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Even on a very rainy day, the little solar panel provides sufficient energy to make one of the LEDs blink. If a spot receives enough light for a plant to grow, this little circuit will be happy as well. The tiny alternating current it uses to measure soil resistance (less than a microwatt) doesn't attack the sensor poles, and isn't even noticed by the plant.
Even without galvanic effects, many metals dislike damp soil. Their surface corrodes, causing extra resistance to be measured. That's why the Green Thumb PCB was specially treated. First a layer of tin was chemically applied to all copper traces, and then the sensor poles were electroplated with pure gold.
Prototypes of the Green Thumb are entering their second summer, and have been doing a good job for various plants. Nevertheless it is an experimental sensor. One would expect that some plants want the soil to be moister than others, and that the electrical resistance also depends on the chemical composition of the soil. So far, these factors haven't been very noticeable. The density of the root network has more impact. When a plant needs a larger pot, the Green Thumb becomes less reliable.
In any case it is necessary to keep tabs on water usage when a Green Thumb is first put into operation. If demand seems too low, then stick it in less deeply, or turn the setscrew a little to the left.
The Green Thumb guarding my lemon geranium needed no adjustment at all. Cuttings or adult plant, rainy winter days or bright sun - I look only at the LEDs, and give the plant some water if the red one blinks. Not just near the sensor but all around the pot and just enough to make the Thumb switch to yellow. The plant is very happy with this treatment.
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