HTU21D
The main chip of this item is GY-213V-HTU21D
The HTU21D is a low-cost, easy to use, highly accurate, digital humidity and temperature sensor.
This sensor is ideal for environmental sensing and data logging and perfect for a weather stations or
humidor control systems. All you need is two lines for I2C communication and
you’ll have relative humidity readings and very accurate temperature readings as a bonus!

There are only four pins that need to be hooked up in order to startusing this sensor in a project.
One for VCC, one for GND, and two data lines for I2C communication. This breakout board has built in 4.7k
pull up resistors for I2C communications.If you’re hooking up multiple I2C devices on the same bus,
you may want to disable these resistors.Dimensions: 0.6 x 0.6"Documents
Schematic
Eagle Files
Datasheet(HTU21D)
Hookup Guide
HTU21D Library
GitHub

Package Included:

1 XI2C GY-213V-HTU21D Temperature&Humidity Sensor

HDC1080
The main chip is GY-213V-HDC1080(Can use SHT21 HTU21D SI7021 chip to made the same sensor)
This I2C digital humidity sensor is a fairly accurate and intelligent alternative to the much simpler Humidity and Temperature Sensor - SHT15 Breakout It has a typical accuracy of ±4% with an operating range that's optimized from 10% to 80% RH. Operation outside this range is still possible - just the accuracy might drop a bit. The temperature output has a typical accuracy of ±0.2°C from -20~85°C.
The HDC1080 sensor chip has 2 address-select pins, so you can have up to 4 shared on a single I2C bus. It's also 3-5V power and logic safe so you don't need any level shifters or regulators to use with a 5V or 3V microcontroller.
Please note: TI has indicated that there's a 'settling' effect for the humidity and that you will need to re-hydrate the sensor once you receive it. To rehydrate it, place it in a location with 85% humidity for 24 hours or 60% humidity for 10 days.
The HDC1080 is a digital humidity sensor with integrated temperature sensor that provides excellent measurement accuracy at very low power. The HDC1080 operates over a wide supply range, and is a low cost, low power alternative to competitive solutions in a wide range of common applications. The humidity and temperature sensors are factory calibrated.
Features
Relative Humidity Accuracy ±2% (typical)
Temperature Accuracy ±0.2°C (typical)
Excellent Stability at High Humidity
14 Bit Measurement Resolution
100 nA Sleep Mode Current
Power Pins:
Vin - this is the power pin. Unlike many sensors, this chip can be powered by 3-5 VDC, so there is no voltage regulator on board. Simply power the board with the same power as the logic level of your microcontroller - e.g. for a 5V micro like Arduino, use 5V. For a 3V ARM processor, use 3V
GND - common ground for power and logic
I2C Logic pins:
SCL - I2C clock pin, connect to your microcontrollers I2C clock line. 3-5V logic OK
SDA - I2C data pin, connect to your microcontrollers I2C data line. 3-5V logic OK
 Optional Pins
These are pins you don't need to connect to unless you want to!
RDY - This is the interrupt/'ready' pin from the HDC100x. The chip has some capability to 'alert' you when data is ready to be read from the sensor. We don't use this pin in our library but it's available if you need it! It is open collector so you need to use a pull-up resistor if you want to read signal from this pin.
A0 A1 - These are the address select pins. Since you can only have one device with a given address on an i2c bus, there must be a way to adjust the address if you want to put more than one HDC100X on a shared i2c bus. The A0/A1 pins set the bottom 2 bits of the i2c address. There are pull-down resistors on the board so connect them to Vin to set the bits to '1'. They are read on power up, so de-power and re-power to reset the address
Package Included:

1 X Low Power I2C GY-213V-HDC1080 High Accuracy Digital Humidity&Temperature Sensor