Analogue-digital interface for low-cost sensors in low-power sensing networks

The ever-increasing application of sensor networks in many different fields is causing a growing demand of low-cost passive sensors for monitoring physical variables such as temperature, pressure or ambient humidity. These sensors need a conditioning circuit that allows an easy interface to a microcontroller, taking advantage of the full range of the sensor and reducing the microcontroller requirements. This paper presents a conditioning system designed to transform the output of low-cost resistive sensors to a digital value. The system consists of a voltage to frequency converter circuit and the implementation of a frequency to code algorithm programmed on a low-power microcontroller. The conversion circuit was designed to use the full frequency range available, providing a good resolution while the resulting quasi-digital signals are compatible to the logic levels of a standard low-power microcontroller. The algorithm provides a suitable accuracy at low requirements in microcontroller resources.