A capacitive soil humidity sensor: Part 3

Initially I imagined the probe as a sleek device with a wire coming out that one would stick in the soil, if necessary completely. This would mean that the circuit pcb would be an integral part of the PCB that  formed the plates, but as always, things go different in practice. For one thing , the piece of PCB I intended to use just wasnt long enough. Second, my PCB etching possibilites are temporarily impeded. Third, I wanted to add an LDR, meaning that I had to have some sort of  translucent cover.

I constructed the circuit on a small piece of stripboard:

IMG_20160225_163125 i2cprobe

With regard to the LDR, sure that is nonsense. If I want to add an LDR I  could just as well add it to my base station. But I am not adding it because I have to, I am adding it because I can and because I wanted to get some experience in programming an I2C slave. An LDR might not be so usefull but in future I may want another sensor, e.g.a sensor that reads if there is really water flowing from the irrigation tube.

Also I had decided on putting the capacitor plates back to back, but as I did not have double sided PCB I just used two pieces glued and  soldered(!) together.
So My BOM was rather simple:

  • 2 equal size pieces of PCB  Size depends on what you have, but do not make them too small. I used 12×3 cm.
  • 1 piece of 0.5-1.0 cm plastic for a baseboard. I used an old cutting board.
  • 1 clear/translucent cover, I used the lid of a whipped cream spray can.
  • 1 piece of  thin 4 wire cable, length depending on your need

pcbprobe2I glued the two piecesof pcb back to back. drilled a hole in all 4 corners and through soldered a piece of wire through each hole, thus anchoring the plates together. I removed some copper around the solder so it would become an island isolated from the rest of the plate. (See picture).

Soldering the plates together in the corners may not be necessary if you decide to electrically isolate them from the soil with e.g. shrink tube.

Eventually I will place an NTC on the plate as well after it is covered with Shrink tube. Esthetically it might be better to put the NTC under the shrink tube, but that could create an air pocket.


baseplateI made the base plate from a o.5 cm thick piece of soft plastic. Cut out a round shape  with a 5.5 cm diameter to fit the base of my  clear dome and  made a slit 3 x0.3 cm into that in which the PCB  fits snug. made a round hole for the connecting cable.




I soldered two wires on the top of the PCB, one on each side. Soldered wires on an NTC, insulated those, attached the NTC to the bottom of the PCB with the wires leading to the top and then covered the PCB with shrink wrap.

probeEventually the probe looks like this (picture)probe3


8 thoughts on “A capacitive soil humidity sensor: Part 3

  1. Hi,
    interesting project, but I have some questions regarding your moisture sensor.
    The two PCBs glued together (or even if you would use a double sided PCB) already form a capacitor – you have two conducting plates held apart by some insulating material (the resin). The distance between the two plates is rather small compared to the “way” the electric field has to go around the two plates through the soil. So, even without any soil, water or other material around the plates, there is already a base capacity of the sensor, which would form the main part of the total sensor capacity. Do you have some figures or hints as of how much the sensor capacity will change when put into moist soil? Is it just a few percent, or much more? What is the sensitivity, i.e. how well could you differentiate between dry soil, wet soil and intermediate moisture levels?

    Best regards, Philipp

    1. Philipp, you are right, but even if they were not glued together but just on a double sided piece of PCB there would already be a dielectrum. There are people that say that it is unwise to have the ‘opposite’ plates of a capacitor ‘back to back´ but that only plates that are connected to the same electrical side of the capacitor should be back to back as is in most professionally made sensors that have an intrinsic pattern of islands forming the plates of the capacitor. I am not sure if that is true. A similar thing can be said if the plates were just next to eachother on a single sided piece of PCB: they are so close to echother then that the gap already forms a dielectrum.

      You ask for numbers. Well as it happens I am comparing a couple of types/shapes of probes and hope to have the numbers soon. I had some trouble with making them waterproof. My current sensor is isolated with heat shrink, but as I am comparing against a probe with a very thin isolation I thought it would be fair to have a thinner layer on my capacitor as well, but that attempt has been a bit frustrated so I am not finished testing yet.
      I probably wont be able to give you percentages like : “the in between dielectrum capacitance is so ans so much % of the total capacitance”, but in fact that is not such an interesting number. But what I can do is give an idea of the sensitivity. Obviously if the major part of the capacitance is formed by the in between plate dielectrum, the impact of water in the surrounding soil will be less.

      My experience till now in the field + initial comparative testing show that there is adequate sensitivity, but that it might be less than the sensitivity of a probe that is made by a PCB service. That is not really a big problem as my goal was to make a quick and cheap sensor that works and that is what it does. I am not so much interested in te various grades of soil moisture as well as that it is above or below a certain level. Nevertheless it is always possible to stick in another sensor that is made by a pro pcb service. Such as the green sensor in this image sensors The yellow one is one of mine as I am comparing various sensors. When I am done I will publish the data either here or in instructables and most likely both

      1. Hi,

        thanks a lot for your reply. I was doing some testing with a very simple sensor – just two parallel tracks of copper on a PCB, 5 mm wide, 60 mm long and 3 mm apart, and of course insulated. This gave some distinct reaction when directly immersed in water, and also some reaction in moist soil. If I would put it directly on a wet sponge, it also “worked”, but when held already 2 or 3 mm above the sponge there was almost no reaction. So it seems that the sensing range is very small. I will try some different approach and also with different shapes to see if I can increase the sensing range to a few more millimeters.

        Best Regards, Philipp

      2. 2-3mm of pure air is quite a distance but shld still be able to sense something. your sensor is indeed very small. I will be testing a sensor like that to but it will be 1x9cm per plate.

        I was wondering what u used to isolate your sensor?

  2. I want to use the sensors in my plant pots on my balcony, so the sensor can’t be much larger. For insulation of the sensor I used just a double layer of acrylic paint, but I dont have any long term experience with that – I’m still in the experimental phase.

      1. I did some measurements and calculations on my sensor described above:
        – sensor capacity in free air: around 1.6 pF
        – sensor capacity in water: 88 pF
        – sensor capacity in wet soil: 86 pF
        – sensor capacity in moist soil: between 47 pF and 85 pF, depending on the amount of water in the soil

        I haven´t measured in dry soil – I will do that later, I just need to wait until it has dried out completely.

      2. well that is a measurable change, but mine did better at 2nF in water and about 300pF in minimally moist soil
        Not sure if the measurement is linear but it seems enough to distinguish between dry and wet soil. The only problem you may have at this level is stray capacities

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