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In this paper, we propose a novel approach to respiratory rate measurement using resonance tube to enhance the performance of microphone inserted and fixed at the end of the tube to catch breath sound signal from the mouth and/or nose. The signal is amplified and passed into envelope detector circuit after which it is compared with a suitable reference voltage in comparator circuit to generate a pulse train of square wave synchronized with the respiratory cycle. A simple algorithm is developed in a small microcontroller to detect rising edges of each consecutive square wave to calculate respiratory rate together with analysis of breathing status. In order to evade noises which will cause errors and artifacts in the measuring system, the reference voltage is creatively designed to intelligently adapt itself to be low during expiration period and high during inspiration and pause period using the concept of resolving contradiction in the theory of inventive problem solving (TRIZ). This makes the developed device simple and low-cost with no need for complicated filtering system. It can detect breath sound as far as 250 cm from the nose and can perform accurately as tested against End Tidal CO2 Capnography device. The result shows that the developed device can estimate precisely from as low as 0 BrPM to as high as 98 BrPM and it can detect shallow breathing as low as 10 mV of breath sound.

Novel Approach to Respiratory Rate Measurement Using Resonance Tube with Contradictory Thresholding Technique