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Specialized Sensor Series #3 – RFID

Part 3 Of A Series Of Three Articles Looking At Flow Sensors, Water Quality Sensors, And RFID.

by Mike


At Phidgets, we try to make a wide variety of sensors available to our customers. If we can’t make the sensor ourselves, we try to find a manufacturer so we can sell a third-party sensor to fill the gap. The purpose of this series of blog posts is to highlight a specific class of unique sensors and walk through what options are out there and which ones will work with your Phidgets devices.

This post discusses Radio Frequency Identification (RFID) readers and tags. RFID tags are electronic devices that broadcast a unique identification number over radio frequency when prompted by a nearby reader. The reader receives the ID and makes it available to the software in the system, which typically uses the unique ID to access information about the object attached to the tag. Tags come in all shapes and sizes, from cards to keychains to wristbands. For example, if your office has ID cards that are used to open doors by waving them near a sensor on the wall, it’s very likely that it’s using RFID. Phidgets Inc. sells an RFID reader and a modest selection of compatible tags, but there’s a huge variety available in the online marketplace. We’ll go into the details in order to determine which tags are compatible with a particular RFID reader.


The modulation frequency of the tag and reader’s radio signal is the most important factor that separates RFID systems. This is because many factors depend on the tag frequency, including read distance, price, and what kind of tags are available. Most RFID systems fall under one of three classifications: low frequency, high frequency, and ultra-high frequency.

Low Frequency (LF) : 120 – 150 kHz

Low frequency tags and readers are commonly used for personal projects because of their widespread availability, low reader cost, and lack of regulation. Tags that operate in the LF band usually have a read distance of about 10cm. Other common uses include animal identification, door access control, and hospital patient identification. Phidgets Inc. sells an LF RFID reader/writer: the PhidgetRFID.

High Frequency (HF): 13.56 MHz

The main advantage of moving up to HF RFID is the increased read distance. Depending on the antenna used on the reader, read distances of up to 1 meter can be achieved. Another advantage of higher frequencies is that the antenna on the tag doesn’t need to be as big. This opens up new options for the shapes and sizes of tags you can use. HF radio communication is regulated, so there may be a power limit on your reader’s antenna for public use depending on where you live. Another downside of higher frequency tags is that they are more susceptible to interference from nearby metal and water. HF tags and readers see use in personal projects as well as book tracking in libraries.

Ultra-High Frequency (UHF): 433 MHz

Again, increasing the frequency of the reader yields greater read distances; for UHF readers and tags, you can read at 10 meters or more. UHF is often used for vehicle identification in parking lots and toll booths, and other large-scale applications. UHF readers cost significantly more than LF, so you’ll only see them used in applications that absolutely require the read distance advantage.

Higher Frequencies

RFID that operates at frequencies higher than UHF are all lumped into the “microwave” category. In most cases, they have a similar read distance to UHF, but also have the fastest read speed. Microwave can often be found in industrial applications.


Each RFID tag has a chip and an antenna. The chipset of an RFID tag determines many of its features, including

  • Which protocol(s) the tag can communicate to readers with
  • Whether the tag is writeable, or read-only
  • Whether a writeable tag can be “locked” after writing
  • Encryption and security options

The special features of the tag’s chip can only be used if they’re also supported by the RFID reader you’re using.


Often the chipset of a tag is confused with the protocol it uses to communicate. This misconception is due to the fact that some chipsets use their own protocol with the same name. Most of the time, tag sellers will simply list the protocols and features of the tag, since most customers are not familiar with the capabilities of a specific chipset. The protocol describes the exact data pattern that the tag will use to transmit its data during a read event. If you’d like to have an in-depth look at the EM4100 protocol used in many LF tags (including those sold at Phidgets), Priority 1 Design has a fairly straight-forward explanation of the details.

Form factor

RFID tags come in a variety of shapes and sizes depending on the task they’re designed for.

Cards and keychains

These are usually used for membership or access control for customers. For example, a physiotherapy office may use a RFID membership card to quickly bring up your treatment history instead of using a filing system. Some gas stations use a keychain that you can tap to pay for gas quickly.


These are commonly used with patients in hospitals or other medical facilities, particularly one where the patients aren’t capable of communicating important medical information to the staff.


Some RFID tags come in sticker form, which makes them ideal for tracking books in a library. This type of tag is usually only available for tags in the HF band or higher, because LF tags are less efficient and require a thicker antenna.

Special Application

Because of the wide range of applications where RFID is useful, a number of specialized tags are being made available. From leg rings that can be used to identify birds to nails and screws that can be tapped into trees, there are plenty of tags being specially designed.


In conclusion, the factors that determine compatibility between an RFID tag and a reader are as follows:

  • The reader must support the modulation frequency of the tag
  • The reader must be designed to communicate in the protocol that the tag uses
  • The tag’s chip must have any special features you plan on using (such as writing capability, encryption, or anti-collision), and the reader must also support these features.

For example, in order for a tag to be compatible with the PhidgetRFID Read-Write, it would need to operate in the LF band (125 to 150 kHz), communicate with a supported protocol (EM4100 series, ISO11785, FDX-B, or PhidgetTag).

As a quick aside, you may have heard of a technology called “NFC” or “Near Field Communication”. NFC is similar to RFID in the sense that it also uses readers and tags to identify objects, but NFC moves some of the complexity into the tag. In NFC, instead of just chirping back an ID when read, the tag communicates back and forth with the reader and usually stores its own data. In some cases, the tag/reader dichotomy disappears and you end up with two identical devices communicating wirelessly (for example, two NFC-enabled phones exchanging information). Generally, NFC isn’t compatible with RFID because they use different communication protocols, but if you can match up the protocol and frequency, you may be able to achieve compatibility.

This quick overview should help you decide what kind of RFID will work for your project, and how to find compatible readers and tags. Thanks for reading and don’t hesitate to comment below.