Revealing invisible objects with a projector

Hi! I need to reveal some object in my game with a special light (or just inside a circle radius). However I'm not a shader developer and can't find any built-in shader in Unity for my purpose.

I suppose that I could use a projector and apply a shader to blend alpha to the desired materials but I have no idea where to start looking for info or examples for it.

Could you please help me or give me some clues?

Many thanks!