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The availability of rapid and low-cost instruments to detect magnetic nanoparticles (MNPs) concentrations is vital in giant magnetoresistance (GMR)-based biosensors. This paper reports a new setup for a simple GMR sensor using the commercial chip AAL024 as a transducer. It was combined with a basic differential amplifier and microcontroller to acquire digital output voltages for the detection of green-synthesized (GS)-Fe₃O₄ MNPs as a label and streptavidin-coated MNPs in biosensor applications. As a characteristic feature of Fe₃O₄, the GS-Fe₃O₄ MNPs displayed a cubic inverse spinel structure. The average GS-Fe₃O₄ particle size was 11 nm and they exhibited soft ferromagnetic behavior with a saturation magnetization (M_S) of 55.5 emu/g. Owing to the presence of phytochemical components in the Moringa oleifera (MO) extract, the M_S of GS-Fe₃O₄ was lower than that of Fe₃O₄. To study sensor …