Your requirements are far from complete (the story is too short) to give reasonable advise. can you tell more about where it is installed (e.g. specific room, or must it be used mobile etc)What is it s purpose e.g. lock the garage.etc
I hope I am providing sufficient detail.
QuoteI hope I am providing sufficient detail.No, How near is near in feet/meters?You can consider wifi, 433MHz, Xbee, bluetooth - these all have different ranges (from 1-30 meter max)longer ranges -> you might need gsm shields and GPS.
By "entered it's range" do you mean that the two devices have gotten close enough to be able to exchange data, or are you thinking that you can measure the distance somehow?