Methods I Rejected: Resistive Sensors

Methods I Rejected: Resistive Sensors

Soil Moisture Sensor Project

Methods I rejected: Resistive Sensors

The first type of sensor I reviewed and rejected are the most common DIY soil moisture sensor – resistive sensors.

Resistive sensors are useless in an automated watering system. Resistive sensors are toys which come up quickly on Google. They are incredibly easy to build and don’t need a science or engineering degree to understand.

They rely on the fact that moisture levels in the soil affect the flow of electricity; the higher the soil moisture content, the lower the resistance to the flow of electricity. Resistance to the flow of electricity is usually provided by an electronics component called a resistor. The value of the resistor is measured on ohms, named after a dead guy called Georg Simon Ohm. You can use soil as a resistor; you simply stick two pieces of metal (or other probes) into the soil, hook a battery and resistance meter up to the probes, and then measure the resistance to the flow of current through the soil.

Throw your probes away

Once you are done measuring the resistance of the soil to the flow of electricity, throw your probes away. I’m a bit rusty on my chemistry, but I remember that the movement of electrons from one side to another is called a galvanic, or redox reaction. When you are running current between two probes, you are moving electrons between the probes. This causes one of the probes to corrode. How long it takes your probes to corrode will depend upon how much current you have run through them, what they were made from, and what is in your soil.

But what about alternating current?

It is possible to use alternating current (where the current flows back and forth – just like what comes out of your wall power points) instead of direct current (where the current continuously flows in one direction – such as from a battery) between those electrodes. This means that the galvanic reaction that occurs if the current is continuously flowing in one direction will be reversed each time the current changes direction. You could also use graphite electrodes (because graphite does not corrode), but then you are still running a current through your soil and I do not know what effect this will have on the minerals in the soil (not to mention any poor worms who just so happened to be passing through at the time you want measure the moisture content of the same bit of soil that the worm was enjoying).

You could take the electrodes out of the soil once you have measured the moisture content, but if you have a friend to do this for you while you are on holiday you could just ask them to water the garden.

How do resistive sensors work?

To find out how resistive sensors work, look at how a voltage divider works. A voltage divider is simply a battery with two resistors connected in series (end to end).

http://umdberg.pbworks.com/f/1456081980/SeriesCircuit.jpg

All of the voltage of the battery has to be expended once the current of electrons have passed through both resistors. How much of the available voltage is used up by one of the resistors depends on the resistance (measured in ohms and signified by the Greek letter omega) of that resistor compared to the total resistance of all resistors in the circuit. The amount that is expended across each resistor is worked out by dividing the value of that resistor by the sum of resistors in the circuit, and then multiplying the result by the voltage supplied to the circuit.

For example: our circuit has a 10 volt battery, a 200 ohm resistor, and a 800 ohm resistor.

  • 200 ohm resistor: 200 / (200 + 800) = 0.2 x 10v = 2v used up by this resistor,
  • 800 ohm resistor: 800 / (200 + 800) = 0.8 x 10v = 8v used up by this resistor.

All 10 volts must be expended by the time that the current makes its way around the circuit. This rule holds for any voltage divider circuit, regardless of the number or  value of the resistors.

In resistive sensors, the electrodes in the soil are just a resistor with an unknown resistance that is connected in series with another resistor of a known value. You apply a known voltage to the circuit and measure how much voltage is expended between the electrodes in your soil. If the soil is wetter, the resistance to flow of current will be lower, and less voltage will be expended. This voltage is inversely proportional (when one goes up, the other goes down) to the amount if moisture in the soil.

The best way to learn is to read many different ways of explaining the same concept before coming up with your own. So this, this, and this are all good links to writeups on voltage dividers.

Example soil resistive sensor

In an example resistive sensor using soil as one resistor, lets assume that:

  • the resistance of dry soil is 900 ohms and the resistance of wet soil is 100 ohms
  • the known resistor is 100 ohms
  • the battery is 10 volts.

For wet soil, the voltage across the soil is: 100 ohms / (100 ohms + 100 ohms) * 10 volts = 5 volts

For dry soil, the voltage across the soil is: 900 ohms / (900 ohms + 100 ohms) * 10 volts = 9 volts

The voltage across the probes in the soil will be lower if the soil is wetter.

Conclusion

In short, resistive sensors are great way to experiment with a microcontroller and any associated circuitry that you have bought. But their downsides mean that they are not useful as part of your automated watering system.

After rejecting resistive sensors, I spent time research methods that are based on the dielectric properties of soil: time domain reflectometry, microchips that measure capacitance, low pass filters and measuring how long it takes to charge a capacitor.

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

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