[This post was part of a CrashSpace mailing list discussion on a "proximity t-shirt": a shirt that would light up or similar when other similar t-shirts were nearby. People were wondering how good RFID was at localized detection of tags.]
Okay so I’m a big RFID nerd, did a lot of consulting work using it. So here’s a quick brain dump.
Regular passive RFID is designed for identification not localization. The RFID tags can be reliably read only to within a few centimeters. But the readers are cheap. You can get 128kHz (LF) and 13.56MHz (HF) RFID readers for $20-40 and the reader chips themselves for under $2. RFID tags that work with these systems are around $1. These systems typically cannot handle multiple tags in the reader’s field at a time.
UHF (900MHz-2.4GHz) passive RFID readers can read up to a few meters, and the tags can be a $0.05 in large quantities. The readers can get pretty expensive though: >$1000. These are the systems used by Walmart et al to read a palette of Mach3 razors as they transit the warehouse. And by the marathon race timers. The standard is called EPC, if you’re interested. These systems can handle a few hundred tags in the reader’s field, but read time goes down exponentially with tag count.
“Active RFID” has ranges up to hundreds of meters. The term “active RFID” is a bit loose, since one can describe a WiFi laptop or a cellphone as active RFID tag. Really it just means an RF radio system that transmits a unique ID using its own power source. There are active RFID versions of all the above technologies. Eric’s suggested use of the RF Link boards is essentially an active RFID beacon. One of my favorite active RFID designs is OpenBeacon (http://www.openbeacon.org/ ). It uses the ubiquitous Nordic RF chips (used in almost every wireless keyboard & mouse) Sparkfun has a ton of Nordic boards to play with.
“Localization” of RFID tags can mean two things. For normal passive RFID, the tag is “located” when a reader sees it. It’s a boolean: sees it / doesn’t see it. This is often called “proximity detecton”. So one way to approach localization is to just have a lot of readers. True localization (knowing where in a reader’s field-of-view a tag is) is pretty tricky. The main issue is just finding how far away an RF source is. The simplest is signal-strength (”the louder you are the closer you are”), but that falls prey to the non-homogeneity of the environment: in free space it would work; in a room full of RF-absorbing humans, it fails. If you’re really savvy, you can do time-of-flight calculation. The reader sends out a ping and measures the time it takes to receive the tag’s echo ping. This requires nanosecond-accurate clocks on the reader (speed of light is very fast) and falls prey to multipath distortion (reflections off the environment). And then you need multiple antennae for a single region to do triangulation. It’s hard, but RFID vendors are starting to release stuff.
Want to hook up a Wii Nunchuck to an Arduino but don’t want to cut up the cord on your Nunchuck? Yeah me too. So I made some of these:
It’s a small PCB that adapts the Wii Nunchuck connector to standard 4-pin header. I call it the “wiichuck adapter”. It plugs directly into the Arduino, no wiring necessary. You can get one too for $4.
Available from the following wonderful shops:
- FunGizmos.com. FREE DOMESTIC SHIPPING. International shipping for $1 more.
- Tinker.it (UK)
- Little Bird Electronics (Australia)
- Sparkfun. Ships domestic & internationally. Be sure to order header pins too!
- Freduino.eu (EU)
With BlinkMs available for purchase soon, I figured it would be neat to show what a large collection of them can do. BlinkMs were created from my desire to have a “smart LED” that did its own tri-color PWM. I didn’t want to build a real-time system to control the PWM of several hundred RGB LEDs. And existing LED controllers didn’t meet my needs. I wanted something that knew a bit about color and color patterns and could be networked together into clusters. Thus BlinkM. A single BlinkM is fun, but the real utility is seen when you have several of them on the same I2C network.
Below are two movies of the lighting system in WineM, our smart RFID winerack. Each place a bottle goes contains a essentially BlinkM and RFID reader controlled via an I2C master.
WineM prototype at NextFest
This video shows WineM in use. A handheld web device allows one to select and display different facets of the wine collection. In the video, first all wines are shown, colored by varietal, then Cabernet wines are selected. Within the Cabernets, color them by year. Select 2002 as the year and only the 2002 Cabernets are shown. Then color those by price. Finally, select the 2002 cabernets that are between $20-$30. You’re left with the perfect wine for tonight’s meal. Also shown is the visual indication when bottles are added or removed.
