Hi there Nikos,

A Hall Effect sensor is used to either count or measure the duration of a magnet going by.
It is typically mounted near a drive shaft or wheel. A small magnet is attached to the moving part.
There are two basic methods of caclulating the speed of the magnet going by. One is to count the number of times it goes by in a given time frame. Or, measure the amount of time it takes to go by in a single revolution.
Both methods have tradeoffs. Research the COUNT and the PULSIN commands.
Interfacing the calculator display is way too wide open. There are so many variables it's hard to address.
Good luck as you climb the PBP learning curve.
Bill H