Using oscillators to measure soil moisture….

Using oscillators to measure soil moisture….

Soil Moisture Sensor Project


By now you may be thinking “Brendan, you have an awesome beard. But are you really going to develop your own method?”

Thankyou. Yes I am.

“Where have I seen an oscillator before?”

Radio transmitters all have one circuit in common: an oscillator. The term “oscillator” just refers to a circuit that can create a regular series of pulses. You are surrounded by circuits that are oscillators. Your car’s indicator light is connected to an oscillator with a very low frequency, about once per second. Your microwave has an oscillator that produces radio waves with a frequency of 2,450MHz. Since the output from some oscillators is so regular, the phone or computer you are reading this on will have little oscillators that help to keep the time, the same as the oscillator in the watch you are wearing. What I hoped was that I could create an oscillator whose frequency would change depending upon the capacitance of the soil.

Types of oscillators

I found three types of oscillators whose output frequency changed with varying capacitance: relaxation oscillators, resistor-capacitor (RC) and inductor-capacitor (LC, if you thought “IC”, sorry, it’s actually LC, I don’t know why).

I quickly rejected relaxation and RC oscillators because they work best at low frequencies. There are a few projects that have used this type of circuit, all of which operate below 1MHz (“Diy bloke“, Boston university, Lucky resistor). While they don’t measure how long it takes to charge up the capacitor, their output is dependent upon how long it takes for that capacitor to charge up. We’ve already read why we are not interested in how long it takes for a soil capacitor to charge up.

LC Oscillators

This left LC oscillators. LC oscillators work by storing electrical energy in the capacitor and inductor. The inductor and capacitor won’t store this energy at the same time but will pass a very small amount of energy back and forward to each other. When one is storing the energy, the other is releasing its stored energy to the other one.

In short, both of these components respond in a different way to changes in the flow of electricity. All an LC oscillator does is take advantage of the fact that they behave differently and amplifies the fact that current is passed back and forth.

If you want to know why capacitors and inductors don’t store energy at the same time, head over to Google and search for the term “LC tank circuit”. But be warned, if you want to know how an LC oscillator works, you will need to learn about Barkhausen criterion, loop gain, phase shift, integrals, and maths…. lots of maths.

There are a few types of LC oscillators that are all named after someone. Four names kept popping up: Hartley, Colpitt, Vackar, and Clapp. When deciding between them…I’m not an electrical engineer, I just use google. What I found was that the Colpitt was more stable than the Hartley and there is relatively little written about the Vackar (probably because they were not British or American). The Clapp is variation of the Colpitt that has a variable capacitor which can be used to tune the circuit.

So, by this point I have decided to use something called a Clapp oscillator (which should lead to a round of applause). Okay, so finding this wasn’t too hard…

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