software/ampel-firmware/co2_sensor.cpp

#include "co2_sensor.h"

namespace config {
  // Values should be defined in config.h
  uint16_t measurement_timestep = MEASUREMENT_TIMESTEP; // [s] Value between 2 and 1800 (range for SCD30 sensor)
  const uint16_t altitude_above_sea_level = ALTITUDE_ABOVE_SEA_LEVEL; // [m]
  uint16_t co2_calibration_level = ATMOSPHERIC_CO2_CONCENTRATION; // [ppm]
#ifdef TEMPERATURE_OFFSET
  // Residual heat from CO2 sensor seems to be high enough to change the temperature reading. How much should it be offset?
  // NOTE: Sign isn't relevant. The returned temperature will always be shifted down.
  const float temperature_offset = TEMPERATURE_OFFSET; // [K]
#else
  const float temperature_offset = -3.0;  // [K] Temperature measured by sensor is usually at least 3K too high.
#endif
  const bool auto_calibrate_sensor = AUTO_CALIBRATE_SENSOR; // [true / false]
}

namespace sensor {
  SCD30 scd30;
  int16_t co2 = 0;
  float temperature = 0;
  float humidity = 0;
  String timestamp = "";
  int16_t stable_measurements = 0;
  uint32_t waiting_color = color::blue;
  bool should_calibrate = false;

  void initialize() {
#if defined(ESP8266)
    Wire.begin(12, 14);  // ESP8266 - SDA: D6, SCL: D5;
#endif
#if defined(ESP32)
    Wire.begin(21, 22); // ESP32
    /**
     *  SCD30   ESP32
     *  VCC --- 3V3
     *  GND --- GND
     *  SCL --- SCL (GPIO22) //NOTE: GPIO3 Would be more convenient (right next to GND)
     *  SDA --- SDA (GPIO21) //NOTE: GPIO1 would be more convenient (right next to GPO3)
     */
#endif

    // CO2
    if (scd30.begin(config::auto_calibrate_sensor) == false) {
      Serial.println("Air sensor not detected. Please check wiring. Freezing...");
      while (1) {
        led_effects::showWaitingLED(color::red);
      }
    }

    // SCD30 has its own timer.
    //NOTE: The timer seems to be inaccurate, though, possibly depending on voltage. Should it be offset?
    Serial.println();
    Serial.print(F("Setting SCD30 timestep to "));
    Serial.print(config::measurement_timestep);
    Serial.println(" s.");
    scd30.setMeasurementInterval(config::measurement_timestep); // [s]

    Serial.print(F("Setting temperature offset to -"));
    Serial.print(abs(config::temperature_offset));
    Serial.println(" K.");
    scd30.setTemperatureOffset(abs(config::temperature_offset)); // setTemperatureOffset only accepts positive numbers, but shifts the temperature down.
    delay(100);

    Serial.print(F("Temperature offset is : -"));
    Serial.print(scd30.getTemperatureOffset());
    Serial.println(" K");

    Serial.print(F("Auto-calibration is "));
    Serial.println(config::auto_calibrate_sensor ? "ON." : "OFF.");
  }

  //NOTE: should timer deviation be used to adjust measurement_timestep?
  void checkTimerDeviation() {
    static int32_t previous_measurement_at = 0;
    int32_t now = millis();
    Serial.print("Measurement time offset : ");
    Serial.print(now - previous_measurement_at - config::measurement_timestep * 1000);
    Serial.println(" ms.");
    previous_measurement_at = now;
  }

  void countStableMeasurements() {
    static int16_t previous_co2 = 0;
    if (co2 > (previous_co2 - 30) && co2 < (previous_co2 + 30)) {
      stable_measurements++;
      Serial.print(F("Number of stable measurements : "));
      Serial.println(stable_measurements);
      waiting_color = color::green;
    } else {
      stable_measurements = 0;
      waiting_color = color::red;
    }
    previous_co2 = co2;
  }

  void startCalibrationProcess() {
    /** From the sensor documentation:
     * For best results, the sensor has to be run in a stable environment in continuous mode at
     * a measurement rate of 2s for at least two minutes before applying the FRC command and sending the reference value.
     */
    Serial.println(F("Setting SCD30 timestep to 2s, prior to calibration."));
    scd30.setMeasurementInterval(2); // [s] The change will only take effect after next measurement.
    Serial.println(F("Waiting until the measurements are stable for at least 2 minutes."));
    Serial.println(F("It could take a very long time."));
    should_calibrate = true;
  }

  void calibrateAndRestart() {
    Serial.print(F("Calibrating SCD30 now..."));
    scd30.setAltitudeCompensation(config::altitude_above_sea_level);
    scd30.setForcedRecalibrationFactor(config::co2_calibration_level);
    Serial.println(F(" Done!"));
    Serial.println(F("Sensor calibrated."));
    ESP.restart(); // softer than ESP.reset
  }

  void logToSerial() {
    Serial.print(timestamp);
    Serial.print(F(" - co2(ppm): "));
    Serial.print(co2);
    Serial.print(F(" temp(C): "));
    Serial.print(temperature, 1);
    Serial.print(F(" humidity(%): "));
    Serial.println(humidity, 1);
  }

  void displayCO2OnLedRing() {
    if (co2 < 250) {
      // Sensor should be calibrated.
      led_effects::showWaitingLED(color::magenta);
      return;
    }
    /**
     * Display data, even if it's "old" (with breathing).
     * Those effects include a short delay.
     */
    if (co2 < 2000) {
      led_effects::displayCO2color(co2);
      led_effects::breathe(co2);
    } else {
      // >= 2000: entire ring blinks red
      led_effects::redAlert();
    }
  }

  /** Gets fresh data if available, checks calibration status, displays CO2 levels.
   * Returns true if fresh data is available, for further processing (e.g. MQTT, CSV or LoRa)
   */
  bool processData() {
    bool freshData = scd30.dataAvailable();

    if (freshData) {
      // checkTimerDeviation();
      timestamp = ntp::getLocalTime();
      co2 = scd30.getCO2();
      temperature = scd30.getTemperature();
      humidity = scd30.getHumidity();
    }

    //NOTE: Data is available, but it's sometimes erroneous: the sensor outputs zero ppm but non-zero temperature and non-zero humidity.
    if (co2 <= 0) {
      // No measurement yet. Waiting.
      led_effects::showWaitingLED(color::blue);
      return false;
    }

    /**
     * Fresh data. Log it and send it if needed.
     */
    if (freshData) {
      if (should_calibrate) {
        countStableMeasurements();
      }
      logToSerial();
    }

    if (should_calibrate) {
      if (stable_measurements == 60) {
        calibrateAndRestart();
      }
      led_effects::showWaitingLED(waiting_color);
      return false;
    }

    displayCO2OnLedRing();
    return freshData;
  }
}