This is only a preview of the January 2004 issue of Silicon Chip. You can view 33 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Studio 350 Power Amplifier Module":
Items relevant to "High-Efficiency Power Supply For 1W Star LEDs":
Items relevant to "Antenna & RF Preamp For Weather Satellites":
Items relevant to "Lapel Microphone Adaptor For PA Systems":
Articles in this series:
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High-efficiency
power supply
For 1W Luxeon Star LEDs
Looking for a highly-efficient switchmode
power supply to run a 1W Luxeon Star LED
from batteries? This easy-to-build design lets
you use a pair of 1.5V “D” cells and includes
brightness control to further extend the
battery life.
By PETER SMITH
L
AST MONTH, we described a
simple linear supply for driving
Lumileds’ 1W Luxeon Star LEDs. Designed with low cost and simplicity in
mind, it is ideal for experimentation as
well as general-purpose fixed lighting
applications.
The downside to this simplicity
24 Silicon Chip
is that it’s not very energy efficient.
However, for portable and emergency
lighting applications, efficiency is
of paramount importance. In a lowefficiency lighting setup, much of the
available energy is consumed by the
power supply itself, where it’s dissipated as heat.
Conversely, an efficient supply
transfers the majority of the input
power to the output, thereby maximising battery life.
This high-efficiency switchmode
design can drive a single 1W Luxeon
Star for more than 20 hours (continuous use) from a pair of alkaline “D”
cells. It also includes a brightness
control which, when set to the lower
end of the scale, will extend useful
battery life many times over.
The PC board is the same size as
two “D” cells side-by-side, making it
ideal for use in lanterns, emergency
lights, beacons, etc. We envisage it
being used anywhere that a portable,
reliable and ultra-long-life light source
is required.
It can drive green, cyan, blue and
royal blue as well as white 1W LED
www.siliconchip.com.au
Main Features
•
•
•
•
•
High efficiency (>85%)
Brightness control
2 x ‘D’ cell powered
20+ hours continuous use
Drives white, green & blue Stars
Fig.1: when the switch closes, inductor current increases with time,
storing energy in its magnetic field.
varieties, most of which are available
locally from the Alternative Technology Association (see panel).
Step-up DC-DC conversion
In common with our 2-cell LED
torch design (SILICON CHIP, May 2001),
the circuit is based around a MAX1676
step-up DC-DC converter IC. These
devices were originally designed for
use in mobile phones and the like.
Our circuit requires a step-up converter in order to boost the battery voltage, typically between 2.4V to 2.8V,
to the higher 3.3V (nominal) required
by the LED. Step-up conversion also
assures maximum LED brightness
over the lifetime of the batteries. To
understand how this works, let’s first
look at a few of the basics.
Fig.2: when the switch opens, the magnetic field collapses. The
inductor’s energy is discharged into the capacitor and load via the
diode.
Boosting the battery voltage
The basic components of a step-up
converter consist of an inductor, transistor (switch) and diode – see Fig.1.
When the switch closes, the input voltage is applied across the inductor. The
current flow (i) ramps up with time (t)
and energy is stored in the inductor’s
magnetic field.
When the switch opens (Fig.2), an
instantaneous voltage appears across
the inductor due to the collapsing
magnetic field. This voltage is of the
same polarity as the input voltage,
so the diode conducts, transferring
energy to the output.
Fig.3 shows where these basic parts
fit in our design. As you can see, most
of the step-up circuitry is contained
within the MAX1676. Q1 acts as the
switch, with Q2 replacing the series
diode. Q2 acts as a synchronous rectifier, eliminating forward voltage losses
and therefore improving efficiency.
Output control
The MAX1676 converter uses a
current-limited pulse-frequency
modulation (PFM) technique to maintain output regulation. Essentially,
the switch is driven with a minimum
www.siliconchip.com.au
Fig.3: this diagram shows the basic elements of the power supply.
Most of the step-up circuitry is contained within the MAX1676 chip,
including the switching transistor and rectifier.
Fig.4: On the bench, our prototype powered a Star for over 20 hours on
“D” size alkaline cells. Even at 0.6V/cell, the supply was still pumping
out more than half a watt (about 160mA). Almost full power is delivered
to the LED down to 1.8V. This means that you’ll get high brightness
over the entire life of a set of rechargeables. Converter efficiency was
measured at 90.1% with a 3.0V input, with a total circuit efficiency
(input to output) of 85.5%.
pulse width, variable-frequency signal
(up to 500kHz), which increases as battery voltage decreases. For a detailed
description of its operation, check out
the Maxim datasheet, available from
www.maxim-ic.com.
