grauzone has created an ATTNode v3 addon board for the MH-Z19C CO2 Sensor. The addon has an onboard 3,3V voltage regulator to supply the node while providing the needed 5V to the sensor (directly from a 5V supply). It also incorporates 2 WS2812B RGB LEDs to signal current LoRa status as well as the current CO2 Level. Details on the hardware can be found on Github .
While adding the code for the CO2-Sensors I also extended the firmware to support multiple sensors. For example a BME280 and a CO2-Sensor can be added in parallel. See the README.md in the Firmware Repository for further details on how to use it.