We present here the design and fabrication of a self-powered and autonomous fringing field capacitive sensor to measure soil water content. The sensor is manufactured using a conventional printed circuit board and includes a porous ceramic. To read the sensor, we use a circuit that includes a 10 kHz triangle wave generator, an AC amplifier, a precision rectifier and a microcontroller. In terms of performance, the sensor’s capacitance (measured in a laboratory prototype) increases up to 5% when the volumetric water content of the porous ceramic changed from 3% to 36%, resulting in a sensitivity of $S=15.5$ pF per unity change. Repeatability tests for capacitance measurement showed that the ${\theta }_v$ sensor’s root mean square error is 0.13%. The average current consumption of the system (sensor and signal conditioning circuit) is less than 1.5 $\mathsf{\mu }$ A, which demonstrates its suitability for being powered by energy harvesting systems. We developed a complete irrigation control system that integrates the sensor, an energy harvesting module composed of a microgenerator installed on the top of a micro sprinkler spinner, and a DC/DC converter circuit that charges a 1 F supercapacitor. The energy harvesting module operates only when the micro sprinkler spinner is irrigating the soil, and the supercapacitor is fully charged to 5 V in about 3 h during the first irrigation. After the first irrigation, with the supercap fully charged, the system can operate powered only by the supercapacitor for approximately 23 days, without any energy being harvested.

Thermoelectric generators (TEG) can harvest solar energy during the day using solar flat panels. They can also benefit from the use of a material that stores solar energy to generate additional power at night, when the panel cools down and the energy stored in this material travels back, through the TEG. The soil can be used as the material that stores solar energy, but the performance of such systems, with the heat sink buried in the soil, depends on the ambient and the soil temperature, parameters which can change drastically with the latitude of the location where the TEG is installed. We present an experimental study with the comparison of the potential energy that can be collected from a TEG system with heat sink buried at different depths and at different latitudes: Campinas, Brazil − ${22}^{\circ }$ 54 ${}^{\prime }$ 20 ${}^{″}$ S; and Mataró, Catalonia, Spain − ${41}^{\circ }{32}^{\prime }{17}^{″}$ N. The potential of energy harvesting calculated during 32 winter days in Campinas is 72% of the total calculated during 205 days in Mataró. Experimental results obtained from a complete TEG system showed that in Campinas, during one day, it was possible to store 34.11 J of electrical energy in a supercapacitor. Notably, we demonstrate that the energy generated during the night by the heat stored into the soil can be as high as the energy generated during the day.

Solar radiation and human activity generate ubiquitous temperature gradients that could be harvested by thermoelectric generators (TEGs). However, most of these temperature gradients are in the range of very few degrees and, while TEGs are able to harvest them, the resulting output voltages are extremely small (a few hundreds of mV), and DC–DC converters are necessary to boost them to usable levels. Impedance matching between TEGs and DC–DC converter plays a fundamental role in the energy harvesting efficiency. Therefore, it is essential to determine the output power of the system in different configurations, in order to decide on the optimum TEG connection. Here, we present an electronic circuit to measure the maximum power that can be harvested with low-voltage TEGs connected to a DC–DC converter. The developed circuit is an electronic controlled load that drains the maximum current from the output of the DC–DC converter while maintaining its output voltage at the maximum allowed value. Using a mechanical set-up able to apply precise low temperature gradients between the hot and cold side of the TEGs, experimental data using different configurations of TEGs are obtained. The measured results show that, for ultra-low voltages, the TEG ensemble’s output impedance plays an important role not only in the amount of the energy scavenged, but also in the onset temperature of the energy harvesting.

Heat dissipation sensors operate based on the temperature dependence of the transient heat conduction within the soil, which is a function of the soil characteristics and its water content. After a heat pulse with controlled energy is applied to a heater, it is possible to show that the maximum temperature rise Δ TM measured in the temperature sensing element can be related to the volumetric water content of the soil θv [m³ , m⁻³]. The design and fabrication of a low-cost soil moisture multiprobe heat pulse sensor system using conventional printed circuit boards and surface-mount devices is presented. The proposed sensor is free of the needles' deflection problem present in conventional multiprobe sensors and is manufactured using conventional off-the-shelf electronic components. A precision low-power electronic signal conditioning circuit, using an instrumentation switched-capacitor building block, was developed and successfully used in the prototype. Due to an energy-efficient topology for the sensor and a low-power signal conditioning circuit, the average current consumption of the system (with one measurement per day) is only 3 μA. To demonstrate the feasibility of the concept, a prototype of the sensor was tested in soils with volumetric humidity in the range from θv = 0.05 m³ , m⁻³ to θv = 0.41 m³ , m⁻³ and, with a very low heating energy pulse (3 J), showed a sensitivity, normalized by the total energy applied, Γ = 211 x 10⁻³ °C , m³ , m⁻³ , J⁻¹. Compared with a button heat pulse probe sensor which has Γ = 192 x 10⁻³ °C , m³ , m⁻³ , J⁻¹, the developed sensor shows a higher normalized sensitivity.

We present a capacitive smart sensor based on printed circuit board (PCB) technology to measure the amount of water content in a paper pulp at the wet end of a paper machine. The developed sensor incorporates in the same PCB the signal processing circuits. It is a handheld portable device, and its output is sent to the reading equipment using a Bluetooth wireless connection, providing to the sensor’s operator ease of mobility around the wet end of a paper machine. The prototype was tested in a laboratory, using a wire mesh to emulate the end of a paper machine, and we were able to measure and easily detect when it reaches the water content in the range of 90% to 92%, as required in the paper fabrication process. Standard deviation of the capacitance measurements at various moisture levels is four orders of magnitude smaller than the mean. The smart sensor was tested in the 20°C to 40°C temperature range, in a paper pulp with a gravimetric water content of 91%. Since the variation of capacitance with temperature is practically linear, we propose a simple linear compensation equation to correct the observed sensitivity with the temperature. To keep the signal processing circuits small, low cost, simple, and robust, a novel direct interface sensor to microcontroller circuit technique was used to make the capacitive measurement, allowing for measuring small capacitance deviations without high-frequency oscillators. It was shown that it is possible to integrate the signal processing circuits in the top layer of the PCB interdigitated sensor without adding noise or degrading the performance of the capacitive sensor.

We present the design and fabrication of a reconfigurable smart-sensor for measuring moisture content (MC) and temperature in grains stored in silos, using a capacitance fringing field interdigitated sensor and using a resistance temperature detector (RTD), both implemented on a single conventional printed circuit board. The sensor was tested in laboratory, using corn kernels with percent MC in the range of MC $= 8\%$ to MC $= 32 \%$ and, using a simple relaxation oscillator and a frequency-to-voltage converter, we measured a capacitance variation of $\Delta C = 4.99$ pf, with a sensitivity $S = 0.21 \, \rm {pF}/\%$ MC. The RTD sensor, which uses the copper resistance of the PCB tracks, was characterized in the 10 $^\circ \text{C}$ – $50~^\circ \text{C}$ temperature range and the measured results presented a very linear behavior ( $R^{2} = 0.99888$ ) when compared to the measured temperature values using a commercial sensor (LM135). A modified bridge circuit was developed, where a linear behavior of the differential output was obtained. The developed configuration allowed for the independent adjustment of both the offset and the gain in the output voltage $V_{\text {out}}$ of the bridge. The bridge signal processing circuit was calibrated using an endpoint method, and the measured points between the calibration points presented a maximum endpoint nonlinearity error of $|E_{\text {nl}}| =\,\,0.64\,\,^\circ \text{C}$ .