When the battery voltage falls
below about 1.8V, the output power
decreases markedly due to the high
input to output voltage differential
(see Fig.4). For example, with only
0.5V per cell, a step-up ratio of about
3.3:1 would be required to achieve
full power. Assuming about 75% efJanuary 2004 25
Fig.5: the complete circuit diagram
for the power supply. Two CMOS
7555 ICs modulate LED brightness
by controlling the step-up converter’s
shutdown pin.
ficiency, this means that the supply
would have to pull around 1.4A from
the (already) flat batteries. And with
increasing cell resistance, this simply
wouldn’t be possible.
As you can see, reducing output
power towards the end of battery life
is actually desirable, as it allows the
supply to almost drain a pair of alkaline cells. This reduces wastage and
provides a useful amount of light for
much longer.
Filament lamp circuits can’t hope to
match this result. To prove the point,
try your torch batteries with this supply when they’re almost knackered
– you’ll be amazed at the brightness
of the LED compared to the incandescent bulb!
Circuit description
The complete circuit diagram for
the power supply appears in Fig.5. It
consists of two main elements – the
step-up converter (no surprises here)
and two 7555 timers (IC1 & IC2). The
timers are part of the brightness control circuit, which we’ll come back to
in a moment. First, let’s complete the
description of the step-up converter.
In a standard application, the
MAX1676 (IC3) requires very little
external circuitry to form a complete
step-up power supply. However, in
order to regulate output current (rather
than output voltage) for our LED load,
we’ve added a few components to the
feedback loop.
Transistors Q2 & Q3 amplify the
current sense voltage developed across
the parallel 1Ω resistors. These two
transistors are connected in a current
mirror configuration, with Q2’s base
and collector connected to IC3’s 1.3V
reference output. Therefore, a known
current flows through Q2. This is used
to generate 175mV at the emitter of
Q2 and by current mirror action, Q3
attempts to maintain the same voltage
at its emitter.
The MAX1636’s internal error amplifier compares the feedback voltage
on pin 1 with a 1.3V reference. If it is
less than 1.3V, the voltage at the output
(pin 10) is increased, whereas if it is
more, the voltage is decreased. This
26 Silicon Chip
www.siliconchip.com.au
has the effect of increasing or decreasing the current through the LED.
Q3’s collector controls the voltage
on the feedback pin, acting much like
a common base amplifier. When its
emitter voltage equals 175mV (for
350mA through the LED), the collector will be at 1.3V and the loop is in
regulation.
Trimpot VR1 provides a means of
adjusting the LED current to the desired 350mA, thus accommodating
component tolerances. Zener diode
ZD1 clamps the output to a maximum
of 6V to protect IC3 should the LED
fail or be inadvertently disconnected.
The 5.6nF capacitor between the output and feedback pins ensures loop
stability.
Low-battery detection
Both rechargeable (NiCd/NiMH)
and alkaline battery types can be used
with the power supply. Alkaline batteries are a good choice for intermittent
use, as they have a low self-discharge
rate.
On the other hand, rechargeables
work well for continuous use. Their
lower internal resistance and relatively flat discharge curve provides
a higher average level of light output
over the discharge period compared
to non-rechargeables.
Unlike non-rechargeables, it’s important not to totally discharge NiCd
and NiMH cells. Repeatedly doing so
substantially reduces cell life. To help
avoid this problem, the power supply
includes low-battery indication.
When the voltage on the MAX1626’s
low-battery comparator input (pin 2)
falls below an internal reference voltage (1.3V), the comparator’s output
(pin 3) goes low. This switches on
transistor Q4, illuminating the “Low
Battery” LED.
A simple voltage divider connected
to the comparator input sets the trip
point to about 1.8V (0.9V per cell).
When running on alkalines, the LED
provides a useful indication of battery
condition.
Brightness control
The brightness of a LED can be
varied by varying the current through
it. However, rather than varying the
absolute level, Luxeon recommends
pulse-width modulating (PWM) it
instead. This results in a much more
colour-uniform light output, right
down to minimum brightness.
www.siliconchip.com.au
Fig.6: this is the waveform across the LED with VR1 at mid-position. A 180Hz
PWM frequency ensures that the LED appears to be always on. Note that the
waveform is not a perfect square wave due to the time constant of the output
filter capacitor.
