CIRCUITS

+ 3.3V

Ground

XBee transmit

XBee receive

FTDI receive

F

FTDI transmit

Fig. F: XBee module on the breakout board, wired to a general-purpose USB-to-TTL serial adapter. Fig. G: The Simple Processing application graphs accelerometer readings and counts a collision when

After configuring all the mobile radios, use the following command to configure the base station radio to talk on network AAAA as radio 0:

ATRE, IDAAAA, MY0, BD7, WR\r

Then, using your serial terminal program, you’ll need to change the port’s data rate from 9,600 to 115,200, to match the radios’ new rate.

3. Program and test the system. Using Rob Faludi and Dan Shiffman’s XBee API library for Processing (available from shiffman.net), I wrote a program that reads the values from multiple sensor modules, then graphs them and periodically logs them to a comma-delimited file. It also makes sounds whenever there’s a collision and keeps track of how many have occurred (Figure G). See makezine.com/14/diycircuits_roller for my code.

4. Test the system in vivo. You have to position the module where it won’t hurt the skater if she lands on it. We tried the top of the helmet, the hip, the ankle, the shoulder, and the cleavage. The shoulder was difficult to secure to, and the radio signal got lost in the cleavage. The helmet turned out to be the safest, but the hip and

128 Make: Volume 14

G

H

they suddenly change. Fig. H: Gotham Girls fresh-meat skaters Dainty Inferno, Miss American Thighs, and Dinah Party with motion sensors. Special thanks go to them for volunteering as test subjects for this project.

ankle gave the widest range of readings and the most pronounced hits.

Padding makes the modules safer, but too much dampens the sensitivity of the accelerometer. Bubble wrap worked well for tests, but ¼" foam is more effective.

What Next?

Fun though it is, roller derby is not the only easy application for these XBee radios. You can attach any sensor that outputs voltages up to 3.3V, and control any device with the same input range. They can exchange data from computers and microcontrollers as well. I’ve used them to read remote photovoltaic arrays, make toxic gas sensors, and control mechanical chimps — and my students and colleagues at N YU have gone much further with them.

Thanks go to Rob Faludi, Dan Shiffman, and Kate Hartman for technical collaboration on this project, and to the Gotham Girls for their cooperation.

Tom Igoe is an associate arts professor at the Interactive Telecommunications Program at New York University. He teaches about physical interaction design, and wants to work with monkeys someday.