My friends and I are currently working on an automated watering project , and we still have to find the fitting sensor. The easiest solution would be to use a humistor, but since we are measuring the moisture of soil, we figured to use one of these. Can such a device be used outside of Arduino, requiring only sufficient supply voltage. Note: This is the first stage of the project and we intend to use only discrete components now. Later on the arduino will be invoked, and these hygrometers could be used again.
Well, you could contact the seller and ask for a datasheet, or you could take a look at this datasheet which appears to be the same (minor differences.)
Those modules appear to put out a voltage proportional to the moisture of the dirt.
So, you should be able to use them without an Arduino.
The datasheet says that the sensors are prone to corrosion and so not suited to being put in the soil and left for long periods.