To realise PWM control, it’s just a
matter of switching the LED current on
and off at a fixed frequency and varying the duty cycle (on/off time) to vary
the brightness. By using a high enough
frequency, the switching effects are
invisible due to the long persistence
of the phosphors (in white LEDs) and
the natural integration of the eye.
On the power supply board, two
7555 CMOS timers (IC1 & IC2) form
the core of the PWM circuitry. The
first 7555 (IC1) is configured as a
free-running oscillator. Its frequency
of oscillation (about 180Hz) is set by
the 680kΩ and 100Ω resistors and the
10nF capacitor on pins 2, 6 & 7.
The 100Ω resistor in the capacitor’s
discharge path is much smaller than
the 680kΩ resistor in the charge path,
resulting in a very narrow positive
pulse from IC3’s output. This is used
to trigger the second 7555 (IC2).
IC2 is configured as a monostable,
with the positive pulse width on the
output (pin 3) made variable by 1MΩ
trimpot VR1. Near the maximum pot
setting, the positive pulse width is
longer than the period of IC1. This is
where transistor Q1 comes in – it is
needed to discharge the 5.6nF timing
capacitor, effectively retriggering IC2
and allowing a 100% duty cycle at
the output.
The fixed frequency, variable pulse
width (PWM) output from IC2 is applied to the MAX1676’s shutdown pin.
When this pin goes low, the chip stops
switching and goes into low-power
mode. Fig.6 shows the waveform
across the LED at VR1’s mid position.
As shown, this results in a 55% duty
cycle or thereabouts.
Power for the 7555 timers and associated circuitry is provided via
Schottky diodes D2 & D3. By sourcing
power from the output as well as the
input sides of the circuit, we ensure
that the signal level applied to the
MAX1676 shutdown pin tracks the
output voltage and remains valid under all conditions.
Readers familiar with last month’s
Experimenter’s Power Supply circuit
may wonder why we’ve used a different (and more complicated) PWM
circuit for this design. The answer is
simple – this circuit must operate at
much lower voltages (down to 1V), and
therefore we cannot afford the diode
losses in the timing network. Note also
that we’ve used 7555 (CMOS) timers
rather than 555 (NMOS) versions,
which saves power and ensures lowvoltage operation.
Reverse battery protection
Most SILICON CHIP designs include
a diode in series with the DC input
for protection against accidental
January 2004 27
Fig.7: three SMD components go on the bottom
side of the PC board and these must be mounted
before anything else.
Fig.8: a close-up section
of the bottom side of
the board, showing just
the area of interest for
the SMD components.
Note how IC3’s leads
are positioned precisely
in the centre of the
rectangular pads.
Fig.9: follow this diagram
when assembling the top
side. Don’t miss any of
the links (there are 10 in
all), and take care with
the orientation of the ICs,
diodes and electrolytic
capacitors.
You will need fine (0.5mm) solder
and a temperature-controlled iron to
solder in the SMD components.
•
Temperature-controlled soldering
iron.
• 0.8mm (or smaller) micro-chisel
soldering iron tip.
• 0.76mm desoldering braid (“SoderWick” size #00).
• 0.5mm (or smaller) resin-cored
solder.
• Needle-nose tweezers.
• Damp sponge for tip cleaning.
• Small stiff brush & alcohol/cleaning
solvent.
• Magnifying glass and bright light
for inspection.
In addition, the job is made easier
with the aid of SMT rework flux, which
is available in a 10cc syringe from
Altronics (Cat. H-1650).
Note: the ICs used in this project
are static-sensitive. We recommend
the use of a grounded anti-static wrist
strap during board assembly.
Bottom side assembly
supply reversal. However, a series
diode in this circuit would seriously
compromise efficiency and running
time. Therefore, we’ve settled for a
reverse diode (D1) across the input
terminals.
A reversed supply will cause large
current flow through D1 and, in the
case of high-energy rechargeable cells,
will quickly destroy it. In many cases,
the diode will fail “short circuit”,
protecting the expensive (and hard to
replace!) step-up converter IC.
This is assuming, of course, that
the batteries are only momentarily
reversed. Leaving them connected
for any length of time will cause heat
damage to the board, or worse. If you’re
concerned about this possibility, then
28 Silicon Chip
install a 2A quick-blow fuse in series
with the positive battery lead.
