I need to build a device with three parts one is a transmitter and the other the receiver and when ever the two are within a certain predetermined range a signal is sent to the third via a buzz. My question is how do I achieve this and what type of circuit would that be called so I can download some schematics on it.

One thought would be using RFID tag on one device, reader on second device. Second device could also have microprocessor like Arduino with simple program and then transmitter (such as Xbee) to receiver (matched Xbee pair) on third device. There are lots of online Arduino forums and resources that could help.

Or use a bright led light that the other sensor can read and register brightness as distance. Would have to sample ambient each time to set baseline for comparison

Hi computabugz and welcome, if you just need a proximity device why not just buy?Inductive Proximity Sensors Overview (http://www.automationdirect.com/adc/Overview/Catalog/Sensors_-z-_Encoders/Inductive_Proximity_Sensors_-z-_Proximity_Switches)
Or is this a school project?

I got the impression that this needed to discern two specific objects, not one object and any light (@tyterhun)source, or any metallic (@parttime)object--is this correct?