Simulate a barcode reader

dragolollo

Hello,
I have the following problem: a conveyor on which 3 different kind of boxes (same lenghts and widths but different heights) are moved; when a box reaches the end of the conveyor, sensors must recognize which kind of box it is. Now, since the boxes have different height, I have used 3 different line sensors (at different heights) to simulate a bar code reader. Can it be possible to simulate it in another way and/or maybe using only one sensor?
Thanks!