Autonomous Soil Water Content Sensors Based on Bipolar Transistors Encapsulated in Porous Ceramic Blocks

• Published in: Applied sciences 2019-03, Vol.9 (6), p.1211
• Main Authors: Carvalhaes-Dias, Pedro, Morais, Flávio, Duarte, Luís, Cabot, Andreu, Siqueira Dias, J.
• Format: Article
• Language: English
• Subjects: bipolar transistor; Calibration; embedded circuits; Energy harvesting; Energy management; Heat; Heat conductivity; heat dissipation soil moisture sensors; Heat transfer; Irrigation; Irrigation systems; Laboratories; low-power circuits; Moisture content; porous ceramic; Porous materials; Power management; Semiconductor devices; Sensors; Soil management; Soil temperature; Soil water; soil water content; Thermal conductivity; Transistors; Water content
• ISSN: 2076-3417; 2076-3417
• DOI: 10.3390/app9061211

We present an autonomous sensor to measure soil water content that uses a single heat pulse probe based on a transistor encapsulated in a porous block. The sensor uses a bipolar junction transistor, which performs as both a heating and temperature-sensing element. Since the sensor depends on a porous block to measure the matric potential of the soil, it does not suffer from accuracy problems if the contact between the probe and the soil is not perfect. A prototype of the sensor showed a temperature variation of Δ T = 2.9   ∘ C when the porous ceramic was saturated with water. The sensor presented an almost linear behavior for small changes in the matric potential of a red latosol when tested in the 1-kPa and 35-kPa pressure range, showing a sensitivity of S = 0.015   ∘ C/kPa. The ultra-low power signal conditioning circuit can read the sensor’s temperature with a resolution of approximately 0.02   ∘ C, so the matric potential can be read in increments of at least 1.33 kPa. When powered only by a 2-F supercapacitor from the energy-harvesting system, the interrogation circuit is able to take one soil water content measurement per day, for eleven days.