I've been an RFID software/firmware specialist for the past 8 or 9 years, and I've been thinking (on and off) about developing an RFID project for one of the netduino platforms. As far as Ive seen, no one has really done much of anything with RFID on the netduinos.
For your application, you would need to use a UHF band around the 900Mhz range, most likely C1G2 (class one, generation 2) - which can read passive (unpowered) tags up to about 30 meters (100 feet).
Most C1G2 readers provide a values known as RSSI (Relative Signal Strength Indicator) in the response from a tag, but it is not as straightforward as you might think, to use this value to determine actual distance - especially using only one reader. UHF signals are notorious for bouncing around and arriving via circuitous routes (this is called "multipath") - so you wouldn't be able to rely on that measurement too closely.
But if you want to experiment with UHF/C1G2 RFID there are many off the shelf products you can play with. One that Ive had my eye on is this one: http://www.digitimes...D=4455&tag=2035
It's a C1G2 USB dongle - you could plug it into your mobile phone and then the software to do what you wanted to do would be relatively easy (although, because of multipath, it wouldn't be 100% reliable as a distance measurement)
There are also non-USB C1G2 modules that someone could use with the netduinos, and a simple little SPI or TTL driver, such as this one: http://www.rfidinfo....02954448600.pdf - wiring one of those up to a standard STM8 module should be trivial, provided you can scour through the forums to find the correct information on how to build the STM8 standard firmware - something Ive yet to be able to do.
- JerseyTechGuy likes this