Review of previous sensor projects

Review of previous sensor projects

Soil Moisture Sensor Project

Review of previous projects

In the first post of this series post I briefly outlined the soil moisture sensor project and what I wanted. This post is my review of previous sensor projects.

If there wasn’t already a wheel that I could use, I’d rather give it an upgrade instead of reinventing it. Both amateur and professional gardeners have published lots of ways to measure soil moisture, but I found that some amateurs did better in telling others how their system works.

In learning how these sensors worked, I had to learn about two main ways that soil moisture is measured; either resistance to the flow of current through the soil, or something called the dielectric properties of the soil. In each section I will try to explain what these terms meant to me. And if you are reading them hoping to study for an electrical engineering exam. …. let’s just say that the sum of my electrical engineering knowledge comes from Dick Smith’s Fun Way Into Electronics, Google, and my mate Paul.

If you want to know more about how soil responds to electricity read here.

When reviewing these projects¬†I rejected some of these methods outright. Some I rejected because they are less accurate than sticking my finger… in the soil (get your head out of the gutter). And I don’t have a rich relative who is close to… I mean a patron who has left me the income stream from a trust fund to fuel my visions for lush gardens.

How I measured soil moisture in this project

If you would like to skip to the good bit, I ended up using the soil as a variable capacitor that regulates the output of a Clapp Oscillator.

Methods I rejected

If you want, you can read about the methods I rejected:

What I learnt from reviewing previous soil moisture sensors

MethodIndicative cost to develop
(coffee as the reference)
Indicative cost to develop
(tea as the reference)
Ease to developAccuracy
Resistive methodsThe instant coffee that your office provides.Anderson's Quality Tea
(comes in a box of 10,000 tea bags)
A 6 year old could do it in an hour.Rubbish
Low frequency capacitive methods
(e.g. measuring capacitor charging times, Analog Device microchips, cheaper low-pass filters)
A cappuccino.BushellsAn afternoon off work.Questionable
Higher frequency capacitive methods
(e.g. Texas Instruments microchips, more expensive low-pass filters)
Your own coffee machineA selection of teas from T2Lots of time spent reading and a supportive family.Accurate
Time domain reflectometryYour own cafe.A trip to Sri-Lanka / Japan etc.You'll need a Bachelor of Science with Honours and a few monthsMost accurate

From the table, I found that there is a trade-off between developing a DIY system that is low cost but is also accurate. I decided to go with the higher frequency capacitive measurements since this offered accuracy at a cost that I am willing to incur.


To go back to the main page: Main Page.

Leave a Reply

Your email address will not be published. Required fields are marked *