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LED
Driver
This white LED driver will drive 30 white
LEDs in six groups of five from a 12V source
such as an SLA or car battery. It can be switched
on and off manually or it can switch on automatically
when darkness falls.
By STEPHEN DAVID
W
HILE WE HAVE now published quite a few LED driver
circuits, to date we have not
published a design to drive a bunch
of high-brightness white LEDs. Such
a circuit is now quite desirable as the
price of white LEDs has fallen and
you can have a handful for not a lot
of dollars.
However, white LEDs do present a
problem because they need a higher
drive voltage than monochromatic
types such as red, green, orange etc.
Instead of around 1.8V to 2V or thereabouts, they normally require more
than 3V to produce their rated brightness. In fact, if you are driving a bunch
of them you need to drive them all
at constant current otherwise their
individual brightness tends to vary
markedly.
However, if you only have a 12V
supply available, you can only put two
or maybe three LEDs in series together
with a constant current source and this
leads to poor efficiency.
Where To Buy A Kit
This kit has been designed by Oatley Electronics who own the copyright. The
kit comes in two parts:
(1) K202 which includes the PC board, the driver circuitry, 10 high brightness
white LEDs and two current source transistors; and
(2) K202A which provides 10 high brightness white LEDs and two current
source transistors, so with K202 and two K202A kits you get the full complement of 30 white LEDs.
Pricing is $17 (including GST) for K202 and $8 for K202A. The optional
swivel mounting bracket is $1.00 while postage and packing is $6 within
Australia. Kits may be obtained direct from Oatley Electronics, PO Box 89,
Oatley NSW 2223. Phone (02) 9584 3563; Fax (02) 9584 3561. Website:
www.oatleye.com
www.siliconchip.com.au
The approach in this circuit is to
boost the 12V supply to something
around 21V and this means that we
can have groups of five LEDs, each in
series with their own current source
transistors.
The result is a single PC board with
the drive circuitry and 30 white LEDs.
It can be used for lighting in caravans
and recreational vehicles, emergency
lighting or whatever application you
can think of. Current drain is around
190mA at 12V.
Circuit description
Now let’s have a look at the circuit of
Fig.1. It uses just one IC (a 4093 quad
NAND Schmitt trigger gate package),
a few transistors and diodes, 30 white
LEDs and not much else.
So where is the familiar boost
converter circuit? Answer: there isn’t
one or least not one with an inductor
switched by a Mosfet. Instead, there
is a charge pump inverter, comprising
IC1c, transistors Q2 & Q3, Schottky
diodes D1 & D2 and a few capacitors.
It works as follows:
IC1c is connected as an inverter
oscillator and its running frequency
March 2004 73
Fig.1: the boost circuit involving oscillator IC1c and transistors Q2 & Q3 drives a diode pump (D1 & D2) to step up the DC to around 21V.
74 Silicon Chip
of about 30kHz is determined mainly
by the 6.8kΩ resistor between pins 8
& 10 together with the 4.7nF capacitor
at pin 8. This produces a rectangular
waveform (not quite square but pretty
close) at pin 10 to drive complementary switching transistors Q2 & Q3.
The waveform at their commoned
emitters drives a diode pump consisting of two 100µF capacitors and
Schottky diodes D1 & D2. The waveform generated by the circuit can be
seen in the scope photo of Fig.2.
RS flipflop
Oscillator IC1c is controlled by an
RS (Reset/Set) flipflop comprising the
two NAND gates IC1a & IC1b and this
is controlled by pushbutton switches
S1 and S2. Normally, this has its pin 4
low and pins 1 & 6 are pulled high via
470kΩ resistors. Momentarily closing
S1 (ON) pulls pin 6 low, causing the
flipflop to change state so that pin 4
now goes high to enable IC1c which
now oscillates at 30kHz.
The 30kHz waveform produced by
transistors Q2 & Q3 drives the diode
pump referred to earlier and this
develops about 21V to drive the LED
columns.
Each column of five white LEDs
is driven by its own current source
transistor which has a 33Ω emitter
resistor. The bases of all six current
source transistors (Q4-Q9) are driven
from pin 4 of IC1b via a 6.8kΩ resistor
and clamped to a maximum of +1.2V
by diodes D3 & D4. Subtract the 0.6V
between the base and emitter of each
transistor and you are left with 0.6V
across each 33Ω resistor, thus setting
the LED drive current to 18mA.
Switching the circuit off is accomplished by pushing the OFF switch,
S2. This momentarily pulls pin 1 low
to toggle the RS flipflop, thus causing
pin 4 to go low. This disables IC1c,
Q2 & Q3 and also turns off the current
source transistors.
Note that there is an interesting
wrinkle to this drive circuit, because
there is no On/Off switch. This means
that the current source transistors must
be turned off otherwise they would
continue to draw current from the
12V supply even when the circuit is
nominally off. The current path may
not be obvious but it is via the boost
circuit’s diodes, D1 & D2.
Auto on/off
As well as using the pushbutton
www.siliconchip.com.au
The optional swivel-mount
unit is fitted directly to the
bottom of the PC board and
allows the “LED Lamp” to
be adjusted to a convenient
angle.
Fig.2: this scope
shot shows the
waveform at the
commoned emitters
of transistors Q2 &
Q3.
Fig.3: the component overlay for
the PC board. Sections can be
snapped off to provide “lamps”
at separate locations.
switches S1 & S2 to turn the circuit
on and off, there is also a facility to
automatically turn the circuit on and
off depending on ambient light levels.
Links L1 & L2 can be used to provide
Auto On and Auto Off respectively and
these features can be used separately
or together.
An LDR (light dependent resistor)
is used to monitor the ambient light
level. When light falls upon it, it pulls
the base of Q1 low, causing pins 12 &
11 of IC1d to go low and its pin 11 to go
high. When darkness falls (or the room
lights go out), the process is reversed.
Depending on whether you have one
or both links connected, you can use
www.siliconchip.com.au
the pushbuttons to turn the circuit
on and off and have it turn on and/off
automatically as well.
Q1 also drives a red high brightness LED (LED1) at very low current,
via a 470kΩ resistor. This is a bit of a
gimmick but it does have the benefit
of showing that this part of the circuit
is working, if you have to troubleshoot it.
Board options
As presented, the PC board is 130
x 47mm and it has three snap-off sections, each carrying 10 LEDs and two
drive transistors. This gives you the
option of having all 30 LEDs on the
board or having three separate LED
“lamps” spread around your tent,
caravan, boat, yurt or whatever. You
would need three wires to interconnect each board section, if you take
that option.
The full board component overlay
is shown in Fig.3 and it shows a full
complement of 30 LEDs (plus red
LED1).
No special order of assembly is
necessary but take care to insert all the
polarised components correctly. Note
the little flat on one side of the LEDs;
this needs to match the screen-printed
overlay on the PC board.
Make sure you connect the supply
wires correctly. Reversing them will
almost certainly cause component
SC
damage.
March 2004 75
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