SMD soldering gear
Referring to the various photos and
diagrams, you can see that the assembly includes three surface-mounted
devices (SMDs) – the MAX1676
converter IC and two 100nF chip
capacitors.
The MAX1676 is supplied in a
tiny “uMAX10” package with 0.5mm
lead spacing. Soldering this little
device can be a challenge – even for
experienced constructors. It must
be mounted first, before any of the
through-hole components.
The following items should be considered essential to the task:
Begin by checking the PC board for
defects. In particular, check for shorts
between pads and tracks around IC3’s
mounting site. The magnifying glass
and bright light will come in handy
here. Use your multimeter to verify
isolation between any suspect tracks.
Next, thoroughly clean the board
with a lint-free tissue (or similar)
moistened with alcohol or cleaning
solvent. The rectangular IC pads must
be pre-tinned and perfectly smooth
(free of solder “lumps”). If you have
SMT rework flux, apply a thin film to
the mounting pads.
Using needle-nose tweezers, grasp
the MAX1676 by its ends and inspect
it closely under a magnifying glass.
Make sure that the leads are all perfectly formed, with equal spacing and
alignment in the horizontal plane. In
www.siliconchip.com.au
other words, they must all line up and
make contact with their respective
pads. Carefully adjust individual leads
if necessary (you may need a second
pair of tweezers).
Place the device in position on the
board, with pin 1 aligned as shown in
Figs.7 & 8 (double-check this!). Now,
using your magnifying glass, make
sure that the device is perfectly aligned
over the rectangular pads. This is very
fiddly and requires patience and a
steady hand!
Next, clean your iron’s tip and apply a small quantity of solder to it.
With your third hand, apply light
downward pressure on the MAX1676
to hold it in position. If the package
moves (which it is liable to do), reposition it and start over.
Apply the tip to one of the IC’s
corner mounting pads, touching both
the pad and IC lead simultaneously.
The solder should “blob”, tacking the
chip in place. Check that the IC is still
perfectly aligned over the rectangular
pads. If it’s not, carefully remove it
and try again.
If you find that the package moves
whenever you try to tack the first pin,
then there is an alternative method.
First, position the IC as described
above and apply your iron to the track/
pad just in front of the IC lead (don’t
touch the lead). Next, feed a little
solder to the tip, and it should flow
along the track/pad and up over the
lead. This method is more successful
when additional flux is used.
Now repeat the same procedure for
the diagonal corner, effectively securing the IC in position. Check alignment
This view shows the fully
assembled PC board. Take
care to ensure all parts are
installed correctly.
again, as we’re about to make this
position permanent!
If you have SMT flux, apply it to
all IC leads and the adjacent tinned
copper areas. You can now solder the
remaining eight leads. Apply heat to
both the pad and lead simultaneously
and feed a minimum amount of solder
to the joint. Do not apply heat to any
lead for more than two seconds!
Despite your best efforts, you’re
certain to get “blobs” of solder and
perhaps even solder bridges between
adjacent pins. Don’t despair – this can
be fixed!
Again, if you have SMT flux, apply
a minimum amount to all IC leads and
adjacent PC board copper. Next, position a length of fine desoldering braid
across the ICs leads and heat with a
freshly tinned iron.
Table 2: Capacitor Codes
Value μF Code
100nF 0.1µF
10nF
.01µF
5.6nF .0056µF
EIA Code IEC Code
104
100n
103
10n
562
5n6
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
No.
1
2
1
1
1
1
1
1
2
1
1
2
2
1
www.siliconchip.com.au
Value
680kΩ
160kΩ
100kΩ
62kΩ
47kΩ
27kΩ
6.8kΩ
3kΩ
470Ω
270Ω
200Ω
100Ω
1Ω
10Ω 5W
4-Band Code (1%)
blue grey yellow brown
brown blue yellow brown
brown black yellow brown
blue red orange brown
yellow violet orange brown
red violet orange brown
blue grey red brown
orange black red brown
yellow violet brown brown
red violet brown brown
red black brown brown
brown black brown brown
brown black gold gold
not applicable
5-Band Code (1%)
blue grey black orange brown
brown blue black orange brown
brown black black orange brown
blue red black red brown
yellow violet black red brown
red violet black red brown
blue grey black brown brown
orange black black brown brown
yellow violet black black brown
red violet black black brown
red black black black brown
brown black black black brown
brown black black silver brown
not applicable
January 2004 29
Fig.10: the full-size PC board
pattern. Check your board
carefully for etching defects
before installing any of the
parts.
