Specifications

14 ExtroNews 13.2 April – June 2002
TECHNICALLY SPEAKING...
Seeing Like An Owl
Some cameras and camcorders can
capture images in total darkness. How does
this work? As stated previously, photo
diodes and the charge-coupled devices
(CCDs) used in cameras respond to a wide
range of photon wavelengths and any
number of photons striking the surface of a
photo sensitive semiconductor produces a
proportional amount of electron charge.
Camera CCD imagers may possess a
spectral response from about 0.4 microns
(blue) to about 1.050 microns (IR range).
Additional processes will allow response to
extend to the extreme ultraviolet range.
Those who may own a popular camcorder
that have the ability to shoot at night are
utilizing such a device. The camcorder
includes an IR LED that provides the IR light
source for close range image capture.
Additional illumination is obtained by the
camera’s sensitivity to the IR radiated by
objects in its view.
Boulders Along The Trail
System designers have many control
interface options. This article discusses IR
control, but what about others? When
would you want a hardwired interface over
a wireless interface? Hardwiring a control
interface is a good, solid approach when
the presence and/or cost of wiring is not an
issue, or the environment contains so many
interference variables that may affect
wireless operation. This type of control
connection that makes use of the IR
protocol is called an unmodulated IR
system.
Most times, IR control involves
directionality. The IR transmitter must be
pointed line-of-sight at the receiver within a
few degrees of normal. So, some variability
in control reliability can occur. Systems
designed with IR flooding transmitters,
repeaters, and wide angle transmitters
and/or receivers tend to be less directional
and can overcome this limitation to a great
extent. Controlling any device within an
environment of high ambient light,
including unshielded incandescent light
sources, can be challenging for an IR
interface. Ambient light sources will tend to
desensitize the IR receiver. Continuous
levels of infrared ambient energy will cause
the receiver’s AGC system to decrease
receiver gain, thus making the system less
sensitive to remote IR transmitters.
Fluorescent lamps typically have not
emitted large amounts of IR energy.
Historically, fluorescent lamps have required
rather large ballast transformers which
develop enough high voltage to cause the
fluorescent tube to ionize internal gases to
create ultraviolet emissions that energize
the phosphor coating on the glass. The
light energy bands emitted by those lamps
are not as rich in IR as incandescent lamps.
Today, however, there are new, compact
fluorescent lamps replacing incandescent
light bulbs of various sizes. Many of these
light bulb replacements are very compact.
How does this affect IR control?
Drive power for these lamps is being
generated by very compact switching
power supplies operating as high speed
power inverters. Much like a regular switch
mode power supply, many of these power
inverters operate around 40 kHz which is
within the operating frequency of many IR
systems. In addition, the switching
frequency combining with small amounts of
the lamp’s IR energy create an interfering
signal that may cause false triggering of IR-
controlled systems. At a minimum, this
energy can interfere with an IR control
transmission that may confuse the IR
receiver.
Playing Five-Card Protocol
There are about five common IR remote
control system protocols. Each utilizes some
format of modulated carrier for data
encoding. Virtually all equipment
manufacturers use one of the five
protocols. The carrier frequency is typically
between 30 – 40 kHz, with a large
percentage of the remotes using 38 or
40 kHz. The use of the carrier supports the
ability of the receiver to be tuned to that
specific frequency, thereby enhancing the
immunity of the system to external noise or
interference. Most receivers are tuned to
about +/- 2 kHz of the carrier frequency.
Pulse Width Modulation (PWM) is
commonly used to denote the difference
between a data "1" and "0." In other
words, the ON versus OFF time for
communication of a zero is specifically
different than that for a one. When the IR
transmitter is actively communicating, it
sends a burst of the carrier frequency that
coincides with the required ON time
followed by the required OFF time, or no
burst, to signify either a one or zero. Refer
to Figure 2 to see the relationship between
the basic data and the way in which it is
transmitted via modulated carrier. The
Figure 2. Data is transmitted via bursts of carrier ON and carrier OFF.