[solved] First time PCB creation, example: soil sensor capacitor probe

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1

Hi,

I want to design my first PCB project. Part of it will be a soil sensor probe, e.g. here. Schematic and PCB layout attached.

As far as I understand, I add tracks to the top and bottom layer in parallel (see pic attached). Nevertheless, I'm little confused.
What is the best layout for that? Just two massive areas? Or winding tracks like this (from previous link)?

I try to achieve something like that one.

Besides my practical question, maybe someone can help me understand why the capacitance changes despite the dielectric does not change inside the PCB-capacitor. At least this is how I understand that this sensor works.

Thanks & Best

soil-example.png
soil-example.png1122×793 27.6 KB

2

The two plates opposite each other (with PCB material between) are a capacitor. But the moisture in the soil creates a third plate. You could consider it like wrapping your thing in foil - that must affect the capacitance although the effect will be smaller than the base-load capacitance. It's not really "between" but it is close enough to affect the capacitance.

The "intersecting combs" capacitor will probably see a larger effect due to the shorter distance between the two copper plates and the soil "plate". I would build that one first if I was going to do it.

3

Thanks for the explanation and your thoughts about the plates.
I will give it a try :slight_smile: If I have space left on the PCB I'll try a "solid" probe, too.

Best

4

I ordered both soil sensor types. Tests show no practical difference between the two.

Best