Parts List
1 PC board, code 11101041,
68mm x 62mm
1 L8 ferrite toroid, 19 x 10 x
5mm (L1) (Jaycar LO-1230)
2 2-way 2.54mm terminal blocks
(CON1, CON2)
1 3-way 2.54mm SIL header (JP1)
1 jumper shunt
2 8-pin IC sockets
1 2 x “D” cell holder
1 SPST power switch to suit (2A
contacts) (S1)
1 300mm length (approx.) 1mm
enamelled copper wire
4 M3 x 10mm tapped nylon
spacers
4 M3 x 6mm pan head screws
Semiconductors
2 7555 CMOS timers (IC1, IC2)
1 MAX1676EUB step-up DC-DC
converter (IC3) (Altronics)
1 1N5404 3A diode (D1)
2 BAT46 Schottky diodes (D2,
D3) (Jaycar ZR-1141)
2 PN200 transistors (Q1, Q4)
2 2N3904 transistors (Q2, Q3)
1 3mm high-intensity red LED
(LED1)
1 1W Luxeon Star LED (white,
green, cyan, blue or royal
blue)
Capacitors
2 100µF 50V low-ESR PC electrolytic (Altronics R-6127)
1 100µF 16V PC electrolytic
2 100nF 50V monolithic ceramic
2 100nF 50V SMD chip (0805
size) (Altronics R-8638)
3 10nF 63V MKT polyester
2 5.6nF 63V MKT polyester
Resistors (0.25W, 1%)
1 680kΩ
1 6.8kΩ
2 160kΩ
1 3kΩ
1 100kΩ
2 470Ω
1 62kΩ
1 270Ω
1 47kΩ
1 200Ω
1 27kΩ
2 100Ω
2 1Ω 0.25W 5%
1 10Ω 5W 5% (for testing)
Trimpots
1 1MΩ miniature horizontal trimpot (VR1) (Altronics R-2486B)
1 5kΩ miniature horizontal trimpot (VR2) (Altronics R-2479B)
Miscellaneous
Hot melt glue or neutral cure
silicone sealant
30 Silicon Chip
You will probably find that it’s easier
to heat two or three leads at once. The
idea is to remove all of the solder blobs
and bridges, leaving bright and wellformed solder fillets between leads
and pads.
As before, do not apply heat to any
lead for more than two seconds and
allow about 20 seconds between applications for the IC to cool! Once you’ve
done that, remove all flux with the
cleaning fluid and brush and inspect
the result under a magnifying glass.
Redo any joints as necessary.
Once you’re happy with your work,
use a multimeter to make sure that
there are no shorts between adjacent
pads and tracks. This step is very important; a hairline solder bridge can
be difficult to spot by eye!
Before moving on to the top side of
the board, solder the two 100nF chip
capacitors in place (see Figs. 7 & 8)
and install the insulated wire link. The
link can be fashioned from a length of
0.7mm tinned copper wire insulated
with heatshrink tubing or similar.
You’ll need to form a gentle bend into
the link so that it doesn’t obscure the
holes for the capacitor leads. Trim the
link ends flush with the surface on the
top side of the board.
Top side assembly
Now for the top side assembly. First,
fit an M3 x 10mm tapped Nylon spacer
to each corner of the PC board. This
will help to protect the SMD parts
while you’re installing the remaining
parts.
Using the overlay diagram (Fig.9)
as a guide, begin by installing all the
wire links using 0.7mm tinned copper
wire. Note that some of these links go
underneath components (IC1 & IC2,
for example), so they must be
installed first!
Next, install all of the 0.25W
resistors, followed by diodes
D2, D3 and ZD1. Be sure to
align the cathode (banded)
ends as shown.
All remaining parts can
now be installed in order of
height, leaving the large 100µF
capacitors and inductor L1
until last. Be careful not to mix up the
two different transistor types.
Winding the inductor
The inductor (L1) must be handwound. To do this, wind 6.5 turns of
1.0mm enamelled copper wire onto
the specified ferrite toroid. The wire
must be wound as tightly as possible
and spaced evenly over the core area
(see Fig.9 and the photos).
The start and finish should be
spaced about one turn apart. Trim
and bend the wire ends to get a neat
fit into the PC board holes. That done,
use a sharp blade to scrape the enamel
insulation off the wire ends. The ends
can then be tinned and the completed
assembly slipped into position and
soldered in place.
