Two componet sensing device

[reglem]

Hi, I'm new in electronics and don't know all the right words and stuff! But... I do need some help in the right direction. I want to build a sensing device to sense when device "A" is close to device "B". Like triggering a switch when device "A" is close to device "B". Must be small and light and running on small batteries. Is this possible??? Any advice will be great!!!

I seem to be asking something that is impossible!!

There must be a way... there's always a way!!

[Twidget]

there are a lot of ways. But that all depends on the application. So you need to be more specific.