I really don't know much about RFID so this is not only an arduino feasibility question but an RFID feasibility question. I want to be able to detect if an RFID is within about a 6 to 10 ft radius of my device. It doesn't have to be super accurate, but should be at least within that range. I could maybe go down to 5 ft but not lower.
I figure I could have to scan on low power for the RFID to save energy, then increase the power only if necessary to find it(I would like to eventually make this thing portable but I realize that would require something other than an atmega328, another question for another day though).
The only thing is from my very light research it appears I need a UHF RFID to get that kind of range. Does that make it way more expensive? The tags would NEED to be passive. And the project is pretty useless if they are expensive.
An RFID tag contains a small chip. The RFID reader generates a magnetic field around an antenna. That magnetic field is all that powers the chip to do it's thing. When the RFID chip does it's thing, the magnetic field is altered, and the reader senses the change in the magnetic field to determine the data sent by the chip.
The distance that the RFID chip can be excited is a function of the power in the magnetic field. Since magnetic field strength drops off logarithmically based on distance, RFID systems that can read chips beyond a few millimeters require very strong fields, very precise antenna designs, and lots of power.
I figure I could have to scan on low power for the RFID to save energy
Scan what on low power? The presence of an RFID tag/chip can not be detected unless the power is high enough for the range the card is from the reader.
then increase the power only if necessary to find it
It? What is it? The RFID tag/chip? No, it doesn't work that way.
And the project is pretty useless if they are expensive.
They? The tags? No, the tags can be cheap. They are, after all, passive. It is the high power/precisely designed antenna based RFID system that is expensive.
Buy whatever it is you are trying to protect a good passive key lock, and use it.
My goal would be to eventually make this thing handheld. I know that at that point I would no longer be able to use the arduino, but that aside could a handheld battery powered device be used to read an RFID tag from at least 5 ft away? Or is that not currently possible?
I don't think you can get that kind of distance from passive RFID and a hand-held reader. Most passive RFID works in the range of an inch or less. To get longer range I think you would be limited to how small you could make the antenna and still get the necessary sensitivity.
Perhaps an active RFID system such as the OpenBeacon design would get the distance you want without too much expense. It has been used to track conference attendees at several conferences. It would be interesting to see how much they would cost to produce in quantity.