CIRCUITS
+ 3.3V
GND
Xbee radio
Vcc
TX
RX
TXen
RESET
PWM0/RSSI
PWM1
NC
DTR
A GND
AD0/DIO0
AD1/DIO1
AD2/DIO2
AD3/DIO3
RTS/AD6/DIO6
Ass’t/AD5/DIO5
Vref
SLP
CTS/DIO7
AD4/DIO4
1. Build the
3.3V power pack skater modules.
The module circuit is
simple (Figure A). The
accelerometer’s outputs
connect to 3 of the
XBee’s inputs, and both
are powered by a 3.3V
battery power module.
Tilting the accelerometer changes the voltage,
and sudden changes,
like from a fall or a hit,
produce a sharp spike.
Plan your layout
before you solder, and
bend connected wires
and leads toward each
other underneath the
board, to make soldering easy. With the XBee
breakout board, I soldered only the corners and the
pins I used, for ease of removal (Figures B–E).
Accelerometer
ST
Z
Y
X
GND
3.3V
Fig. A: Module wiring: The accelerometer’s X, Y, and Z outputs connect to the
XBee’s analog 0, 1, and 2 inputs.
MATERIALS
For each skater module:
Digi XBee 802.15.4 (formerly Series 1) radio
part #XB24-ACI-001 from digi.com, $19
3.3V regulated power module #VPack3.3V_AA_ 1
from Bodhi Labs, bodhilabs.com, $11
3-axis accelerometer module #SEN-00849 from
SparkFun, sparkfun.com, $30
Small perf board that you can snap in half to
mount 2 radios, such as RadioShack #276-148
Breakout board for XBee SparkFun #BOB-08276
2mm 10-pin sockets ( 2) SparkFun #PRT-08272
Breakaway female headers 5 pins for each
module, SparkFun #PRT-00115
Breakaway male headers 2 pins for each module,
SparkFun #PRT-00116
Scrap wood or enclosure You need something
strong to mount the circuit boards and batteries onto that the skaters can wear. I bolted the
boards to scrap wood with #4-40 screws. You
could also get fancy and build a nice casing.
For the base station:
Computer
XBee-to-USB module Droids part #990.002 from
droids.it, $33, or New Micros #USB-XBEE-DONGLE-CARRIER from newmicros.com, $39.
You can also use a general-purpose USB-to-TTL serial adapter like SparkFun #BOB-00718,
$15, and wire it to the XBee on a breadboard.
For the skaters:
Contact your local branch through the Women’s
Flat Track Derby Association ( wftda.org) and
have plenty of beer on hand.
2. Configure the radios.
All the USB adapters listed at left use FTDI’s USB-to-serial chip, so you’ll need to install the drivers
from ftdichip.com. An XBee adapter lets you just
plug the radio into your computer. With a generic
USB-to-serial adapter, connect your computer’s
transmit pin (TX) to the adapter’s receive (RX) pin,
and its RX pin to the adapter’s TX pin. Connect the
adapter’s 3.3V output to the radio’s voltage in, and
connect both of their grounds together (Figure F).
With the radio and adapter connected, run a serial
terminal program on your computer to check for the
new port. In Mac OS X you can use ZTerm (homepage.
mac.com/dalverson/zterm), and with Windows use
Hyper Terminal (under Start → Programs → Accessories → Communications). The port should show up
with a name like /dev/ tty.usbserial-A5001rNq in
OS X, and COM4 or a higher number in Windows.
You may also need to upgrade the firmware on
your radios. For instructions, search for “firmware
upgrade example” on Rob Faludi’s excellent XBee
blog at faludi.com/category/xbee.
With your terminal program, configure and open
the port for your USB adapter. In OS X, open ZTerm’s
Settings → Connection menu item and configure
your connection as 9600-8-None- 1 with no flow
126 Make: Volume 14
