The SCD4x Library provides an interface to interact with Sensirion SCD4x true CO2 sensors using the I2C protocol in Arduino.

The SCD4x CO2 sensors come with a default auto-calibrate mode that assumes the lowest CO2 level from the past week as 400ppm. However, it's crucial to be aware that this auto-calibration can result in inaccurate readings if the sensor is placed in an environment that doesn't reach 400ppm CO2 within a week. This issue is particularly important to note when using the sensor for scientific purposes.

To address this issue, it is recommended to permanently disable the auto-calibration mode by using the provided code snippet:

#include "scd4x.h"

SCD4X co2;

void setup() {
  // Initialize the SCD4x library
  co2.begin();

  // Disable auto-calibration
  co2.setCalibrationMode(false);

  // Save the settings to EEPROM
  co2.saveSettings();
}

void loop() {
  // Your code here
}

Please exercise caution when using the saveSettings() command as it writes to the EEPROM, which has a limited lifespan of approximately 2000 write cycles before potential failure.

Despite the auto-calibration issue, the Sensirion SCD4x CO2 Sensors remain an excellent choice for monitoring indoor air quality, as they have proven to be more accurate than other popular eCO2 sensors.

• Supports multiple I2C busses: scd4x.begin(Wire1);
• No extra dependencies required
• Implements only necessary functions
• Uses doubles (64-bit floating-point numbers) for accurate data calculations

• not all functions are implemented
• not compatible with other SCD4x Arduino libraries
• only tested with the esp32

#include "scd4x.h"
SCD4X co2;
double CO2 = 0, temperature = 0, humidity = 0;

Wire.begin();
co2.begin(Wire);
co2.startPeriodicMeasurement();

while (co2.isDataReady() == false) {
	vTaskDelay(50 / portTICK_PERIOD_MS); //check every 50ms
}

if (co2.readMeasurement(CO2, temperature, humidity) == 0) {
	Serial.printf("%4.0f,%2.1f,%1.0f\n", CO2, temperature, humidity); //nice formatting of data
}
vTaskDelay(4750 / portTICK_PERIOD_MS); //new data available after approx 5 seconds

• The SCD4x sensor can draw up to 205mA peaks at 3.3V (only 18mA average), so ensure you have a robust power source (the above schematic has been tested to work).
• You only need to solder the 6 shown pins and the thermal pad on the underside to get it working.
• Unfortunately, soldering these sensors with a soldering iron is not easy; an oven or hotplate seems to be the only way.
• Watch out for a temperature offset if you place the sensor in a case or enclosure.

Based on the work of Raphael Nestler and everyone at Sensirion AG. Originally created by Raphael Nestler in 2021. https://github.com/Sensirion/arduino-i2c-scd4x

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