As far as I can find, documentation for GY-MCU681 is the same as for GY-MCU680V1. I downloaded it from here.

These are marketed as being compatible. I question this, they may be compatible at the serial UART interface; however, by this parts list they clearly differ at the I²C interface.

Part list of GY-MCU681 and the drivers that I used with the I²C interface.

• SGP40 - VOC sensor
  • https://github.com/DFRobot/DFRobot_SGP40
• SHTC3 - temperature and humidity sensor
  • https://github.com/sparkfun/SparkFun_SHTC3_Arduino_Library
• BMP280 Marked 01V (lot number) KU (K identifies BMP280), barometric pressure sensor
  • https://github.com/Erriez/ErriezBMX280 - Library builds without warnings and works!
• I²C interface:
  • Onboard termination: Resistance from pins SDA and SCL to ground, each read 10KΩ.
  • The instructions for selecting I²C mode are ambiguous "soldering the PS solder joint." The GY-MCU681 has two jumper positions PS0 and PS1. I bridged PS0 and it has been working. I am able to access all three devices through I²C.
  • Useful bus scanner https://github.com/farmerkeith/I2CScanner
• XC6204, Marked 4B2X, LDO Volt Regulator 3.3V
• ??, Unknown processor with markings scratched out. Smaller pin count, 20, than STM32F05186 (32) used on GY-MCU680V1.

Part list of GY-MCU680V1

• BME680
• STM32F05186, processor
• 5 to 3.3 Volt regulator

The GY-MCU681 may be the result of a chip shortage. It appears to incorporate multiple discreet function chips to do the job of the multifunction BME680.

Below is an example of the seller's description. It does not appear 100% accurate. I don't see a BME680 or MCU680 on my board. And, You would not need the SGP40 if you had a BME680.

• GY-681 BME680 MCU680 indoor air IAQ SGP40 temperature and humidity air pressure sensor module GY-MCU680V1
• A low-cost air detection module with a low operating voltage of 3-5v and a small size.
• The working principle is to read the BME680 sensor data through the MCU, and obtain the algorithm, temperature and humidity, atmospheric pressure, IAQ indoor air quality, and resistance value.
• The module has two ways to read data, namely serial port (TTL level) or direct chip I²C communication mode.
• The product has high precision and high stability. It is possible to directly output utility data and omit the algorithm.
• The baud rate of the serial port is 9600bps and 115200bps. There are two modes of continuous output and interrogation output, which can adapt to different working environments.
• Connect to all microcontrollers and computers
• When soldering the PS solder joint, the module is in the chip's own IIC mode, at which point the MCU does not participate in the operation and does not consume current. Can be used as a simple BME680 module.
• The IAQ indoor air quality algorithm is transplanted according to the routine of the BME680 chip official website. It takes a certain time to power on the test, and the data will be normal for about 5 minutes.

Parameters

• Temperature measurement range: -40°~ 85°
• Humidity measurement range: 0% ~ 100%
• IAQ measurement range: 0?~500
• Air pressure measurement range: 300 ~ 1100hpa
• Response frequency: default 3 seconds each time
• Working voltage: 3~5 V
• Average operating current: 5mA
• Working temperature: -40°~ 85°
• Storage temperature: -40°~ 125°
• Size: 12mm × 30mm
• Using the chip: ME680+STM32

• Estimated CO₂
  • I see this often referenced as being equivalent to CO₂; it is not.
  • Most sensors are highly sensitive to one gas and have minor sensitivity to other gases, like CO₂. When you assume the primary level is insignificant, you can use the measurement to estimate the CO₂ level.
  • I am guessing here. The cheap meters that have one sensor and display several gases present a value for each gas which assumes none of the other gases are present.

• SCD-40 and SCD-41 are for measuring CO₂ levels datasheet, Adafruit