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My original soil moisture monitor is shown in Figure 1. Moisture from the soil of a potted plant would enter the a housing through a tube containing a STM-332U Temperature Sensor module configured with a HSM-100 Humidity Sensor module to determine the moisture level of the soil.
It was not clear that the humidity sensor would provide a good indication of the soil moisture so I added a soil resistance measurement to the soil moisture monitor (see Figure 2). The soil resistance measurement is made using a 10 k Ohm resistor and a soil resistance probe connected as a resistor divider connected between Switched Power (SWPWR) and ground. The center tap of the resistor divider is connected to the ADIO0 input of the STM-332U temperature sensor. The soil resistance probe is made out galvanized nails that as space close together. The two nails act as a variable resistor in the presents of water and ions in moist soil.
Temperature Sensor Modifications
I modified the STM-332U as shown in Figure 3. A 1206 size resistor was soldered between ADIO0 and ground. Wires with a connector on the end were soldered to ADIO0 and SWPWR. The connectors would allow the soil resistance probe to be easily attached. To protect the module form surges a 1k Ohm series resistor was added between the soil resistance probe and the connectors.
Temperature Sensor Configurations
The STM-332U was configuration was modified using DolphinView as shown in Figure 4. This module was set up to report temperature, humidity, set point, and occupancy (A5-10-10 EPP). I also reduced the threshold for reporting changes in humidity and set point. This did not provide a very fast update rate so I also set the module to wake up and report every 16 seconds.
Figure 5 shows the physical modifications to the STM-332U temperature sensor and the HSM-100 humidity sensor before and after it is installed.
Figure 6 shows the smallest transparent box I could find. It was in stock and available for immediate delivery. I though it was rather big from the specs at the time I ordered it but it turns out to be very well made and a good size for this prototype. I could not find rigid plastic pipe and fittings of the small enough diameter for the box at my local hardware store so I used flexible tubing and a press on fitting to couple the tubing to the box. To support the tubing and the soil resistance probe, I used a ¼ inch threaded rod.
1k Ohm protection resistors were soldered into each lead of the cable going to the soil resistance probe. These resistors would protect the temperature sensor IC from static discharges. The soil resistance probe was made from galvanized nails. Heat shrink tubing was used to protect the solder connections and some of the tubing was used to hold the nails in place. The wires from the nails were run through the tubing into the box. Tie-wraps were used to hold the tubing to the threaded rod.
With the moisture monitor assembly complete, the modified STM-333U and HSM-100 were attached to the soil resistance probes and placed in the housing (see Figure 7).
The soil moisture monitor is approximately 2 feet long (see Figure 8) and should be easily placed in most medium to large potted plants.
Moisture Monitor Tests
Figure 9 shows my first soil test. The soil moisture monitor was placed into a previously opened bag of potting soil. The FHEM reports the set point at 141 which would indicate a soil resistance of around 10 K ohms (9.98k produces a set point reading of 130). For some reason, FHEM would not report both the set point and humidity in real time. I am still troubleshooting my FHEM configuration to determine the cause.
I used DolphinView to view real time changes in the humidity and soil resistances. Figure 10 shows my test to determine how well the two moisture monitor techniques work. I cleaned the soil probe after the soil test and placed it a shallow bucket of clean tap water and let it sit for a while. The set point reading for soil resistance ranged between 174 and 177. The humidity sensor reported 62% during this test.
I clean and dried the soil probe and let it sit for a while. The set point reading for soil resistance was 0 as expected and the humidity reading spiked to 66% and slowly settled to around 59% (It’s raining here today).
Then I placed the moisture monitor into the soil of an actual potted plant. The set point reading jumped up to 181 and the humidity reading is about 59%.
Figure 11 shows the moisture monitor operating and measuring the soil resistance and humidity of a potted plant.
The soil moisture monitor was able to measure humidity and soil resistance, but the sensitivity is very course. The humidity sensor in this configuration is not effective at all at determining the soil moisture. My guess is that the moisture flow into the plastic box housing is too low. The soil resistance probe worked way better than I expected but still need work to know what soil moisture levels make for a health plant   .
 Wikipedia, Soil Resistivity
 GardonBot, Soil Moisture Sensor
 UMassAmherst, Measuring Soil Moisture