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There have been an number of circuits discussed here to detect soil moisture. Most use conductivity of the soil though there is one reference to using a galvanic cell to generate a voltage. What is the preferred method? AND none of the circuits have any protection from charged soil (e.g. from a lightening strike -- of course taking a direct hit is probably un-surviveable). Some even fed the output directly into a micro controller which sounds like a nice cloud to ground strike within a mile would likely cook the MCU.

So, best type (direct measurement of conducted current? 555-based? AC? DC? galvanic cell? ??) Others?

I will read the output with a micro controller so anything that might be included in the MCU need not be duplicated in the sensor circuit.

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If you were to use one of the 555-based measuring circuits, you could use an optical fiber line to get the output of the 555 into your microcontroller.

I use a lot of stuff from Industrial Fiber Optics. They sell jacketed 1mm plastic fiber along with matching transmitters and receivers.

I'd send power out to the soil sensor from its own wall wart power supply and also run a length of fiber with the power wires.

Even if the soil sensor gets smoked from a nearby lightning strike, your controller should still be safe.

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  • \$\begingroup\$ That would be a pretty expensive approach to protecting the MCU. I don't know how protected the inputs to a modern MCU are. Maybe it just isn't necessary. My thoughts were just that taking something buried in the ground directly to an MCU input seemed a little unsafe. Am I over-cautious? \$\endgroup\$ – SchroedingersCat May 30 '15 at 13:08

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