This is a library for the GY-21 Board (HTU21, SHT21 and Si7021 Sensors).
It runs on ESP8266/ATMega Devices using Arduino IDE.
These boards use I2C to communicate.
https://www.arduino.cc/en/Guide/Libraries
Go to Example or:
1 - #include <GY21.h>
2 - make an object, for example: GY21 sensor;
3 - call Wire.begin(); on setup()
4 - call sensor.GY21_Temperature(); or sensor.GY21_Humidity(); to get the values
Be Aware: It takes ~55ms to get the values
https://github.com/JonasGMorsch/GY-21/blob/master/documentation/HTU21.pdf
https://github.com/JonasGMorsch/GY-21/blob/master/documentation/SHT21.pdf
https://github.com/JonasGMorsch/GY-21/blob/master/documentation/Si7021.pdf