PROJECTS
●●●●
■ THIS MONTH’S PROJECTS
High Power LED Strobe . . . . . .32
Lithonia Emergency Light . . . . 38
Four Channel Datalogger . . . . . 42
■ LEVEL RATING SYSTEM
To find out the level of difficulty
for each of these projects, turn
to our ratings for the answers.
●●●● . . . . Beginner Level
●●●● . . . . Intermediate Level
●●●● . . . . Advanced Level
●●●● . . . . Professional Level
High-power LEDs
have received much
attention as of late,
and jumped in the
last few years from a
laboratory curiosity
into mainstream
applications. We all
have seen LED
flashlights (with their
bright bluish white
light) and its LEDs
incorporated into
some luxury auto
brands.
32
January 2008
BUILD A
H SIGTH-RPOOWEBR LEED
The reason for this rise in popularity is very simple: as light sources
measured in lumens per watt, they are
extremely efficient. These devices easily surpass incandescent lights, even
halogen-filled ones. They also seriously challenge fluorescent and high-intensity discharge lighting (which
themselves have undergone extensive
efficiency improvements). LEDs offer
a few extra features: low voltage operation, small size, and solid-state
ruggedness. These qualities make
them very attractive for portable and
automotive environments.
This doesn’t mean that high
power LEDs are not without
problems. Unlike their lower rated
brethrens — which max out at 20 or
30 mA — these monsters start at
300 mA, with 700 and 1,500 mA
devices available, and even higher
rated devices in the pipeline.
With so much power concentrated into such a small volume, heat
becomes a major concern. With the
LED itself being a semiconductor, heat
substantially reduces both the operating life and the efficiency of the
device. Thus, thermal management is
paramount in a successful application.
LEDs have an additional advantage that none of the light sources
discussed above can match: the
ability to be turned on and off
almost instantaneously. Only Xenon
flash tubes have better response
times, but their high voltage has
precluded them from being used in
applications such as cell phone
cameras, where high power white
LEDs have become ubiquitous. As I
needed to build a strobe circuit, I
decided I should give them a try.
The Challenges are
in the Details
Starting a new project is similar
to hiking an unknown trail the first
time. One quickly finds unexpected
challenges, some of which are tough
to solve. But solving those challenges make it all worthwhile, and
hopefully, make you a little wiser.
This project did not disappoint.
I had originally attempted to
build a strobe employing the conventional high-voltage capacitor and
Xenon-tube combination — circuits
which have been around for a long
time — and rapidly noticed the drawbacks. For one thing, it is very easy to
exceed the tube’s maximum power
dissipation at higher repetition rates,
causing it to crack. For another, the
simple inverter circuits that convert
from the battery level voltage to the
required high voltage simply do not
have enough energy to recharge the
flash capacitor in 1/100 second or
less. These units are designed for
non-repetitive applications, with
typical recharge times of two or three
seconds — maybe longer — and are
two orders of magnitude too weak.
As I pondered how to design
the required high power converter,
a thought came to mind: This was a
perfectly good example for a radical
new solution. In this case, employ
the newer, high power white LEDs.
