by Chris Woodford. Last updated: July 9, 2013.
How many times have you walked through a store's doors and—to your extreme embarrassment—set off the anti-theft alarm? It's surprisingly easy to do, even when you've paid for your item and had it "deactivated" at the checkout. Anti-shoplifting alarms use a technology called RFID (radio-frequency identification), but the same technology has many other uses too, from tracking pets to collecting fares from bus passengers. Let's take a closer look at this cunning new technology and find out how it works!
Photo: A typical RFID gate in a shop doorway. There's another one of these on the other side of the door. Sometimes these scanners are disguised so you can't see them, but more often they're designed to act as a very visible deterrent to shoplifters.
Radio or wireless is a way of transmitting energy through empty space—that is, instead of using a wire cable. The energy is carried by invisible waves of electricity and magnetism that vibrate through the air at the speed of light. The basic science and the practical technology of wireless communication was developed in the second half of the 19th century. During the early 20th century, "radio" came to mean audio programs beamed through the air from giant transmitters to cumbersome electronic boxes sitting in people's homes. When inventors found a way of sending pictures, as well as audio, television was born. Today, all kinds of things work using the same wireless technology, from digital radio and television to cellphones (mobile phones), and wireless Internet.
Radio and television involve sending radio waves in one direction only: from the transmitter at the radio or TV station to the receiver (the radio or TV set) in your home. Wireless Internet and cellphones are more sophisticated because they involve two-way communication. Your cellphone, for example, contains both a radio receiver (to pick up an incoming signal from the person you're speaking to) and a radio transmitter (to send your voice back to the other person). Radar is another technology that uses radio waves. Planes and ships fitted with radar transmitters send out beams of radio waves and listen for echoes—reflected radio beams bouncing back off other planes and ships nearby. Anti-shoplifting devices are a little bit like radar: they beam radio waves out into a store in the hope of catching a stolen book or CD as it passes by. But how exactly do they work?
Photo: Right: An example of a "race track"-type anti-theft RFID tag like the one used in book labels. You can often see these by holding library paperbacks up to the light.
How RFID works
Imagine your mission is to design an anti-shoplifting device using some old radio sets you found in the garage. You could build something a bit like a radar (with a combined radio transmitter and receiver), sit it by the shop doorway, and point it at people passing by. Radio waves would pass out from your transmitter, bounce off people walking past, and then reflect back to your radio receiver. The trouble is, this wouldn't actually tell you anything useful, because everyone would reflect the radio waves in exactly the same way! You wouldn't know whether people were shoplifting or not, because there would be no way to distinguish shoplifters from ordinary customers or people who hadn't bought anything at all. What you'd really need would be for shoplifters to reflect radio waves in a different way to everyone else. But how?
Anti-shoplifting devices have cracked this problem. As well as having a transmitter and receiver at the doorway, every item in the store contains a concealed RFID "tag". In bookstores and libraries, the tags are very discreet labels, stuck to one of the inside pages. In record stores, the plastic shrink-wrap may have an RFID label stuck onto it, or CDs may be locked into large plastic cases with RFID tags built into them, which can be removed only be a special tool at the checkout. In clothes stores, there is typically a chunky plastic tag bolted onto each item with a sharp metal spike. Some of these tags are concealed so you can't spot them. Others are deliberately very obvious and easy to see—so they deter you from stealing. The gates on the doorway are another very visible deterrent to shoplifters.
If you walk through the doorway without paying for something, the radio waves from the transmitter (hidden in on one of the door gates) are picked up by the coiled metal antenna in the label. This generates a tiny electrical current that activates a tiny, concealed chip or other electronic component buried in the label). The chip modifies the incoming radio waves and sends them back out again in a slightly different form (either by modifying the waves or by transmitting a unique identifier code). The receiver (hidden in the other door gate) picks up the modified signal (or identifier code) and sounds the alarm. Why doesn't the alarm sound when you pay for something? You may have noticed that the checkout assistant passes your item over or through a deactivating device. This destroys the electronic components in the RFID label so they no longer pick up or transmit a signal when you walk through the gates—and the alarm does not sound.
Photo: You can see the metal tracks of the antenna in this RFID tag quite clearly in this photo. It was quite hard to peel this out of the book I bought without damaging it. RFID tags need to be stuck firmly and relatively hard to remove in a shop.
Passive and active RFID tags
Anti-shoplifting labels are called passive RFID tags. Instead of containing batteries, they work entirely by responding to the incoming radio waves from the doorway transmitter. There is just enough energy in those radio waves to activate the RFID chip. Passive tags can send and receive signals only a few meters—enough to cover a doorway, but not much more. An alternative form of RFID technology, known as active tags, contain more advanced chips and batteries. They can send and receive signals over much greater distances.
What does an RFID tag contain?
Passive RFID tags contain just three components:
- The antenna—catches incoming radio waves and sends them back out again.
- The chip—generates a unique identifier code for the particular tag.
- The substrate—the backing material (typically paper or plastic) to which the antenna and chip are fixed.
As you can see from this photo, most of the space in an RFID tag is occupied by the antenna: the oval-shaped tracks around the edge. The antenna needs to be this big both to pick up radio waves from the transmitter and (because there are no batteries) to convert them into energy to power the chip. The chip itself is tiny—sometimes as small as the point of a pencil.
Anti-shoplifting tags are sometimes even simpler than this: instead of containing a chip and generating a unique identifier code, they just modify the incoming radio waves and retransmit them in a different, easily detectable form. These tags are sometimes known as transponders or chipless RFID tags.