Nov 192009

[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 ( ). 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.

 Posted by at 10:11 am
Jun 162009

I have one of those USB-based logic analyzers that needs Windows software to make it go. I had been doing Windows-in-a-window with VMWare, but it’s kind of a pain. If I were to use a real Windows laptop, I’d need a shelf or something for it. I wanted it above my oscilloscope, which meant a laptop stand that was taller and wider than most. Time for the laser cutter!

(click for larger)

My design requirements were:
– assemble without any tooling or fasteners
– fit on a single 1’x2′ sheet of the 1/4″ plywood I already had
– be stable enough to hold a 7lb laptop
– be wide & tall enough for the oscilloscope to fit underneath and be usable.

The 1/4″ (0.20″ really) plywood is cheap, from a big box hardware store. I think I paid $10 for a 4’x8′ sheet of it, and they nicely cut it down to 2’x4′ sheets for me.

It ended up fitting pretty exactly on the 12″x24″ cutting bed of the laser.

If you want to make your own based off this, here are the vector files:

What prodded me into doing this was the great Cardboard Laptop Stand I saw the oomlout guys had just received. Also, there are so many great DIY laptop stands on Instructables, they created a whole category for it.

 Posted by at 11:55 pm
Apr 112009

I’ve been exploring various types of gearmotors. DC motors by themselves spin too fast and have low torque. Gearmotors are motors with a gearbox that slows down the high speed of the motor and produces higher torque. Most gearmotors are pretty expensive though. I want a really cheap, almost throw-away, source of gearmotors. It turns out cheap servos can be made into continuous rotation gearmotors.

Modding servos for continuous rotation is not a new hack. You can find many examples of it. You can even buy a nice continuous servo made by Parallax. But I wanted a micro servo version. I’ve been getting cheap servo motors from Hobby City, and they have several super-tiny servos for less than $4. The ones I use here are the Hextronic HXT500 available for $3.49 each.

Here’s how to modify one of those servos to make it into a tiny little gearmotor.
Continue reading »

 Posted by at 12:10 am
Mar 072009

DIY iPhone headphone extension

Most cars nowadays (especially rentals, which I’ve been in a lot lately) have 1/8″ AUX inputs for an MP3 player.  When I had just an iPod, this worked great.  Now that I have an iPhone I found my standard 1/8″ to 1/8″ extension cable  that I’ve had in my cable go bag wouldn’t work.  There’s no way I’m going to spend upwards of $20 on an iPhone adapter cable to get around Apple’s design mistake.   Five minutes with the Dremel and I’ve got a handy DIY iPhone adapter cable (and lots of little rubber bits all over the place)

 Posted by at 7:26 pm
Jun 172008

BlinkMs are a lot of fun by themselves, but they’re also little network devices, each having its own address on an I2C network. Here’s where I think BlinkM can really shine since it makes controlling multiple RGB LEDs pretty easy. For Maker Faire, I wanted to show off this facet by having a single Arduino control a dozen or so BlinkMs on a single I2C bus. The result is shown in the little video below.

Read on for how this was put together.

Continue reading »

 Posted by at 2:43 am
Apr 282008

I just received some colorful tiny mini-breadboards from They are pretty great. Now quickie ideas prototyped with Arduino can be even smaller than the “1¢ Arduino under-shield”.

They appear to be the same quality as the other breadboards I have, just different color plastic. I can already tell the colors will help me differentiate projects, which all tend to look alike from 10 feet away. Normally when you buy these from Digikey or similar places, these little ones cost $7 a piece. FunGizmos has them for $5.40. And that’s cheap enough to get a few. Note that all these tiny breadboards don’t have the side power busses like the larger breadboards do. That’s the price you pay for tininess.

 Posted by at 3:51 pm