SMAKN HTU21D Temperature & Humidity Sensor Breakout Board Module 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 youll have relative humidity readings and very accurate temperature readings as a bonus! 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 youll have relative humidity readings and very accurate temperature readings as a bonus!