In the following we present a low-cost optical system for cracks evolution monitoring. The transducer principle is based on the variation of light transmitted between two facing fibers with their axial distance, exploiting the unique light-collecting capabilities of large-core polymer optical fibers. The characterization shows that the working range of the transducer is up to 3.5cm, with a resolution of 10$m and a repeatability of 5$m. With our current custom-developed control unit, up to four transducers can be arranged to provide a multi-axial displacement sensor or to simultaneously monitor four points for distributed sensing. A closed-loop light modulation/detection scheme is implemented to reduce the environmental noise sensitivity. The control unit is interfaced to a PC via USB port or via GSM/GPRS modem to automatically send periodical reports of the measurements and to issue warnings in case of displacements above a given threshold. Extensive tests in an environmental chamber have been carried out in order to extract calibration curves and to compensate for day-night and summer-winter temperature fluctuations.