This thesis addresses the development and validation of a low-cost system for
detecting biological signals using functional near-infrared spectroscopy (fNIRS).
fNIRS is a non-invasive optical method that enables monitoring of changes in tissue oxygenation and perfusion based on light absorption at specific wavelengths.
The aim of this work was to design a robust and affordable sensor suitable for
basic biomedical measurements, such as detecting heart rate and responses to res-
piratory manipulations. The developed system is based on two light sources (660
nm and 940 nm), a photodetector, and two separate sound cards serving as inter-
faces for signal generation and acquisition. The mechanical design of the sensor
features a 3D-printed housing, allowing stable and reproducible placement. The
software, developed in the LabVIEW environment, enables synchronous generation
of the modulation signal, data acquisition, and lock-in demodulation, thereby
improving the signal-to-noise ratio and allowing the detection of weak biological
signals. The experimental work consisted of two targeted tests. The first test
aimed to validate the device’s operation by detecting heart rate and comparing
the results with a reference device. The second test evaluated the device’s re-
sponsiveness to controlled physiological changes induced by breath-holding. Both
tests were complemented with electrical engineering and biomedical analyses of
the acquired signals, enabling a multi-layered interpretation of the results. The
results demonstrate that the developed fNIRS sensor reliably detects changes in
optical density corresponding to the cardiac cycle and hemodynamic responses
during breath-holding. The device design enables stable measurements even with
minimal components, confirming its suitability for further development. The conclusions highlight the device’s potential for research and educational purposes,
while also providing a basis for future upgrades, such as multi-channel systems
or portable solutions for home monitoring of vital functions.
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