The HTU21D is a low-cost, easy to use and high-accuracy digital humidity and temperature sensor module. This sensor is ideal for environmental sensing and data logging and perfect for a weather stations or humidor control systems. You only need to connect two I2C lines, so that you can read the temperature and humidity value accurately. You also can use it for test by soldering 4 pins. Two I2C lines, one is VCC, the other is GND. This board has built-in 4.7k pull-up resistors for I2C communication.