Autonomous soil moisture sensor based on nanostructured thermosensitive resistors powered by an integrated thermoelectric generator

An autonomous single heat pulse probe porous ceramic soil moisture sensor powered by a thermoelectric generator (TEG) is presented. The sensor uses nanostructured thermosensitive resistors fabricated on the same ceramic substrate of the TEG. The nanostructured resistors, fabricated by printing PbS quantum dots, present a very high thermal coefficient (−16 × 103 ppm/°C) and, used in a bridge configuration with conventional precision and low thermal coefficient SMD metal film resistors, result in a high sensitivity temperature sensor. A laboratory prototype of the sensor showed a voltage variation of 2.4 mV in the output of the bridge when the volumetric water content of the soil changed from 5% to 40%. To complete the autonomous system, we designed an ultra low-power electronic interrogator which, when powered only by the 3 F supercapacitor of the integrated TEG energy harvesting system, was able to take daily measurements up to 5 days without harvesting energy.