Different types of transmitter receiver systems??

DUMB FRESHER POST ALERT !!

Hi all.
I'm not the best with electronics, okay with the basics and I love to take things apart, sometimes I put them back together so bear with me.

I've tried looking for different types of transmitter/receiver systems but can't seem to find much info anywhere.
Whether there aren't that many systems out there or I'm just searching incorrect terms I don't know but a list of different types of systems and descriptions is what I need.

If anyone can do one better than that and narrow it down to what types will suit my application, it would be most appreciated.
I'm in need of some sort of system that will have a powered control unit and multiple separate units (as small as possible and ideally unpowered) that will trigger an audio/optical alarm in the main powered unit when any of these multiple units comes within a specified range. But here's the thing that may take it a shelf higher, ideally, I will need to be able to adjust the range of signal on the main powered unit.

Can anyone help on this one or know of anyone that can design me this type of system? Paid, of course.

Thanks.
Dean.

 

Thanks bluejets, sounds like just the technology I'm after.
I'll have to do some digging and see what comes up.

In the meantime, if anyone has knowledge of any other similar types of technology, I'd love to know about them.

Cheers.

 

Actually, now you mention it, I suppose Tesco must use the same system on their trolleys as the wheels lock on if you push them past the boundary of one of their car parks.

I don't make a habit of stealing Tesco trolleys by the way!!