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I'm currently working on a irrigation system for which I would like to build a low cost soil moisture sensor. I would like to measure the moisture levels of a given environment based on a capacitance.

enter image description here

So far I was unable to find a differential capacitance sensor (see fig.I)

I had a look at multiple touch sensors controller/button controller/you name it. They all had a ground plane and did not measure in differential mode. Does anybody know a differential capacitance sensor suitable for my purposes.

Note: Due to volume restrictions in my design, I am unable to use a sensor with a ground plane per measurement plane. Also, due to flexibility in the design (see fig. II) I can adjust the copper tube to match the capacity range of the sensor and can therefor not specify such a range.

Note 2: The copper tube inside the plastic tube should function as measurement pads. (see fig. II)

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  • \$\begingroup\$ Seems this fixture bears some similarity to ECG methods, which are well-established. Perhaps the mid-electrode could be driven with an AC-probe signal with respect to a remote ground? \$\endgroup\$ – glen_geek Mar 4 at 17:43
  • \$\begingroup\$ thanks, maybe there is. Im gonna have a look at those applications. However, I havent mentioned that i would like to use several more electrodes, maybe around 10 - 20 in total. Threrefore I do not want to add a ground electrode for ecery meassurement electrode. \$\endgroup\$ – Matthias Waelti Mar 4 at 19:02
  • \$\begingroup\$ what about a 555 mutlivibrator? it might be enough for the purpose. it can convert the capacity into frequency, which is easy to measure with a timer \$\endgroup\$ – G. B. Mar 4 at 19:33
  • \$\begingroup\$ This may be a possible solution. But I was looking for a more advanced approach with an IC which already offers some features and is accessible over I2C, SPI etc. \$\endgroup\$ – Matthias Waelti Mar 5 at 13:30

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