Soil Moisture Sensor Update

Saturday, January 8, 2011 by darco
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I ended up building the prototype soil moisture sensor boards I mentioned in a previous post, and... it works! Somewhat.

This is what it ended up looking like:


The sensor section of the board I coated with black Plasti-Dip to protect it from moisture, and the results turned out rather well. In later versions I'll probably be potting all of the circuity in either Plasti-Dip or epoxy.

It turns out my method for trying to get two zones to work properly on the limited pin count of the ATTiny13a was not effective—instead of getting independent values for each depth, it acted like a single large sensor. In light of this observation, I've decided to jettison the cleverness and just focus on a single-zone capacitive moisture sensor design—at least for now. I also removed the ground plane as well, as it doesn't seem to be helping the readings. Here it is the new single-zone version:




I've ordered a some of these boards from BatchPCB with my last order, so when I get the boards back (in about a month) I'll post again to describe how they turn out. I'm pretty confident that they will work well, and I'll likely end up making a bulk order of the boards some point soon thereafter.

I'll post some code once I get it working well, but if you would like some idea of the theory behind how it works, read up on AVR's QTouch library. While I'm not actually using QTouch in my code for this project, their documentation provides some good background theory behind how capacitive sensing works. Worth a read.