• MCU: U1, ESP32-S3-MINI-1-N8
• Framework: Arduino via PlatformIO
• Interfaces: I2C CO2 sensor/display, RGB LED, native USB CDC serial

Ini
[env:esp32-s3]
platform = espressif32
board = esp32-s3-devkitc-1
framework = arduino
monitor_speed = 115200
upload_speed = 921600
build_flags =
    -DARDUINO_USB_MODE=1
    -DARDUINO_USB_CDC_ON_BOOT=1
lib_deps =
    sensirion/Sensirion I2C SCD4x@^1.0.0
    adafruit/Adafruit SSD1306@^2.5.13
    adafruit/Adafruit GFX Library@^1.11.11

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

#define I2C_SDA_PIN        4
#define I2C_SCL_PIN        5
#define OLED_RESET_PIN     9
#define LED_RED_PIN        6
#define LED_GREEN_PIN      7
#define LED_BLUE_PIN       8
#define BOOT_PIN           0

#define SCREEN_WIDTH       128
#define SCREEN_HEIGHT      64
#define OLED_ADDR          0x3C
#define SCD41_ADDR         0x62
#define SENSOR_INTERVAL_MS 5000

Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET_PIN);
SensirionI2CScd4x scd4x;

static uint32_t lastReadMs = 0;
static uint16_t lastCo2ppm = 0;
static float lastTemperatureC = NAN;
static float lastHumidityRh = NAN;

void setRgb(bool redOn, bool greenOn, bool blueOn) {
  // D1 is common-anode: LOW turns a channel on, HIGH turns it off.
  digitalWrite(LED_RED_PIN,   redOn   ? LOW : HIGH);
  digitalWrite(LED_GREEN_PIN, greenOn ? LOW : HIGH);
  digitalWrite(LED_BLUE_PIN,  blueOn  ? LOW : HIGH);
}

void setAirQualityLed(uint16_t co2ppm) {
  if (co2ppm < 800) {
    setRgb(false, true, false);   // good: green
  } else if (co2ppm <= 1200) {
    setRgb(true, true, false);    // moderate: yellow/red+green
  } else {
    setRgb(true, false, false);   // poor: red
  }
}

const char* airQualityLabel(uint16_t co2ppm) {
  if (co2ppm < 800) return "GOOD";
  if (co2ppm <= 1200) return "MODERATE";
  return "POOR";
}

void drawDisplay(uint16_t co2ppm, float tempC, float rh) {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.print("USB CO2 Air Monitor");

  display.setCursor(0, 18);
  display.setTextSize(2);
  display.print(co2ppm);
  display.print(" ppm");

  display.setTextSize(1);
  display.setCursor(0, 43);
  display.print("Air: ");
  display.print(airQualityLabel(co2ppm));

  display.setCursor(0, 55);
  display.print(tempC, 1);
  display.print("C  ");
  display.print(rh, 0);
  display.print("%RH");

  display.display();
}

void showError(const char* line1, const char* line2) {
  setRgb(true, false, true); // magenta = fault
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println(line1);
  display.println(line2);
  display.display();
  Serial.printf("ERROR: %s %s\n", line1, line2);
}

void initRgb() {
  pinMode(LED_RED_PIN, OUTPUT);
  pinMode(LED_GREEN_PIN, OUTPUT);
  pinMode(LED_BLUE_PIN, OUTPUT);
  setRgb(false, false, false);
}

bool initDisplay() {
  pinMode(OLED_RESET_PIN, OUTPUT);
  digitalWrite(OLED_RESET_PIN, HIGH);
  delay(10);
  if (!display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDR)) {
    return false;
  }
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println("Starting CO2 monitor");
  display.display();
  return true;
}

bool initScd41() {
  scd4x.begin(Wire);

  uint16_t error;
  char errorMessage[256];

  error = scd4x.stopPeriodicMeasurement();
  delay(500);

  uint16_t serial0, serial1, serial2;
  error = scd4x.getSerialNumber(serial0, serial1, serial2);
  if (error) {
    errorToString(error, errorMessage, sizeof(errorMessage));
    Serial.printf("SCD41 getSerialNumber error: %s\n", errorMessage);
    return false;
  }
  Serial.printf("SCD41 serial: %04X-%04X-%04X\n", serial0, serial1, serial2);

  error = scd4x.startPeriodicMeasurement();
  if (error) {
    errorToString(error, errorMessage, sizeof(errorMessage));
    Serial.printf("SCD41 startPeriodicMeasurement error: %s\n", errorMessage);
    return false;
  }
  return true;
}

bool readScd41(uint16_t &co2ppm, float &tempC, float &rh) {
  uint16_t error;
  char errorMessage[256];
  bool dataReady = false;

  error = scd4x.getDataReadyFlag(dataReady);
  if (error) {
    errorToString(error, errorMessage, sizeof(errorMessage));
    Serial.printf("SCD41 dataReady error: %s\n", errorMessage);
    return false;
  }
  if (!dataReady) return false;

  error = scd4x.readMeasurement(co2ppm, tempC, rh);
  if (error) {
    errorToString(error, errorMessage, sizeof(errorMessage));
    Serial.printf("SCD41 readMeasurement error: %s\n", errorMessage);
    return false;
  }
  if (co2ppm == 0) return false;
  return true;
}

void setup() {
  Serial.begin(115200);
  delay(300);
  Serial.println("USB CO2 Air Monitor starting");

  WiFi.mode(WIFI_OFF);
  btStop();

  pinMode(BOOT_PIN, INPUT_PULLUP);
  initRgb();
  setRgb(false, false, true); // blue during startup

  Wire.begin(I2C_SDA_PIN, I2C_SCL_PIN);
  Wire.setClock(100000);

  if (!initDisplay()) {
    Serial.println("OLED init failed");
  }

  if (!initScd41()) {
    showError("SCD41 init failed", "Check I2C/power");
  } else {
    setRgb(false, true, false);
  }
}

void loop() {
  const uint32_t now = millis();
  if (now - lastReadMs >= SENSOR_INTERVAL_MS) {
    lastReadMs = now;
    uint16_t co2;
    float tempC, rh;
    if (readScd41(co2, tempC, rh)) {
      lastCo2ppm = co2;
      lastTemperatureC = tempC;
      lastHumidityRh = rh;
      setAirQualityLed(co2);
      drawDisplay(co2, tempC, rh);
      Serial.printf("CO2=%u ppm, T=%.2f C, RH=%.2f %%\n", co2, tempC, rh);
    } else if (lastCo2ppm == 0) {
      showError("Waiting for SCD41", "warming up...");
    }
  }
}

1. Install PlatformIO.
2. Connect USB-C to J1.
3. Build: pio run
4. Flash: pio run -t upload
5. Monitor: pio device monitor
6. If flashing fails, hold SW3 (BOOT), tap SW4 (RESET), release SW3, then retry upload.