► Schematic diagram of the
circuit
►
Design calculation
According
to the electrical characteristics in the datasheet, the minimum value of R1 is,
(5V-1.7V)/50mA = 66 ohm
Therefore, we set R1 as 100 ohm.
Empirically,
when the reference voltage of OP-Amp in non-inverting input was set as 3.5V
(maximum value in the spec), the output voltage of OP-Amp was 3.1V for black
tape, 1.7V~2.0V for green tape, and 0.8V for white board. The output of the OP
amp was connected to Port AD, and the reading was 600~650 for black tape,
300~400 for green tape, and 120~140 for white board at the maximum reading of
1023.