MIKROE Air quality 4 Click is an advanced air quality sensing device that combines multiple metal-oxide sensing elements on a chip to provide detailed information of the air quality parameters. This click can output a TVOC value readings as well as the CO2 equivalent concentration readings in an indoor environment. Besides these compensated attributes, the click can also output raw values of H2 and Ethanol in the air, that can be used to calculate gas concentrations, relative to a reference concentration.
The sensor used on Air quality 4 Click is actually a complete system of sensors on a chip, featuring I2C interface, a temperature controlled micro hotplate, and two compensated and processed indoor air quality signals. It is produced using the proprietary CMOSens® technology, which provides very detailed information and robustness against air contamination. These attributes make Air quality 4 Click a perfect solution for various air conditioning systems, ventilation systems and other IoT applications where accurate, detailed and reliable air quality readings are required.
The sensor IC used on Air quality 4 Click, is the SGP30 from Sensirion, a multi-pixel gas sensor for indoor applications. This sensor is actually a system of several metal-oxide sensing elements on a chip (pixels), used to measure and process readings of various gasses in the air and output them in a form of two complementary air quality readings - Total Volatile Organic Compounds (TVOC) [ppb] and CO2 equivalent signal [ppm].
The traditional metal-oxide (MOX) sensors suffer from sensitivity and responsiveness degradation, after being exposed to siloxanes over time. Siloxanes are specific compounds commonly found in the air, which negatively affect the MOX sensors. To overcome this problem, the SGP30 sensors are made by utilizing the advanced MOXSens® Technology, which provides these sensors with the unmatched robustness against siloxanes contamination. This results with unprecedented long-term stability and accuracy.
The SGP30 uses a dynamic baseline compensation algorithm and calibration parameters, to provide accurate readings of TVOC and CO2eq properties. These baseline values of the compensation algorithm can be stored externally and read back into the device with the appropriate I2C commands and can be used in an event of power down or restart. For the first 15 seconds after the I2C initialization command, the sensor is in an initialization phase of collecting sensor measurements and baseline compensation data and will return fixed values of 400 ppm CO2eq and 0 ppb TVOC, when measurement command is attempted.
The device also allows air humidity corrections over the results for both the compensated and raw values. However, an absolute humidity sensor should be used for obtaining the humidity reading. Readings from SHT click or any other similar humidity measuring device can be used for this purpose. The appropriate I2C command will store the value into the on-chip memory and perform the corrections, accordingly.
There are additional components on Air quality 4 Click, that are used to provide necessary voltage levels for the SGP30 sensor IC. This sensor uses not so common voltage level of 1.8V, so to provide a proper communication with 3.3V and 5V capable MCUs, a small LDO voltage regulator has to be used, along with the level shifter, which provides required logic voltage levels. The SPX3819M5, a small 500mA LDO voltage regulator is used to provide the power supply of 1.8V for the SGP30. For the level shifting, the PCA9306 dual bidirectional I2C bus and SMBus voltage level shifter is used. This level shifter IC allows shifting (converting) the I2C signal levels to the voltage level selected by the VCC SEL onboard SMD jumper. This allows both 3.3V and 5V capable MCUs to be interfaced with the Air quality 4 click.
More detailed information about all the available I2C commands can be found in the SGP30 datasheet. However, all the functions and commands necessary to perform air quality readings, as well as functions used to calculate H2 and Ethanol concentration levels, are contained within the Air quality 4 click libraries, provided with this Click board™. Their usage is demonstrated in the included example application, which can be used as a reference for a custom application design.
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