## 1 Introduction  
Due to space variability [1], precision agriculture and advanced irrigation management systems require the use of a large number of reliable and accurate soil water content sensors [2]. Although many types of soil water content sensors have been developed (time-domain reflectometry [3], tensometers, electrical resistance blocks, electromagnetic conductivity meters [4]), one of the most used low-cost soil water content sensors are the heat pulse probe sensors. The various types of heat pulse probes (button heat pulse probes (BHPP) [5, 6], dual heat pulse probes (DHPP) [7], and single heat pulse probes (SHPP) [8]) operate based on heat capacity measurements.  
The SHPP sensors (discussed in this work) have a heating element and a temperature-sensing device inserted inside a metal cylinder filled with thermally-conductive epoxy, which is put in direct contact with the soil. A heat pulse is applied to the heating element for a fixed period of time (typicallyin the range of 10-30 s), and at the end of the pulse, the temperature rise is measured using the temperature-sensing element.  
The temperature rise in the probe inserted into the soil depends on the thermal conductivity of the material that surrounds the sensor. Since the heat capacity of the soil depends on its water content, the temperature rise measured in the sensing element can be related to the soil water content [9].  
However, if the contact between the probe and the soil is not perfect, the soil water content measurement results can be very inaccurate, and a very efficient solution to this weakness is to manufacture the SHPP sensors encapsulating the heating element and the temperature sensor within a porous block [10,11].  
The operation of porous block soil water matric potential sensors depends on the characteristics of the porous material. When a sensor with a porous block is inserted into the soil, the water flows between the porous ceramic and the soil until an equilibrium is reached, even if the the sensor's contact with the soil is not good. By measuring the thermal conductivity of the porous ceramic filled with water, accurate estimations of the water content in the soil surrounding the sensor can be obtained. The use of a porous ceramic block is so efficient in overcoming the soil contact problems that even capacitive sensors have been fabricated using porous blocks [12].  
Recently, we presented a novel technique for the fabrication of SHPP sensors, where the metal cylinder with the heating element (resistor) and the temperature-sensing element (thermistor or thermocouple) were replaced by a single bipolar transistor, which acts both as the heating and temperature-sensing element [13]. Although there are many advantages in using such a sensor, because it is very low cost and the fabrication is extremely simple (the sensor is actually a commercial transistor), it suffers from the soil contact problem.  
In this work, we present a highly improved version of the single transistor soil water content sensor, obtained by encapsulating the transistor in a porous ceramic block (to mitigate the problems originated from the uneven contact of the sensor's surface with the soil). We also designed a new signal conditioning circuit, providing the possibility of autonomous operation of the system when powered by an energy-harvesting system (such as those presented in [14]), by using a low-power heat pulse and an ultra-low-power signal conditioning circuit.