Sorry for the lack of updates still dealing with leg issues and unfortunately lately the pain has been winning 
.
I was able to get some time in my shop today and did some more testing with my prototype sensor.
Using a 8" sensor probe with ~1 twist per inch I got the following results
| Media | location 1 | location 2 | location 3 |
|---|---|---|---|
| Free air | 17.15kHz | 17.15kHz | 17.15kHz |
| Dry dirt | 16.75kHz | 16.80kHz | 16.70kHz |
| Nominal dirt | 11.5kHz | 10.76kHz | 10.15kHz |
| Wet dirt | 9.13kHz | 9.42kHz | 9.14kHz |
| Glass of water | 8.39kHz | 8.47kHz | 8.42kHz |
| Dried off after water | 17.57kHz | 17.57kHz | 17.57kHz |
Each location above is in the same sample just the probe was removed and move to a different spot in the container.
At a given location the readings stability seemed to be +/- 0.05kHz.
Sensor in free air was very sensitive to what was placed around it (especially humans) but once inserted in the soil 1/2" away from the side of the pot was not significantly influenced by touching the outside of the pot.
Also tried cutting the sensor down to 5" long
| Media | reading |
|---|---|
| Free air | 20.8kHz |
| Dry dirt | 19.2kHz |
| Normal dirt | 12kHz |
| Wet dirt | 10kHz |
In both cases the % difference between bone dry and the other samples are within a few % So looking like the exact size and length of probe does not affect the dynamic range by much. I did noticed that "fluffed" soil will result in dryer readings than compacted. (makes sense less contact with wet soil)
My current circuit setup is running ~66% duty cycle. I adjusted my resistor values to get to 52% and did not see much affect on the output readings.
Next up I want to lay out the sensor board. I think I am going to go with SMT devices to make it easier to encapsulate (has to live in damp conditions) and add provisions for dual sensor probes. That way in shallow pots it will be easy to add a second probe in parallel to the first for increased sensitivity.
While I am waiting on parts and raw cards I am going to try and set up a simple data logger using the prototype circuit and the Psoc 4. Once that is done I want to set up and monitor one plant over a few weeks. One to verify the long term operation of my probe and second to see if there is any significant temperature variations. (will show up as a cyclic wave form on a 24 hour period due to my automatic setback thermometer)
