Hardware Reference
In-Depth Information
Two Flavors of Wireless: Infrared and Radio
There are two common types of wireless communication in most people's lives: infrared
light communication and radio communication. The main difference between them,
from a user's or developer's position, is their
directionality
.
Television remote controls typically use infrared (IR) com-
munication. Unlike radio, it's dependent on the orientation
between transmitter and receiver. There must be a clear
line of sight between the two. Sometimes IR can work by
bouncing the beam off another surface, but it's not as
reliable. Ultimately, the receiver is an optical device, so it
has to “see” the signal. Car door openers, mobile phones,
garage door remote controls, and many other devices
use radio. These work regardless of whether the trans-
mitter and receiver are facing each other. They can even
operate through walls, in some cases. In other words, their
transmission is
omnidirectional
. Generally, IR is used for
short-range line-of sight applications, and radio is used for
everything else; Figure 6-2 illustrates this difference.
Figure 6-2
The signal from the LED at left radiates out in a beam from the LED,
while the signal from a radio antenna like on the XBee radio at right
radiates omnidirectionally.
Transmitters, Receivers,
and Transceivers
There are three types of devices common to both IR and
RF systems:
transmitters
, which send a signal but can't
receive one;
receivers
, which receive a signal but can't
send one; and
transceivers
, which can do both. You may
wonder why everything isn't a transceiver, as it's the most
flexible device. It's more complex to make a transceiver
than it is to make the other two. In a transceiver, you have
to make sure the receiver is not receiving its transmit-
ter's transmission, or they'll interfere with each other and
not listen to any other device. For many applications, it's
cheaper to use a transmitter-receiver pair and handle any
errors by just transmitting the message many times until
the receiver gets it. That's how TV remote controls work,
for example. It makes the components much cheaper.
Keep in mind the distinction between transmitter-receiver
pairs and transceivers when you plan your projects, and
when you shop. Consider whether the communication
in your project must be two-way, or whether it can be
one-way only. If it's one-way, ask yourself what happens if
the communication fails. Can the receiver operate without
asking for clarification? Can the problem be solved by
transmitting repeatedly until the message is received? If
the answer is yes, you might be able to use a transmitter-
receiver pair and save some money.
How Infrared Works
IR communication works by pulsing an IR LED at a set
data rate, and receiving the pulses using an IR photodi-
ode. It's simply serial communication transmitted using
infrared light. Since there are many everyday sources of IR
light (the sun, incandescent light bulbs, any heat source),
it's necessary to differentiate the IR data signal from other
IR energy. To do this, the serial output is sent to an oscil-
lator before it's sent to the output LED. The wave created
by the oscillator, called a
carrier wave
,
is a regular pulse
that's modulated by the pulses of the data signal. The
It's increasingly common in radio applications to just make
every device a transceiver, and incorporate a microcon-
troller to manage the transmitter-receiver filtering. All
Bluetooth, ZigBee, and WiFi radios work this way. However,
it's still possible to get transmitter-receiver pair radios, and
they are still cheaper than their transceiver counterparts.
