My neighbor asked me (a mechanical engineer) to invent a device consisting of a distance meter and a passive target to determine the distance of an object from reader. Usage could be in a home, outside, anywhere. If distance exceeds a selected value, the reader puts out a warning signal. Intent here is to develop a low cost consumer product.
I thought about RFID and did a very quick look, but admit I'm way out of my element here.
I agree with the above comments, but eventually you will come to a programming problem. Just to get you started, you should first check to see what frequency is more appropriate (that depends on the maximum reading distance you need). After that you need to also check that the frequency allows for the evaluation of a distance dependent indicator (like signal strength - RSSI). After these steps you need to concern yourself with the application you wish to build.
