I built a project very similar to this last winter to monitor moisture levels in the soil of plants that I was keeping over winter in my basement. I used GPIO pins to power the sensors similar to what jose can u c described above and performed testing to determine the minimum on time to get readings. I found that 150ms was enough to get stable readings in soils varying from completely saturated and with standing water to very dry (plant leaves starting to show signs of drying out).
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