Bluetooth, ZigBee, and
Other RF Modules
R/C transmitters and receivers are
“channel” based; that is, on one frequency the transmitter controls a certain
number of channels, with each channel
dedicated to a specific function. In the
typical model airplane R/C transmitter,
for example, a joystick controls the up
and down action of a servo connected
to the plane’s ailerons. Another
joystick controls the servo connected
to the plane’s rudder, and so on.
Data modems provide for
completely digital communication. If
you have a laptop PC with a wireless
internet connection or a Bluetooth
headset for your cell phone, you’re
already familiar with modern data
modems. Common types of data
modems used in amateur robots
include Bluetooth and ZigBee (the latter sometimes referred to as 802.15.4,
after the international protocol that
defines its standard). Both require a
set of transceivers for sending and
receiving data along a two-way link.
Depending on the specific device,
effective communication range is
hundreds of feet in free air; somewhat
less indoors where walls, doors, and
ceilings may obstruct the signal.
Bluetooth and ZigBee modules can
be duplex or two-way — as opposed
to R/C transmitters and receivers,
which are only one way. Another plus
in their favor is that they are based on
industry accepted standards and
documentation on their use is widely
available from Internet sources and
manufacturers of the products.
There are a number of Bluetooth
and ZigBee modules directly suitable
for robotics use, such as the Parallax
EmbeddedBlue Transceiver. All they
require is a power source and the
data is provided via a parallel, serial,
or USB interface. Not all RF data
modules use Bluetooth or ZigBee
technology. There are a number of RF
modules that use other technologies,
some standard like 802.1 (Wi-Fi) and
some proprietary. You can choose the
module best for your application
based on whether you need such
features as longer range, true duplex
(two-way) communication, ultra-small
size, and so on. Check the Sources
listing for several companies that specialize in RF digital data transmission.
Alternatives to RF
Purchasing an RF transmitter and
receiver module is one way to provide
a communications link between you and
your robot. In addition, several ready-made products can be hacked for their
RF systems and pressed into use as
radio links between you and your bot.
• Walkie talkie. Many toy walkie
talkies include a “code sender” button
for transmitting Morse code. By
connecting the receiving to an AC
coupled interface and 567 tone
decoder, you can add simple on/off
control of your robot.
• Garage door opener. Try to find a
used one that’s being discarded; the
electronics — the part you want — last
longer than the mechanics. Hack the
receiver to work as an on/off control
for your robot.
• “Keyring” (or keyfob) appliance
control. You can purchase a radio
controlled powered outlet at many
department and home improvement
stores. Hack the module to work with
your robot. The transmitter is a
keyring, with one or two buttons
(some control several modules).
• Wireless car alarm kit. Two- and
three-function wireless car alarm kits
can be retrofitted for controlling a
robot. You can find them at auto
parts stores and weekend swap meets.
Infrared Remote
Control
A radio link isn’t the only way to
wirelessly control a robot. Another
technology for one- and two-way links
is infrared control. This system has the
benefit of low-cost hardware, and it’s
relatively easy to interface to most
microcontrollers used in robotics.
The major components of the robot
infrared remote control system are:
• Infrared remote. Most any modern
infrared remote control will work, but
... remote controls vary considerably in
the signal patterns they use. You’ll find
it most convenient to use a “universal
remote control” (under $10 at a
department store). Specifically, you want
the universal remote to support Sharp
TVs and VCRs, of which 99.99% do.
• Infrared receiver module. The
receiver module contains an infrared
light detector, along with various
electronics to clean up, amplify, and
demodulate the signal from the remote
control. The remote sends a pattern of
on/off flashes of light; these flashes are
modulated at about 38-40 kHz, in
order to reduce interference from other
light sources. The receiver strips out the
modulation and provides just the
on/off flashing patterns.
• Computer or microcontroller. You
need some hardware to decode the
light patterns, and a computer or
microcontroller, running appropriate
software makes the job straightforward.
In operation, you press a button
on the infrared remote which sends a
coded light signal to the receiver
module. The receiver demodulates the
signal and extracts the code sent by
the remote control. The code is a
sequence of binary 0s and 1s, in the
same way a number or letter is represented in a personal computer. Your
microcontroller interprets the binary
sequences as a specific button that
was pressed on the remote control.
Wired Remote
Control
Perhaps the simplest form of
remote control is the wired controller,
such as an Atari, PC, or Playstation
joystick. These connect to your robot
via a set of wires. How you interface
the controller to your robot depends
on the type of control.
SERVO 08.2008 63
