Designing a Clapp Oscillator to measure soil moisture

Designing a Clapp Oscillator to measure soil moisture

Soil Moisture Sensor Project

Designing a Clapp Oscillator

A Clapp Oscillator became the first method I wanted to use to measure soil moisture. The frequency of the oscillator would change depending upon the capacitance of the variable capacitor. (For those who are just tuning in, you can use soil as a variable capacitor by changing the amount of moisture in the soil).

Firstly, a quick search for “Clapp oscillators” showed me lots of different designs. Some used a few components and others had very elaborate designs. I don’t want an incredibly complex design because I am not building a radio. I want something that is simple.

Eventually, I came across design on the All About Circuits forum. It was simple; two capacitors, an inductor, a resistor, and a type of transistor called a JFET (junction field effect transistor). It also ran at a high frequency: 150MHz. Perfect. Or so I thought…

When I went searching for the component that was used as the transistor….

Picture of the 2n5484 with a note that it is obselete
The 2n5484 is obselete…..

…. it was marked as “Obsolete”.

Unfortunately, the internet is good at telling you which part to use. But no so good at telling you why that part is better than any other part. Unless you are an engineer who understands these:

Screenshot of a datasheet
A screenshot of from a weighty tome (engineers call it a datasheet)

Or these:

Another screenshot from a datasheet
More dark arts


Can I design my own circuit?

I could have just bought a few transistors that other people recommended and then tinkered until something worked. Or I could design the circuit on the computer and then test it out. Which is what I did.

I won’t go into any detail on how I simulated the circuit or what program I used. There is already a lot written on the internet about how to do this. And each program has its own idiosyncrasies that will drive you mad unless you are trained to think like an engineer.

The good thing about circuit simulation software is that you can change parts of the circuit with a few clicks of a mouse. You can change a part and re-run the simulation until it works. Which is how I replaced the obsolete part.


Brendan’s thoughts on designing this project (art vs science):

Dark Arts -(*)————————————————————————– Science

Picture of Snape
Yep, I’ll need a bit of dark art to get this going.

Now, time to build a prototype.

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