In this design we used precise temperature and humidity sensor from Sensirion: SHT30 or SHT31 connected via I2C. Sensors are equipped with ALERT output. This output is connected to N-channel MOSFET and controls PTC Heating Element. Huge advantage of this design is ALERT logic which can be programmed via I2C and it's independent from main micro controller. In other words apart of sensor initialization it doesn't require any action from main micro controller.