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Protecting a capacitive soil moisture sensor?

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This is regarding the following sensor:

http://www.dfrobot.com/wiki/index.php?title=Capacitive_Soil_Moisture_Sensor_SKU:SEN0193

It is stated that "The components on this board are NOT waterproof, do not expose to moisture further than the red line. (If you want to protect components from the elements, try using a length of wide heat shrink tubing around the upper-section of the board.)"

I don't have heat shrink tubing wide enough to cover it. I was thinking I could just use some hot glue instead. As in cover the electronics completely in hot glue. Is there any reason to avoid this? Is it likely to work, or to fail?
 

AnalogKid

Well-Known Member
Most Helpful Member
Aerosol conformal coating, either urethane or acrylic. The sensor circuit is a CMOS 555 oscillator plus a diode and lowpass filter to change the output to a DC level proportional to the oscillator frequency. By definition it is susceptible to changes in stray capacitance, so less is more. Mask off the sense area and connector with masking tape, lay down 2 or 3 layers of conformal coating, ***remove the tape*** (yes, not everyone does), and test.

Hot glue might work, but a) it is hot; b) it is thick. As it cools it might stress or even partially remove a component.

ak
 

Tony Stewart

Well-Known Member
Most Helpful Member
I agree, I was looking at Arduino connect to sensor xtal.

Adding layer of thickness, to reduce hydroscopic effects, adds to cap tuning value if <100pF. If touching the tracks shifts the threshold, you might be able to tune it. Since this is based on capacitance ratio, ratios will be affected.

Contact corrosion with water is inevitable unless all gold plated, even then, add GE RTV silicone to seal from air.
 

Mosaic

Well-Known Member
I've tried a few DIY things like spray on clear lacquer, fibre glass resin etc, but I think you need a spec'd conformal that is very moisture resistant. My experience is that the lacquers work ok for a few hours and then degrade the signal as the moisture penetrates.
 
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