Indoor Air Quality Monitor

#include <Wire.h>
#include <U8g2lib.h>
#include <Adafruit_BME280.h>
#include <DFRobot_SGP40.h>

const uint8_t I2C_SDA_PIN = 4;
const uint8_t I2C_SCL_PIN = 5;
const uint8_t OLED_RESET_PIN = 6;
const unsigned long UPDATE_INTERVAL_MS = 2000;

U8G2_SSD1306_128X64_NONAME_F_HW_I2C display(U8G2_R0, OLED_RESET_PIN, U8X8_PIN_NONE);
Adafruit_BME280 bme;
DFRobot_SGP40 sgp40(&Wire);

bool bmeReady = false;
bool sgp40Ready = false;
unsigned long lastUpdateMs = 0;

void drawReadings(float temperatureC, float humidity, float pressureHpa, uint16_t vocIndex) {
  char line[24];

  display.clearBuffer();
  display.setFont(u8g2_font_6x12_tf);
  display.drawStr(0, 11, "Indoor air monitor");
  display.drawHLine(0, 14, 128);

  snprintf(line, sizeof(line), "Temp: %.1f C", temperatureC);
  display.drawStr(0, 28, line);
  snprintf(line, sizeof(line), "Humidity: %.0f %%", humidity);
  display.drawStr(0, 40, line);
  snprintf(line, sizeof(line), "Pressure: %.0f hPa", pressureHpa);
  display.drawStr(0, 52, line);
  snprintf(line, sizeof(line), "VOC index: %u", vocIndex);
  display.drawStr(0, 64, line);
  display.sendBuffer();
}

void drawError(const char *message) {
  display.clearBuffer();
  display.setFont(u8g2_font_6x12_tf);
  display.drawStr(0, 14, "Indoor air monitor");
  display.drawHLine(0, 17, 128);
  display.drawStr(0, 36, message);
  display.drawStr(0, 52, "Check wiring and power.");
  display.sendBuffer();
}

void setup() {
  Serial.begin(115200);
  delay(100);

  Wire.setSDA(I2C_SDA_PIN);
  Wire.setSCL(I2C_SCL_PIN);
  Wire.begin();

  display.begin();
  drawError("Starting sensors...");

  bmeReady = bme.begin(0x76, &Wire);
  sgp40Ready = sgp40.begin(10000);

  if (!bmeReady) {
    Serial.println("BME280 was not found at address 0x76.");
    drawError("BME280 not found");
  } else if (!sgp40Ready) {
    Serial.println("SGP40 did not start.");
    drawError("SGP40 not found");
  } else {
    Serial.println("Air monitor ready.");
  }
}

void loop() {
  const unsigned long now = millis();
  if (now - lastUpdateMs < UPDATE_INTERVAL_MS) {
    delay(10);
    return;
  }
  lastUpdateMs = now;

  if (!bmeReady || !sgp40Ready) {
    delay(10);
    return;
  }

  const float temperatureC = bme.readTemperature();
  const float humidity = bme.readHumidity();
  const float pressureHpa = bme.readPressure() / 100.0F;

  if (isnan(temperatureC) || isnan(humidity) || isnan(pressureHpa)) {
    Serial.println("BME280 read failed.");
    drawError("BME280 read failed");
    delay(10);
    return;
  }

  sgp40.setRhT((uint16_t)humidity, (int16_t)temperatureC);
  const uint16_t vocIndex = sgp40.getVoclndex();

  Serial.print("Temperature: ");
  Serial.print(temperatureC, 1);
  Serial.print(" C, humidity: ");
  Serial.print(humidity, 0);
  Serial.print(" %, pressure: ");
  Serial.print(pressureHpa, 0);
  Serial.print(" hPa, VOC index: ");
  Serial.println(vocIndex);

  drawReadings(temperatureC, humidity, pressureHpa, vocIndex);
  delay(10);
}