This has probably been asked before, but I couldn’t find it after a search.
Here in Houston, we have tollways. You can get a little box called an EZ-Tag to fix on your windshield; it allows you to drive through the tollgates without having to dig for quarters and throw them in the bucket. A sensor reads the tag and automatically bills our account for the toll.
In addition, I have a little card in my wallet to get into the parking garage and the office building where I work. All I have to do is get it within 3-5 feet of a sensor and the door unlocks automatically.
I have a sort-of conceptual idea of how these things work, but I’d like some details, please. TIA…
There’s a small microchip with what amounts to two wires either embedded in them in a coil, or routed elsewhere on or near the chip. These are two antennas.
One antenna is designed to pick up very low frequencies, like in the kilohertz range. The low frequencies are emitted by the reading device. If you’ve ever used the Mobil or Exxon ‘Speedpass’, you’ve used one of these devices. When you put your little tag near the reader, the low antenna in the chip absorbs some of the energy and rectifies it so that you end up with a pulsing DC voltage (small, but nevertheless present) on the chip.
This DC Pulse is what powers the rest of the chip. They don’t have any batteries, which is why they’re so cheap and low-maintainence. The DC pulse goes into a small capacitor which can hold enough charge for the device to power up, read a unique ID programmed into each one, and then transmit that unique ID on the high-frequency antenna coil.
This unique ID is heard by the reader/scanner and verified against some database so that the individual RFID device can be identified. *
I’ve designed systems using RF tags before. They are kinda cool.
There are basically two types, beam powered (also called passive) and battery powered. The battery powered ones have a longer range, but then they also have a battery which needs changed every now and then. Plus, the battery makes them larger.
The beam powered ones come in a couple of varieties. Coincidentally, we were just talking about these at work. The ones I used many years ago took the incoming RF beam and shunted it off to a capacitor. Once the capacitor was charged, a small silicon chip switched on, and used the power stored in the capacitor (kinda like a small battery) to re-transmit an RF signal back to the receiver. All of the beam powered ones use some variation on this theme,
like for example what silenus described.
Chances are your ez-pass is battery powered, and your wallet card is beam powered. Here’s a pretty decent pic of a beam powered unit. Note the coil shaped antenna, which is used both to couple power onto the tag and transmit and receive RF. The actual working guts of the thing are all in that little chip in the middle. Most of the surface area is just the copper antenna. http://www.sensational.ch/web_de/images/lab/rfid/id_tag.jpg
Scientific American ran an excellent explanation (as part of their “Working Knowledge” series) in December 2001. The online article costs $7.95, or you could go to the library and look it up for free.
In addition, some of these automated toll road systems have video cameras to photograph your rear license plate, and use OCR to read the character. If you don’t have your transponder/fob with you, the system can look up your account information from your plate – or send you the traffic ticket if you didn’t subscribe to the system.
(I worked on one of those systems for a Canadian toll road several years back)
I love ez-pass…and they love me back! I have double digit pictures of my car, my license plate and my dearly beloved dog (now departed) looking out the rear window with his tongue hanging out. Believe me, all candid shots, and really quite professionally done.
Good story cept this is a beauracracy…I got double digit letters, with pics, that needed to be answered ONE AT A TIME! Now? I take the time to read what they are telling me in their little box.
These folks are so bad they are good…or is that so good they are bad?
