# RFID project, need some guidance on reading multiple passive UHF RFID tags

Good morning everyone,

I'm currently involved in a senior level capstone project (practical design and implantation course), a requirement for an engineering at my University. Alongside two other engineering majors, we are currently in the process of creating an interface to convert temp data from passive RFID devices onto a web sever (ie an RFID reader to a wi-fi shield via possibly an ardunio).

Part of my shared responsibilities with one of the other engineering students involves the hardware aspect (the RFID reader and the MPU). Doing some independent reading last summer, I was able to get a general understanding of NFC ( a subset of RFID).

The intresting problem I'm pondering is how can interpret multiple RFID devices at the same time without overwhelming the Ardunio. What I'm thinking as possible solution is pulsing the RFID antenna (the number of pulses equaling the number of RFID devices), sending the original signal and the returned (RFID modified) signal through a comparator, then using Boolean logic on the composite signal to get the component data. The component would then be stored and transmitted via Wi-Fi.

Example:

Device 1) 00 00 00 Device 2) 00 00 01 Device 3) 00 00 02 Device 4) 00 00 03

Assumptions, 1) Ardunio is tied into a crystal oscillator tuned ~ 915 Mhz (UHF RFID) 2) Crystal oscillator is tied into a transistor (Magnetic field strength is proportional to the square of current, Biotâ€“Savart's law) 3) RFID device ID's are known are stored on the Ardunio 4) RFID packet lengths are the same length, therefore the not logic applies to the data streams/arrays starting with the Device ID's)

Ardunio sends a pulse to the antenna (duration ~ 5 microseconds) Received signal: Composite signal of Device 1,2,3 and 4 Ardunio then Boolean logics the signal as NOT(2,3,4)

Ardunio sends a pulse to the antenna (duration ~ 5 microseconds) Received signal: Composite signal of Device 1,2,3 and 4 Ardunio then Boolean logics the signal as NOT(1,3,4)

Ardunio sends a pulse to the antenna (duration ~ 5 microseconds) Received signal: Composite signal of Device 1,2,3 and 4 Ardunio then Boolean logics the signal as NOT(1,2,4)

Ardunio sends a pulse to the antenna (duration ~ 5 microseconds) Received signal: Composite signal of Device 1,2,3 and 4 Ardunio then Boolean logics the signal as NOT(1,2,3)

Is there a more effective way of handling this/ am I missing anything?

Thanks, Joe.

915MHz does not use NFC, so I'm uncertain why you mention it. UHF RFID is electromagnetic in nature, with fractional wavelength antennas.

Digital interference like that is going to be destructive.

You need to either use a very directional antenna on the reader (it takes a lot more than just a transistor fed a 915MHz signal and it -doesn't- use a magnetic loop) and tags spread out physically so that you can select them by aiming the antenna at them, or use smart tags that will wait to respond until they've been polled.

thanks polymorph for the clarification, I was thinking plan B being the smart tags (need to contact the manufacturer if they offer the option)

I would have thought though that the 915 mhz band would use a magnetic loop.

Joe M.

No. A half-wavelength at 915MHz is only 16.4cm.