Description:
This is a plant watering alarm. 
You put it into the soil near a plant and it emits a tiny chirp when the soil is dry, reminding you to water the plant. 
As more water evaporates, it will increase the alarm rate.
It uses capacitive humidity sensing as opposed to resistive humidity sensing, this means, it does not make an electric contact with the soil, avoiding corrosion and soil electrolysis and resulting in better accuracy and longer battery life.
This sensor can detect if it’s day or night by measuring the amount of ambient light and will not make noise when it’s dark.