The Soil Temperature Sensor


You are building a soil temperature sensor. We need a waterproof thermometer that we can burry in the soil. The sensor is made up of a small circuit enclosed in a piece of heat shrink (to waterproof it).


Supplies:
(see the parts page)
  • an LM335 temperature sensor
  • a 33K and a 56K resistor
  • a short piece of glue-lined heat-shrink tubing
  • soldering iron and solder
  • lead wires



The circuit for the actual sensor

This is the circuit for our waterproof sensor (not to be confused with the local circuit below).

The resistor in this circuit is optional. You can just get the value of the LM335 raw and use it in calculations (see below). But if we want to know the actual temperature we will need to use a resistor to calibrate the sensor.

If you do want to use a resistor, you can make things simpler to wire up in the garden by soldering the resistor(s) inline with this sensor. See more detailed info below to see why this is technically not correct. This resistor is to calibrate the LM335 temperature sensor. The actual value of the resistor will vary from unit to unit. You will need to calibrate it by trying different resistors.

The ground leg will be tied to the common ground connection with your other sensors. The output leg will need a signal wire to get back to the brain where it will be connected to the local circuit for this module.




Step-By-Step

Here are the steps to building one of these water proof sensors.

Solder the two resistors together.
Attach the two resistors to the ADJ leg of the LM335. Attach a piece of lead wire to the positive leg that will reach to the end of the ADJ leg.
Use a piece of heat-shrink to cover the two-resistor leg. Wrap the end of the positive leg lead wire around the newly covered ADJ leg.
Attach a lead wire to the new combo positive-ADJ leg. Attach a lead wire to the negative leg of the LM335.Use another small piece of heat-shrink to cover the negative lead wire where it connects to the LM335.
Now use a larger diameter piece of heat-shrink to cover the entire package.
While the heat-shrink is still hot, it is still pliable. So you can squeeze the package (careful, it's hot) and help the heat-shrink take the shape of the package. Notice that the tip has been squeezed with pliers.
You can even clip the end to get rid of the extra material. Just be sure not to clip too far and break the seal.
Notice that the end is glued together without opening. This assures complete waterproofing.






The local circuit

Here's the diagram for the local circuit portion of this module.

The circuit is mainly a voltage divider - the soil temperature sensor is one half and the 1K resistor is the other half.

There is also a noise filter - the 0.1uF capacitor going to ground. And of course, an output so the Arduino can take a sensor reading.




And here is what the circuit might look like on your local circuit breadboard.





More detailed info

So technically we should not package the resistor alongside the temp sensor. Here's why:

The reason to make a soil temp sensor is so that we can use it to offset the simple soil moisture sensor. The reason for this is because soil resistance changes with temperature. So to add another layer of accuracy, we can include a temperature sensor near our moisture sensor. Then we can use the temperature to offset the data from the moisture sensor to help mitigate the false dry reading caused by changes in temperature.

Remember that the resistor we are soldering alongside the LM335 is an ADJ (adjustment) resistor. This means that it is there to calibrate the sensor properly. BUT, if resistance changes with temperature, then our adjustment is changing with temperature also. Oops.

This effect is negligible enough for the job this sensor is doing, but for more accurate results you would want to move the ADJ resistor to the local circuit board to shield it from those direct-sunlight temperature effects (even the inside of your house changes temperature).


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