WineM prototype light tests
This is just a light show using the WineM lights. It was taken right after I had assembled and installed everything in the rack. Apologies for the exposure fluctuations.
For a bit more detail on the WineM prototype hardware, see this post.
On Nov 8th I was a guest lecturer at the “Theory and Practice of Tangible User Interfaces” class at UC Berkeley. It’s a physical computing course, about “a new approach to HCI which focuses on the physical interaction with computational media.” The class has both lecture and lab components. The lab section is hands-on experimenting with novel physical interfaces, using Arduino as the core. So of course I have a great fondness for the course.
The talk itself was a sort of summary of the things we’ve been pondering at ThingM. If you’re already familiar with the stuff Mike and I have been talking about for the last several months, there’s not much new. But for me it was interesting to put it all together into one package and attempt to construct a narrative that threads it all together. The talk covered:
Computation as Material
Smart Interface Components
Reversible Hacking
Technology Sketches
Informational Objects
Before my talk I got to see the in-progess work of the students’ final projects to create new kinds of musical instruments using a variety of input devices, Arduino, Processing, and so on. It was great. I was invited to be a guest lecturer by fellow SketcherKimiko Ryokai, an assistant professor at Berkeley’s School of Information. Thank you, Kimiko, for letting me be a part of your class and to meet your students. The class was 40 strong and it’s obvious there’s a lot of interest in the process of making software interact with the real world. I hope more schools follow in this one’s path.
WineM solves the problem of remembering all the information about the wines in a large wine rack, cellar or cabinet, or searching through that data. This smart wine rack makes that information always accessible and updatable.
ThingM will be at Wired’s NextFest conference, showing off a prototype of our WineM product. Come visit us if you’re in LA!
WineM has a bank of RGB LEDs for every cell a bottle can go in. Each cell has a microcontroller implementing a high-power variant of a SmartLED.
There are 576 total LEDs in the WineM prototype, controlled by 52 microcontrollers, bussed together over an I2C bus.
Here’s a little video showing the cell lights in action.
The electronics in each cell look like (click for more details):
Standard through-hole components were used since space wasn’t an issue.
Each cell also has an RFID antenna to recognize the RFID tags on the wine bottles. This lets you recognize which wine bottle is in which cell. A controller board holds the RFID reader and controls up to 12 cells. Four controllers are linked together to in the WineM prototype to control the entire rack:
What are Smart Interface Components? Current interface components, the sensors and actuators that comprise the user interface of the gadgets we use, are dumb. They require specialized domain-specific knowledge to make work correctly, non-trivial processing to use, and in general are a pain. Tiny microcontrollers are becoming cheap enough to embed even at the edges of our hardware designs. A component with local brain can embed some of the domain knowledge and enable a higher level of communication between it and the application processor.
An example presented is a Smart LED. LEDs are dumb. Multi-color LEDs are hard to control. Can we make it better?
Imagine an LED that instead of worrying about PWM and current-limiting resistors, you just give it the HSV or RGB color values via a serial line? “#FFCC22 @ 20% brightness”, you say.
LEDs should be smarter. Sure we have flashing LED assemblies and even rudimentary RGB-flashing discrete LEDs. But LEDs themselves are predominately just dumb lights. There’s no real reason for this. Fab processes for microcontrollers and LEDs aren’t that dissimilar. It should be possible to have both in a single LED-like package.
There is a glimmer of this happening, like the “RGB LED Slow Colour Change” LEDs you can get. You can see an example of these being used on the Embarrasingly Easy CaseMod. Unfortunately you can’t change the cycle time or anything else of how these LEDs work. So let’s make our own. These prototype Smart LEDs will necessarily be larger than a production run, but the size is getting close, giving us a feeling for how we might use them.
Random experiments, circuits, code, rapid prototyping examples, sometimes things to buy, and occasionally tunes by Tod E. Kurt.
Reach me at tod [at] todbot.com
Yay BlinkMs!
BlinkM is a smart LED. Imagine an LED with a tiny computer inside, one that can be any color and have a life of its own. You can buy them now from one of our global distributors.
ThingM
A device studio that lives at the intersections of ubiquitous computing, ambient intelligence, industrial design, and materials science.
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