Apologies for a novice question. I'm looking into designing a system which would detect presence of certain objects on say a shelf. I think (not sure) I need to use "load cells" for that. Is that so? How expensive those things are? Where to get them (I need about 100)? The objects I need to detect weights between 0.5kg to 20kg. It doesn't have to be very precise, just need to detect presence of a single object.

Thanks!