Methods I Rejected: Low Pass Filters

Methods I Rejected: Low Pass Filters

Soil Moisture Sensor Project

Low pass filters:

When reviewing methods of measuring soil moisture I didn’t use low pass filters because they are hard to use to measure soil moisture at a reasonable cost.

You have all heard low pass filters in action, especially if you you are a bassline junkie or are just addicted to bass. If you’ve ever wondered why you have “subwoofers” and “tweeters” on your speakers, it is because each type of speaker is better at creating sounds with different frequencies. Subwoofers are better for keeping your neighbours awake at night with your drum and bass. Tweeters are better for vocals and higher pitches. A “low-pass” filter lets the low frequencies pass through to the subwoofers. Similarly a high pass filter lets high frequencies through to the tweeters.

Simple filters will weaken the frequencies they don’t want passing through. A low pass filter allows low frequency waves to pass through unweakened while weakening higher frequencies. The higher the frequency, the greater the weakening. You can make a low pass filter from a resistor and a capacitor. How much each frequency is weakened is determined by the values of the resistor and capacitor.


A capacitor acts like a resistor when it is exposed to an alternating current. The value of its reactance changes according to the frequency that is passing through it. (If you thought it would be called resistance, don’t feel bad, so did I). When the frequency is higher, the reactance (resistance) is lower. When the frequency is lower, the reactance is higher. If the frequency is zero… the resistance is infinite! A capacitor blocks direct current.

(Note: about nine months later, after trying to use low pass filters, I realised that a filter circuit becomes a voltage divider when it is exposed to an alternating current)

Simple Low and High pass filters


Remember that the amount of moisture in the soil will affect the capacitance. If you use soil as the capacitor in a low pass filter, the soil moisture will affect the cut-off frequency of the filter. As you add moisture to the soil, the soil capacitance increases. Therefore, the cut-off frequency decreases.

I found two projects that take advantage of the fact that high frequencies are weakened by low-pass filters (CHIRP and Vitor). Each of them use a low pass filter to weaken high frequencies. The amount that the frequency is weakened depends upon the cut-off frequency, which changes as soil capacitance changes.

If you decrease the cut-off frequency of a low pass filter, you will further weaken a high frequency signal.
Graph of signal weakening for two cut-off frequencies. This graph is a bit blurry because it is from my notebook.

This graph shows how different cut-off frequencies will affect a high frequency. F1 and F2 are two different cut-off frequencies. Let’s just say that F1 is for dry soil and F2 is for wet soil. We are measuring a frequency that is higher than either cut-off frequency. This means that in dry soil it will be weakened by 40% , but in wet soil it will be weakened by 80%. (In reality, people don’t use percentages when talking about signals, they talk in decibels. Why? Because they always have)

You can determine the cut-off frequency by measuring how much the signal has been weakened after it was travelled through the low pass filter. If the signal is weaker, the cut-off frequency must be lower.  The cut-off frequency is lower because the value of the capacitor in the low pass filter is higher. This means that the soil must be more moist.

This sounds promising….. But…… The drawbacks to these projects are that CHIRP uses a relatively low frequency (1MHz, and we want frequencies of above 70 MHz to minimise the effect of minerals in the soil) and while the Vitor project runs at 100MHz, it uses components that are relatively expensive (>$20 for the main componenets on element14).


Unfortunately low pass filters can be expensive like time domain reflectometry or inaccurate like some microchips.

But I didn’t reject the idea of using soil as a capacitor.

Go back to review of previous sensors, or main page.


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