I recently had to build an infrared close proximity sensor that could tell if something was dropped in a parcel delivery drop box. After some searching, I ended up using a microcontroller and a piece ...

Proximity and inductive displacement sensors suffice for the vast majority of applications ... For accurate details on relative performance specifications, best to consult the datasheets of vendors ...

[mircemk] shows how to create a simple non-contact proximity sensor using little more than an Arduino Nano board, and a convenient software library intended to measure the value of capacitors.