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Green Thumb

Groene Vinger When the red LED blinks, the plant needs water. A blinking yellow LED indicates sufficient or too much water. The solar panel on the back side of the PCB easily provides the necessary 100uA at 1.9V, even on a very rainy day. When there is more sunlight, LED brightness goes up automatically.
The soil resistance is measured using a tiny alternating current (less than a microwatt). To avoid corrosion, the PCB has been tinned and the sensor poles were gold-plated. Prototypes are entering their second summer, and have been doing a good job for various plants.


I don't have them by nature, "green Thumbs." Fortunately, most plants are fairly easy to please. Put them near a window, give them new soil now and then, a larger pot when they grow, and enough water; that usually keeps them going pretty well.

Enough water. How much is that? Two months ago, the lemon geranium (Artemisia abrotanum) on my window sill consisted of a few 10 cm high cuttings. Half a glas of water once every three days sufficed nicely. Now I'm looking at a 60 cm high adult who wants almost ten times as much water. And when the summer sun hits that window, it will need at least three times more.
Giving a plant some arbitrary amount of water every now and then doesn't work too well, because it's in a closed pot. A substatial layer of water will cause the roots to decay. The soil should be moist. That means neither wet nor dry. Difficult to judge, as the top layer can be quite dry even though there is still more than sufficient water below.

An old way to solve this puzzle is a pot with a hole in the bottom, standing on a saucer which serves as water detector. One disadvantage is the limited choice in pots of that type. Moreover, the saucer soon gets dirty. And this method doesn't work at all for large flower boxes. So how about an electronic sensor?

Moistness reduces the electrical resistance of soil. But if you measure the resistance with a simple direct current, one of your sensor poles will be soon covered with a deposit which causes extra resistance (and the corrosion resistance will be seriously tested). Also, it would be inconvenient as well as environmentally unsound if the sensor needed batteries. Just a few years ago it was quite difficult if not impossible to make something usable. Now, modern electronics and small solar panels allow construction of a nice, not too complex and fairly cheap Green Thumb.

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