Using the new method of measurement, described in this post, I could successfully collect some meaningful data. This time, the read values are the exact values of the final sensor without a different kind of oscillator.
I watered the plant at day zero with quite a great amount of water. From there you can see how the frequency slowly rises, while the soil in the flower pot starts to get dry. There is a small measurement error between day two and three. Here I had a short power loss and no data was recorded which resulted in some zero records.
The reason why the water seems to dry up much faster at the end has to do with the weather. It got cold and the air humidity went down, which dried up the soil much faster than in the previous period.
It is very nice to see how stable and predictable the measurements are over this long time period. There are some fluctuations, which may affect the behaviour. In the worst case you get the flash signal only for a short time on and off, until the soil is dry enough to permanently enable it. If I look at the data, this usually will happen over a transition period of two to four hours which seems acceptable.
I am still working to find the best way to seal the foot part of the sensor. Here I found an epoxy coating is the easiest thing to do and get the best results. Now I try different resins and hardener combinations to find the best one for my dipping process.
After this step, I will write the next part of my article series with all the details about this finalising work on the project. Another topic will be the manual for the sensor.
If you have questions, miss some information or just have any feedback, feel free to add a comment below.
Have you noticed any [significant] variations due to temperature changes?
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Not significant ones, but they can be measured. It will give a change about +1kHz if the PCB is fully exposed in the sun. I have to mention, I use 1% special temperature stable resistors for the oscillator circuit for that reason.
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Thanks – I have read quite a bit about this – there are some rather complex circuits for temperature compensation which I am hoping I will not need to concern myself with 😉
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I think it always depends where you plan to use your device. Mine is designed for indoor use, so I expect not a huge temperature impact. If you plan to use it outdoors, you may consider adding a temperature sensor to the board so you can later calculate a compensation for the measurement. Some microcontroller have already a temperature sensor built-in which you can use for this task.
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Yes – Mine will be going into a polytunnel where the temperature swings can be quite extreme – I already have an Environment monitor in the tunnel, but I am considering adding a thermistor or something to a later design of the probe so I can see what happens to the soil. One variant of the device called “chirp” has something like this IIRC.
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