Pipe Tube blockage checking

[DaveEvans]

You could count the seeds. If the count stops, the pipe is blocked.

Ideas in this link. The concepts work for even very small seeds, smaller than the 7mm noted.

Maize seed detector and counter

I am aiming to develop a maize seed counter for one of my projects. The maize seed has an average of 7 mm of diameter. The seed has to pass through the area of 25 mm x 25 mm where the detection is to be performed. I tried mini laser diode coupled with ambient light sensor but that mechanism covers one 13 mm of the width. What are the other possible options that i can try to detect maize seeds efficiently in the said area?