NDIR CO2 Click Board™

Product Code: MIKROE-3134

Due to its large scale of integration, this sensor requires no external components involved in the measurement process, which improves the overall accuracy. Each of these sensors is factory tested and calibrated. The sensor also offers several I/O pins for simplified configuration and operation, saving the MCU from polling the registers. A specialized ALARM pin offers the possibility to trigger an MCU interrupt if the programmed CO2 concentration value is exceeded. These features make the NDIR CO2 Click Board™ a perfect solution for various CO2 measuring applications, such as air conditioning applications, indoor air quality control applications, automatic fresh air exchange and venting systems, and similar. How Does The NDIR CO2 Click Board™ Work? The main component on the NDIR CO2 Click Board™ is the CDM7160, a pre-calibrated single light source, dual wavelength, CO2 sensing system, by Figaro Engineering, INC. Its light source emits the light, which is detected by two IR sensors. One light sensor is placed behind the filter which allows only a part of the IR spectrum affected by the CO2 gas to pass through, while the second sensor is placed behind the filter which passes the IR spectrum of the light which is not affected by the CO2 gas.

This forms a kind of a differential input for the sensor - an integrated MCU will process the received data by differentiating these readings. This allows the absolute value of the CO2 gas concentration to be obtained, but also removes any influences of particles and other disturbances, as they affect both sensors equally. This allows consistent readings over various temperatures, in various environments, including areas rich with corrosive gases (SO2, H2S…), and over a longer period of times (aging). The CDM7160 sensor has the ability to output data in two ways: depending on the status of the MSEL pin, it can use either a UART or I2C communication interface. If this pin is pulled to a LOW logic level, the I2C interface will be selected after the CDM7160 reset cycle. Otherwise, the UART interface will be selected. Since the communication pins are shared between the interfaces (SCL/RX and SDA/TX), they need to be switched to the corresponding pins of the mikroBUS™ whenever the different type of communication is used. Therefore, the Click board™ has a section with three small SMD slide switches labelled as COM SEL. Positioning all three switches to the LEFT position will select the I2C interface while the RIGHT position will select the UART interface. When the I2C interface is selected, an additional pin is available to set up the I2C address of the device. This pin determines the LSB of the I2C slave address and when it is pulled to a LOW logic level, this bit becomes 0. This allows up to 2 different devices to be connected to the same I2C bus. This pin is routed to another SMD slide switch, labelled as the ADD SEL. It is possible to perform two types of calibration for this sensor: zero calibration and the background calibration. The zero calibration is performed in the atmosphere with the CO2 concentration of 0 ppm, while the background calibration is performed in the atmosphere with a nominal CO2 value (400 ppm). Since the sensor is influenced by the sea level and the atmospheric pressure, these calibrations should be performed whenever these conditions are changed. This will allow an increased accuracy of the CO2 concentration readings. The CDM7160 sensor offers a pin labelled as CAL, for an easy calibration: if the CAL pin is pulled to a LOW logic level for about 2 to 11 seconds, a background calibration will be performed. If pulled to a LOW logic level for more than 12 seconds, the zero calibration will be performed. This pin should remain HIGH during normal operation. An internal pull-up resistor ensures that the pin is always HIGH if it remains floating. This pin is routed to the mikroBUS™ CS pin, labelled as CAL The ALERT pin of the CDM7160 sensor is used to trigger an interrupt on the host MCU. By default, it will trigger an interrupt if the CO2 concentration exceeds 1000ppm. The interrupt will be cleared if the concentration drops below 900ppm. These settings can be changed by writing values to the corresponding ALHI and ALLO registers (upper and lower threshold registers). The ALERT pin of the CDM7160 sensor is routed to the mikroBUS™ INT pin. BUSY pin of the sensor provides means to save the sensor from polling sensor registers in order to verify if the device is ready for the communication. By setting an interrupt for the BUSY pin, the MCU can be automatically triggered only when the sensor is ready to accept a new command. A logic LOW level signals the MCU that the sensor is unable to accept a new command. The sensor might be unavailable while processing the data internally, for about 0.3 seconds. This pin is routed to the mikroBUS™ RST pin, labelled as BSY on the Click board™. Besides UART and I2C communication, the sensor offers a 1KHz PWM signal of