Forget Me Not: Episode 11B - Soil Sensor Design

RE: Forget Me Not: Episode 11B - Soil Sensor Design

hlipka — Tue, 30 Sep 2014 18:06:44 GMT

One thing to be aware of: when using the soil sensor with DC current, it will corrode sooner or later. You can use high-quality probes (e.g. galvanized rods or stainless steel), or use alternating current.

One way to do the latter is to connect the propbe not between ground and an input pin, but between two GPIO pins (both with pull-up-resistors). Then for each measurement one pin goes high, and the other is used as input. Next time the other pin goes high, and so on. That way the current flows in alternatrring directions which can greatly reduce corrosion.

Disadvantage: this doesn't play nice with your method of linearizing the input curve. But OTOH: since the relation between soil moisture and resistance is probably not linear, why do you need to linearize the relation between resistance and measured input voltage? I would just go with, lets say, 4 or 5 different moisture level, and the corresponding input voltages. From these values you can do easily linear approximation (two subtractions, one division) to get the value you need.

RE: Forget Me Not: Episode 11B - Soil Sensor Design

DAB — Tue, 30 Sep 2014 17:48:58 GMT

Great post.

Did you consider setting up a resister bridge circuit?

I do like your final circuit. Simple and linear, makes logic decisions easy.

DAB

RE: Forget Me Not: Episode 11B - Soil Sensor Design

fvan — Tue, 30 Sep 2014 06:14:30 GMT

Wow, very nicely done [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-3b373a7f-fe93-4af1-9381-b129b2973dbc/contentimage_5F00_1.png:16:16]

(I confirm figure 1 is broken ...)

Frederick

RE: Forget Me Not: Episode 11B - Soil Sensor Design

mcb1 — Tue, 30 Sep 2014 05:15:17 GMT

Nice job.

It's a much better approach than just measuring the resistance.

One of your images has gone AWOL.

Mark