In this study, we used eight photodiodes (OPT101, manufactured by Burr-Brown, Tucson, AZ, which is a monolithic photodiode with on-chip single supply amplifier) and three multiwavelength light-emitting diodes (LEDs) (L4*730/4*805/4*850-40Q96-I, manufactured by Epitex, Kyoto, Japan) [1]. These LEDs send three different wavelengths at 730, 805, 850 nm, in one package (we used two wavelengths 730 and 850 nm). Since each LED sends two wavelengths, we will need six drivers for three LEDs. However, we can design only one current source; thus, need to use an analog switch to select one of the LEDs to be fed by the constant current source. We used two single-pole/single-throw (SPST) analog switches.

The ATMEGA328P-PU was used to select different LEDs and wavelengths and also to control the main board. Figure 1 shows the arrangement of LEDs and photo detectors [2]. The source-detector separation in our system is 2.5 cm. Using...

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