PCB as a Capacitive Soil Moisture Sensor

Electronics enthusiasts have been designing ways of automating plant care for decades, with mixed results. A traditional weak point of these installations is the resisitive moisture sensor, that is inaccurate and prone to degradation. In this post we will design and fabricate an inexpensive capacitive soil moisture sensor out of a printed circuit board that exhibits none of the weaknesses of its resistive brethren.

The Story So Far

Plant care automation is a popular project in the amateur electronics community. Recent examples include the Garduino system, and the twitter-enabled Botanicalls. Both these systems rely on the fact that the resistance of earth that contains water is lower than the resistance of earth that contains less water. This works, but it has two important drawbacks.

First of all, the electrodes suffer from degradation due to oxidation. The voltage applied to the electrodes in the soil attracts oppositely-charged ions present in the soil or water. These ions then deposit on the surface of the electrode. These oxides are insulators, leading an increasing resistance of the electrodes over time. Second, water is actually a rather poor conductor compared to the impurities it contains. Soil resistance is therefore dependent upon the contamination in the soil more than the moisture content. This means that resistive methods require a calibration step that depends on the soil composition.

Capacitance and Dielectrics, for fun and profit

A parallel plate capacitor

Let's take a standard parallel plate capacitor. The capacitance of this device is dictated by its dimensions (the plate area and the distance between the plates), as well as by the material properties of the substance between the plates, the dielectric. It turns out that water has a very large dielectric constant. A capacitor that has water between the plates would have a capacitance that is 100 times higher than the same capacitor using air or rocks.

This is how we are going to build our sensor: we are going to make a capacitor using the soil as our dielectric. If the soil is moist, the capacitor will contain more water. This will result in a higher capacitance. If the soil is dry the capacitance will be lower. 

Touchscreens on iPads and smartphones use this exact same system. Your finger is 70% water. When you touch the screen the composition of the dielectric changes and you cause an increase in capacitance.

A co-planar plate capacitor

As a basis for the sensor we will use a co-planar plate capacitor, instead of the standard parallel plate construction. That way we don't have to get the soil/moisture to occupy the gap in the middle. The electric field lines extend outward from the plates into the dielectric on both sides.

Designing the Sensor

As we have seen in the previous section, we are looking for a way to create a sensor that consists of isolated conductive plates, separated by a narrow gap. The assembly must be waterproof, as it will be in contact with moist soil for extended periods of time. As luck would have it, there is an inexpensive technology that enables us to build isolated conductors in arbitrary shapes with ridiculously small tolerances. This technology is printed circuit board fabrication. 

We design a custom PCB that contains a number of co-planar plate capacitors in parallel. Capacitance sums in parallel, so this will lead to a larger capacity of the entire assembly and therefore a larger capacity swing when the dielectric changes. We want to measure the dielectric constant of the surrounding medium, not of the FR4 fiberglass the PCB is made of. Therefore, we will place plates of the same polarity on both sides of the sensor. This is in contrast to a parallel plate arrangement, where opposite polarities are used.

The sensor, as designed in Eagle

In the image above you can see the sensor as designed in Eagle. The entire assembly is about 4 inch long and 0.5 inch wide. The traces are identical on the reverse side. The PCB sports pads for a jumper header to connect to your circuit, and a pointy tip for easy insertion into your target soil. Note the relatively large separation between the plates. The capacity is actually inversely proportional to the separation between the plates. In a next revision we'll tighten this up significantly. After all, we're paying for 6 mil spacing, we might as well put it to good use!

The physical sensor, in Laen purple!

I sent it off to Laen's PCB order to have it produced. Two weeks and $15 dollars later I received three copies of my sensor, in a glorious purple color. It was my first board using Laen's order, and I'm really happy how it turned out. I have a feeling it will not be the last.

Soak testing.

To make sure the isolation (soldermask) on the sensor is really waterproof, I'm performing an extended soak test. In order to be usable the sensor needs to have a resistance that is at least an order of magnitude larger than the discharge resistor in the measuring system, even after being submerged for a significant period of time. I'm planning to use a resistor in the mega-ohm range. The resistance needs to remain above 10 mega-ohm or so. After 12 hours in the glass, the resistance is still above the highest range on my multi-meter. So far so good.

After the soak test, it's time to start terrorizing the potted plants again!

You can also download the schematic and the board file, both in Eagle 5 format.

The small print: All rights for the design are reserved, but you are welcome to use it in a non-commercial or educational setting and/or to use is as a basis for your own designs. No warranties of any kind are provided. I am not liable for anything you do with these files. If you want to negotiate different terms, contact me.