help.. rf proximity alarm

hi there, i am trying to build a circuit that will eventualy become an rf proximity alarm of sorts. it must be small and low powered with a transmitter and reciever. if the reciever gets within aproximately 5 to 6 feet of the transmitter it will trigger the alarm. can anyone help please. i used to build a lot of electronic projects and am a mere amature and have been out of the loop for a lot of years. in other words simple circuits are my option. any help would be greatly appreciated.

What do you consider low power? You can buy turnkey transmitter and receiver modules. How much are you willing to spend?