = 0;

const char* airQualityLabel(uint16_t co2ppm) {
  if (co2ppm <= 800) return "Good";
  if (co2ppm <= 1200) return "Fair - ventilate";
  if (co2ppm <= 2000) return "High - ventilate";
  return "Very high";
}

void drawStatus(const char* title, const char* detail) {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 8);
  display.println(title);
  display.setCursor(0, 28);
  display.println(detail);
  display.display();
}

void drawMeasurement(uint16_t co2ppm, float temperatureC, float humidityRh) {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.print("CO2 MONITOR");
  display.setTextSize(2);
  display.setCursor(0, 14);
  display.print(co2ppm);
  display.setTextSize(1);
  display.print(" ppm");
  display.setCursor(0, 39);
  display.print("Temp: ");
  display.print(temperatureC, 1);
  display.print(" C");
  display.setCursor(0, 50);
  display.print("RH: ");
  display.print(humidityRh, 1);
  display.print("% ");
  display.print(airQualityLabel(co2ppm));
  display.display();
}

void setup() {
  Serial.begin(115200);
  Wire.begin(I2C_SDA, I2C_SCL);
  if (!display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDRESS)) {
    Serial.println("SSD1306 OLED not found at 0x3C");
    while (true) delay(1000);
  }
  drawStatus("CO2 MONITOR", "Starting sensor...");
  scd4x.begin(Wire);
  scd4x.stopPeriodicMeasurement();
  delay(500);
  uint16_t error = scd4x.startPeriodicMeasurement();
  if (error != 0) {
    Serial.print("SCD40 start error: ");
    Serial.println(error);
    drawStatus("SCD40 error", "Check wiring");
    while (true) delay(1000);
  }
  drawStatus("CO2 MONITOR", "Warming up...");
}

void loop() {
  if (millis() - lastPollMs < 1000) return;
  lastPollMs = millis();
  bool dataReady = false;
  uint16_t error = scd4x.getDataReadyFlag(dataReady);
  if (error != 0) {
    Serial.print("SCD40 ready-check error: ");
    Serial.println(error);
    return;
  }
  if (!dataReady) return;
  uint16_t co2ppm = 0;
  int32_t temperatureMilliC = 0;
  int32_t humidityMilliPercent = 0;
  error = scd4x.readMeasurement(co2ppm, temperatureMilliC, humidityMilliPercent);
  if (error != 0) {
    Serial.print("SCD40 read error: ");
    Serial.println(error);
    return;
  }
  if (co2ppm == 0) return;
  float temperatureC = temperatureMilliC / 1000.0f;
  float humidityRh = humidityMilliPercent / 1000.0f;
  Serial.printf("CO2: %u ppm, Temperature: %.1f C, RH: %.1f%%\n", co2ppm, temperatureC, humidityRh);
  drawMeasurement(co2ppm, temperatureC, humidityRh);
}