## 6 Conclusions  
A soil water content sensor based on a single transistor encapsulated in a porous ceramic block was fabricated and tested in the laboratory. The sensor was tested with volumetric water contents in the ceramic in the range of \(6\%<\theta_{v}<46.4\%\). With the porous ceramic saturated (\(\theta_{v}=46.4\%\)), the temperature difference measured was \(\Delta T=2.9\,^{\circ}\)C. For a dry ceramic (\(\theta_{v}=6\%\)), the value of the \(\Delta T\) was \(6.3\,^{\circ}\)C. The sensor's sensitivity, calculated in the worst case condition (between \(25\%\leq\theta_{v}\leq 46.4\%\)) was \(S=0.023\,^{\circ}\)C/%. The 15-bit A/D converter used can read the \(V_{BE}\) of the transistor in steps of \(36.6\)\(\upmu\)V, which corresponds to a temperature resolution of approximately \(0.02\,^{\circ}\)C, so \(\theta_{v}\) can be read in increments of at least \(0.87\%\).  
When tested inserted in a red latosol, the sensor presented an almost linear behavior for small changes in the metric potential of the soil, between 1 kPa and 35 kPa. In this small region, the sensor presented a sensitivity of \(S=0.015^{\circ}\)C/kPa. The sensor presented in this work is a great improvement over the previous work because it does not have the soil contact problem (as it is based on a porous block), and the addition of the porous block does not reduce the energy efficiency of the sensor. The novel ultra-low power circuit conditioning circuit developed was designed to be used powered by an energy-harvesting systems. When powered by a 2-F supercapacitor, the signal conditioning circuit operated continuously for eleven days, making one soil water content measurement per day.  
Conceptualization, P.C.-D.; formal analysis, P.C.-D.; investigation, J.A.S.D., P.C.-D., F.J.O.M. and L.F.C.D.; methodology, J.A.S.D., P.C.-D., F.J.O.M. and A.C.; software, F.J.O.M.; supervision, J.A.S.D. and A.C.; validation, J.A.S.D. and F.J.O.M.; writing, original draft, J.A.S.D. and A.C.; writing, review and editing, J.A.S.D. and A.C.  
We thank Hidrosense Ltda for making the measurements of the sensor with the Richards chamber and for supplying the porous ceramic blocks used in the fabrication of the sensor.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; nor in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
A/D Analog-to-digital  
BHPP Button heat pulse probe  
\(C_{Stor}\) Value of the supercapacitor  
DHPP Dual heat pulse probe  
\(E\) Energy in the heat pulse  
\(I_{C}\) Collector current  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensor \#** & \(S_{1}\) & \(S_{1}\) & \(S_{2}\) & \(S_{2}\) \\ \hline
**Condition** & **Saturated** & **Dry** & **Saturated** & **Dry** \\ \hline Meas. \#1 & 865.49 & 859.45 & 866.68 & 859.08 \\ Meas. \#2 & 865.40 & 860.18 & 867.14 & 859.17 \\ Meas. \#3 & 864.89 & 859.86 & 867.09 & 859.22 \\ Meas. \#4 & 865.12 & 860.09 & 866.64 & 858.58 \\ Meas. \#5 & 864.89 & 860.05 & 866.27 & 858.76 \\ Avg. & 865.16 & 859.93 & 866.76 & 858.96 \\ Std. Dev. (\(\sigma\)) & 0.28 & 0.29 & 0.36 & 0.28 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Measured values of \(V_{BE}\) (mV) in two sensors (\(S_{1}\) and \(S_{2}\)) with two different water content conditions: dry and saturated.