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.