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I am interested in making a DIY PCB soil sensor based on 555 timer IC. The circuit works well but I have problems with regard to manufacturing a durable PCB. As I keep the PCB based soil sensor immersed in soil for longer than a month, the PCB starts to discolor and rust. This does not happen for the commercial sensor shown below. Note: even if there is a laminated layer around the copper to protect it, the sensor detects soil moisture fine. However, this lamination is not durable.

This sensor is still in the prototype phase. I would appreciate advice on how I can ensure the durability of the PCB.

I heard that a process called "Conformal coating" can be done. But I do not know what type of manufactures I should refer to have such a PCB manufactured. The generic PCB manufactures seems not to do Conformal coating.

Image of DIY soil sensor on top and a commercial sensor on bottom

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  • \$\begingroup\$ Even conformal coating may not last in soil. I think you will need to pot the entire board in, for example, epoxy. Not sure if the sensor will work after potting. You can conformal coat or pot prototypes yourself. I am sure you can find information online. \$\endgroup\$
    – user57037
    Commented Nov 2, 2017 at 6:26
  • \$\begingroup\$ But the one I have shown below lasted quite a long time with no bad effects on its PCB. I'm wondering what coating they did. \$\endgroup\$
    – Denis
    Commented Nov 2, 2017 at 10:19
  • \$\begingroup\$ How is the upper part (with the exposed components) protected from moisture? \$\endgroup\$
    – τεκ
    Commented Nov 2, 2017 at 21:32

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Is this sensor measuring the moisture all the time, or do you only apply voltage to the circuit when it is needed? I have heard that a lot of soil sensors start to rust very fast if they are under voltage all the time. Have you tried reducing the time of measurement to a minimum? In most cases a couple of milliseconds per hour is enough to get the data that is needed.

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