Falling objects and suggestions on sensors to use

dougp:
Are the objects all unique?

No. They are different by material, size and shape.

wvmarle: What a choice... Fewer (but probably more expensive overall) sensors + a mighty complex bit of software to try to trace back what happened, or a slightly greater number of sensors that simply tell the sketch "I've got a piece here!", and also can instantly tell when one is leaving it's perch. I think I'm a tad less desperate to bring down the number of sensors.

My choice is due to some physical constraints in my environment which make easier install less sensors. Furthermore, less sensor = less wirings and (I hope) a less vulnerable system. Anyway, this is my very first experience with Arduino and I'll keep ready to jump back if something goes wrong. Let me thank you for you warning.

What's at the bottom? Do you even have space to mount 'column' sensors? The emitter at the top will be easy but the receiver at the bottom might be blocked by a speck of dust at the wrong time.

MorganS: What's at the bottom? Do you even have space to mount 'column' sensors? The emitter at the top will be easy but the receiver at the bottom might be blocked by a speck of dust at the wrong time.

That's one of the challenges to be faced, but I'm confident in regular cleaning receivers. Some perplexities about where to mount receivers as some objects falling could damage them.