You can now permanently fix the
inductor to the PC board using a few
blobs of hot-melt glue or neutral cure
(non-acetic) silicone sealant.
Finally, install the two 100µF electrolytic capacitors. Note that they go
in opposite ways around, so be sure to
align the positive leads as indicated on
the overlay diagram.
Test and calibration
Don’t be tempted to hook up your
LED just yet! First, the supply must be
checked for correct operation and the
output current set.
To do this, first connect a 10Ω 5W
resistor directly across the output
terminals. Next, hook up your battery holder’s flying leads to the input
terminals, making sure that you have
them the right way around. Use the
overlay diagram (Fig.9) to determine
the correct polarity.
Note that the leads to the battery
holder should be kept as short as
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January 2004 31
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possible. We’d also recommend replacing the light duty leads (supplied
pre-wired on most holders) with
heavy-duty multi-strand cable.
The next step is to install a jumper
shunt on pins 2-3 of JP1 to disable
brightness control and to set VR2 to its
centre position. Now hold your breath
and plug in a pair of fresh alkaline
batteries.
Measure the voltage drop across the
10Ω resistor. If the supply is working
properly, your meter should read near
3.5V. If it is much lower (say, around
2.3V), then the step-up converter is
not doing its job. Assuming all is
well, adjust VR2 to get 3.5V across
the resistor.
LED mounting
The Luxeon Star’s emitter and collimating optics are factory-mounted
on an aluminium-cored PC board. In
most cases, no additional heatsinking
is required. However, a small heatsink
reduces junction temperature and
therefore ensures maximum LED life.
Just about any small aluminium
heat
sink with a flat surface large
enough to accommodate the Star’s
25mm footprint can be pressed into
service. For example, an old 486 PC
processor heatsink would probably be
ideal. A light smear of heatsink compound between the mating surfaces
will aid heat transfer.
We’ve not provided any specific
mounting details here, as they will
depend entirely on your application.
Keep in mind that the heatsink surface
must be completely flat so as not to
distort the LED’s PC board when the
mounting screws are tightened. You
should also provide strain relief for
the connecting wires.
Note that this supply is suitable for
use with white, green or blue stars but
not red or amber. This is because of
the lower forward voltage of the latter
varieties (2.3V min. versus 2.8V). With
maximum input voltage, the output of
the supply could exceed a red/amber
LED’s forward voltage, with the result
being loss of regulation and probable
damage to the LED.
LED hook-up
Wire up your Star with light to
medium-duty multi-strand cable. Try
to keep the cable length under 150mm
or so. A small copper “dot” near one
of the corner pads indicates the anode
(+) side of the LED.
Next, disconnect the 10Ω “test”
resistor and replace it with the LED
leads. That done, you can power up
and measure the voltage drop across
the paralleled 1Ω resistors. These are
situated next to the output connector
(see Fig.9). If necessary, readjust VR2
to get a reading of 175mV. As described
earlier, this sets the LED current at full
power to 350mA.
By the way, don’t stare directly into
the LED beam at close range, as it is
(according to Luxeon) bright enough
to damage your eyesight!
Note: the current calibration procedure described above should only be
performed after installing a fresh set
of alkaline batteries. If you’re using a
DC power supply instead of batteries,
set the input voltage to 2.80V (never
exceed 3.0V!)
Brightness control
To use the brightness control function, move the jumper shunt to the alternate position (JP1, pins 1-2 shorted).
By rotating VR1, it should now be possible to vary the LED intensity all the
way from dim to maximum brightness.
If required, VR1 can be mounted
away from the PC board. Keep the
wire length as short as possible (say,
no more than about 50mm) and twist
the three connecting wires tightly together. If you’re using a plastic case,
then the metal body of the pot will
probably need to be connected to battery negative to reduce the effects of
SC
noise pickup.
Card No:
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32 Silicon Chip
Where To Get Parts & Stars
A complete kit of parts for this project is available from Altronics for $34.95
(doesn’t include 1W Luxeon Star LED).
1W Luxeon Star LEDs are currently available from Prime Electronics on the
web at www.prime-electronics.com.au or phone (02) 9746 1211. You can
also get them from the Alternative Technology Association at www.ata.org.au,
phone (03) 9388 9311. Detailed technical information on Luxeon Star LEDs
can be obtained from the Lumileds web site at www.lumileds.com
www.siliconchip.com.au
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