I'm not exactly sure I understand your idea--you need high-power on specific frequencies to trigger passive RFID tags--because your RF has to power them. Many are designed to be readable at under 10 inches because they're expecting the RF source to be nearby. So to 'detect' RFID tags of various and unexpected varieties, you'd have to radiate large amounts of energy on many frequencies and at the same time, listen for responses on many frequencies.

The itty-bitty RF signals (three meter's max?) that comprise blue-tooth communication aren't the right frequencies or forms to trigger RFID tags to respond. You can, however, make a big-noisy 'detector' and if you detect something, you can pass that information on to your host using bluetooth.

So your question is a little like "can you detect TV's with a cellphone." If you've got another method to 'see' the TV (your eyes), you can call someone and say, "yes, I see six TV's" but the signals from the phone can't trigger a response from a TV and really the TV's are only designed to repond to signals from TV towers anyway. The cellphone or the bluetooth are only good for communication.

Also, you can try this test with 'active' RFID tags, that respond using their own power, so they don't have to absorb it first--but they still have to be triggered to respond. This would be a little like finding the TV's because they were already turned on and up-loud. Easier to find, but the cellphone didn't find them, you need another method to find them.

Not directly with Bluetooth, unless you make special directly bluetooth compatible RFID tags, which may not be technically reasonable.

What would be reasonably possible is a Bluetooth RFID reader, which would be an input device for your handheld computer.

Bluetooth is a frequency-hopping radio protocol in the 2.40-2.48GHz band with limited power and sensitivity. RFID is a radio-powered passive and single frequency radio protocol in 3 bands: 30-500kHz (2m range), 850-950MHz (30m+ range) and 2.40-2.50GHz (30m+ range).

While bluetooth operates in the same band as the most expensive RFID devices, the protocols are completely different and designed to co-exist without interference or the ability to interoperate.

If your bluetooth device is software-defined (these days, many are), and if the software is user modifiable (good luck on that one), and if the bluetooth hardware can emit a sufficiently powerful pulse to activate the RFID device (unlikely) and if the bluetooth hardware is sufficiently sensitive to detect the reply from the device (unlikely given the 90dB difference in signal strengths between the activation signal and the sideband response), then maybe, just maybe, it can be done. Even then, the vast majority of RFID devices would be in the wrong frequency band.

I think separate RFID hardware would be a far easier path than cracking a software defined radio.

All this technical stuff! Let's try another approach.

Scenario:Freddy Forgetful keeps losing his hat, umbrella, pen, scarf, etc. If it's not physically attached to him, Freddy puts it down in a public place and walks off without it. The only thing Freddy doesn't leave lying around is the cell phone in his pocket.

Idea:Make Freddy a cell phone application that (1) tracks whether a tracking tag is within a 10 foot radius (2) alerts Freddy when the tag is outside the 10 foot radius. Put a tag on Freddy's hat. Now, if Freddy leaves his hat somewhere, his phone will tell him to go back and get it before he gets very far.

Please help Freddy keep track of his stuff!

P.S.: Freddy's phone has Bluetooth. Freddy understands that Bluetooth can wirelessly detect things and junk. Could Bluetooth help make the tracking system Freddy needs to remember his hat?

I had exactly the same idea a couple of years ago. It just can't work with phones as the transmitter because of the huge amount of power needed to cause the RFID to respond. You're basically trying to induce a current in a circuit with a sphere of energy that decreases in strength proportionally to the square of the radius (unless you have a directed antenna). And the current has to be strong enough for the circuit in the RFID to be able to respond. Not to mention all the troubles with the differing frequency bands of Bluetooth/Wifi, the software issues with bluetooth, etc. (Nihil did a good job analyzing this). In other words, your cell phone will, in the conceivable future, always be blind/deaf when it comes to non-active (or non-powered) RFID tags.

The only way this could conceivably work would be to have a transmitter box in each place you go (your house/apt, your car, the bar/restaurant on the corner, the library, work) that "talks" to your cell phone and tells it periodically that the RFID tags that it knows about (pre-programmed into the phone) are still around it. The box would sound an alarm if the phone stopped talking to it (you've left the area) and it still sees your RFID tags. There are an enormous amount of problems with this though ... what if your phone runs out of batteries? What if the phone can't communicate with the box (bluetooth or wi-fi out of range) but the box still manages to communicate with the items you've tagged with RFID? What if someone's box has been hacked into and it breaks into your phone somehow? How feasible is it for every place (including every floor of your office building for instance) to purchase one of these boxes? What if someone uses it for tracking people's whereabouts?

Anyway, it's a good idea because like Freddy Forgetful, I'm very absent-minded with my keys, wallet, cell phone, scarf, etc ... in fact I lost my wallet on Halloween.

This does not necessarily apply to cell phones (a clever concept).I have been tracking rf tag for years. They are cheap now but no one seems to be going in the direction of home use to locate items.This would not only be for the standard items like keys, glasses and wallet, but could include tools, computer parts, documents. etc.Perhaps a hand held device with a data base you could maintain on your computer? This would change my life cause I can never find anything when I need it.Just think of the unfinished projects I could finish!