Imagine a disk spinning like a record player turn table Half of the disk is black and the other is white Assume you have an unlimited number of color sensors How many sensors would you have to plac?

I think it needs only three sensors. Place each sensor at 120 degrees apart. senseor A at 12'O clock position, senser B at 4' O clock position and Senser C at 8' O clock. Before start spinning the wheel identify the location of the colors on the sensers two of them will be one color (say black) and one will be the other color (white). Before start spinning the disc say sensor A and B are at the same color and C is other color. Start spinning the disc whatever direction you like. Keep on reading the colors when the color on A changes read the color on sensor C if C didnt change the color the disc is spinning in the clockwise direction. For anticlockwise read the sensors B and C. ======================================= I would place 2 sensors (A & B) at circumference, place A @12'O clock position and B @3'O clock position. Get the time (TA1) when color of A changes then the time (TB1) when color of B changes and the time (TA2) when color of A changes again. If ((TB1 -TA1) < (TA2-TB1)) then disk is spinning clockwise else anticlockwise. =================== No no - you only need two sensors. You place them close together. Once the first sensor to fire on black followed by the second sensor shortly will indicate the directional rotation of the disk easily. This is common place in industrial automation. They even sell single sensors that have two detectors on them for this purpose.