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Wanna go prawning this summer? You’ll need our
PRAWNLITE
You can pay twenty or thirty bucks a kilo at a fish shop. Or you can
have fun and go prawning yourself. All you need is a dark night, a
suitable lake, lagoon or inlet, a prawn net and a light to attract the
little beggars . . .
P
rawning is fun. And gastronomically rewarding! It’s best done
when the moon is new – the
darker the night the better.
It’s also best done during summer.
Not only are there more likely to be
prawns around (always a good start!)
but you don’t freeze the proverbials
off in cold water!
How do you prawn?
If you’re anywhere around coastal
lakes, inlets or lagoons during the
darker nights of summer, you’re likely
to see people with bright lights and big
nets wandering around the shallows.
Almost certainly, they’re prawning.
66 Silicon Chip
The bright lights attract the prawns
(and also allow you to see ’em, which
is not easy!) and you simply scoop
them up with your prawn net. When
you’ve got enough, you take them
home, boil them for a few minutes and
enjoy your prawn feast.
Well, that’s the theory anyway. The
execution – or reality – might be a bit
different.
(a) there might be no, or very few,
prawns running in the water you’re
Design by Branko Justic
Words by Ross Tester
prawning (technically known as too
high a water:prawn ratio).
(b) you might prawn all night and
end up with not even enough to fill
one hand. (Yeah, speaking from experience here!).
(c) the bloke next to you might end
up with all your prawns ’cos he has a
better prawn light (or he knows what
he’s doing!).
(d) a myriad of other excuses reasons, not unrelated to other forms
of fishing (too moonlit/too wet/too
dry/too cold/too hot/too windy/not
enough wind/wrong tide/etc).
To save face, you call in at the local fish’n’chip shop on the way home.
siliconchip.com.au
(Two tips: buy green prawns. Everyone
will know you’re cheating if you come
home with cooked ones! And for heaven’s sake, lose the wrapping paper).
Attracting (raw) prawns
In times past, most prawners used a
hurricane lamp or gas-powered light
to attract the prawns.
That’s fine, except you can’t put
them under water. And, surprising
as it might seem, that’s where the
prawns are.
What you need is a bright light
which can go under the water surface
– not deep, just enough to spot the
prawns. Generally you’d prawn in
water that’s well under a metre deep
anyway – maybe 300-500mm.
More recently, a lot of prawners have
used waterproof high-power torches.
They work but can be back-breaking.
Or they use a commercial prawn
light. There are many available these
days, most operating from a battery
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(usually a rechargeable) which you
wear in a belt pack.
There are some high power ones
using QI (halogen) bulbs which work
very well but tend to flatten the battery
too quickly.
Commercial prawn lights aren’t
cheap. But with the rather dramatic
reduction in the price of ultrabright
white LEDs in recent months, we
figured they would be a proposition.
As well as bright, they’re efficient,
not wasting too much energy as heat.
So they will give your battery a lot
better run for its money (or charge).
Our Prawnlite
The circuit is pretty simple. A 1kW
resistor (R17) biases three diodes (D1,
D2 & D3) to provide a voltage reference
of about 1.8V. This is fed to the base
of transistor Q17 which is connected
as an emitter follower. Its output
becomes a 1.2V driver to the bases
of the 16 transistors, all of which are
connected as constant current drivers,
each with a load of three LEDs (16 x
3 = 48 LEDs). The constant current
transistors each apply 0.6V to their
33W emitter resistors, resulting in a
LED current of about 18mA.
The LEDs are arranged on a circular
PC board which itself fits into a waterproof glass jar, shining through the
bottom. Presto, one very bright white
light which can go underwater.
Such a light is not just limited
to prawning. If you’re looking for
a general-purpose 12V bright light
(perhaps for camping, etc) then this
would be a beauty.
You could use it for prawning, then
for cooking, then for eating back at
camp!
Half or full power?
The PC board has been arranged so
that you can build the Prawnlite as
either half-power or full power, simply by choosing the number of LED
January 2005 67
Here’s a close-up view of the Prawnlite in
its Salsa Dip container. As you can see,
there is plenty of clearance for the PC board
in this particular jar.
strings you install. In fact, two kits of
parts are available which reflect this
flexibility.
The first kit contains the PC board
and half the LEDs, transistors and
resistors, along with the reference voltage components, allowing you to build
it in that format if you wish. Needless
to say, that’s the economy route.
The second kit contains the remaining LEDs, transistors and resistors
which, when added to the first kit,
enable the full power version to be
built.
As all LED/transistor/resistor
strings are identical, if you build the
half-power version you can choose
which areas of the PC board you
populate, as long as the trios of LEDs
and their associated transistor and resistor are kept together (ie, LEDs 1,2,3;
25,26,27; etc).
The components to the left of the
LED strings, the constant current
circuit, remain the same regardless of
how many LEDs are fitted.
From here on, though, we’ll assume
you are building the complete kit containing 48 LEDs.
The battery
The Prawnlite draws around 290mA
from a 12V supply so there shouldn’t
be too much worry about quickly flattening your battery.
It is designed to run off one of the
medium-sized 12V SLA batteries –
which, of course, could be charged via
a suitable charger from a car battery
when mobile.
(Such a charger was described in
SILICON CHIP in July 1996). Or perhaps
you could use a solar charger – again,
we’ve described one of those in March
2002.
A typical 7Ah SLA, fully charged,
should give more than 20 hours of light
before needing a recharge.
You could run the Prawnlite from
smaller than a 7Ah battery – and that
would mean significantly less weight
to carry, particularly when you’re
standing out in the water. Smaller
SLAs are also generally cheaper.
You could even run this from a bank
of NiCads or NiMH cells (perhaps even
inside the handle?), bearing in mind
that 290mA drain.
Most commercial electric prawn
lights use a battery mounted in a
purpose-built belt. We suggest a battery in a bum bag would be just as
effective – and a lot cheaper!
Just make sure you don’t get the
battery wet – salt water and batteries
do not make good bedmates!
And one more thought: you could
even make up a battery using highcapacity AA cells (say 2 x 10 in series)
48 ultrabright white LEDs shine through the bottom of the
glass jar. It’s not just a prawnlite – this also makes a dandy
general-purpose 12V light as well!
68 Silicon Chip
siliconchip.com.au
all around the cable hole (inside and out) and the screw
holes. In fact, the sealant also acts as an adhesive and takes
some of the stress off the bolts and nuts.
Construction
E
B
C
C8050
R16
Q16
λ
L48
λ
L45
siliconchip.com.au
A
1N4148
A
K
Three diodes and Q17 provide constant voltage drive
to the bases of 16 transistors, assuring constant current
drive to the 16 banks of LEDs.
SC
2005
DIODES: 3 x 1N4148
D3
BLACK
CLIP
LEAD
22nF
PRAWNLITE
RESISTORS: 16 x 33 Ω
TRANSISTORS: 17 x C8050
R1
Q1
22nF
D2
K
D1
A
12V
1kΩ
(R17)
RED
CLIP
LEAD
Same-size
component
overlay. Note
the green
resistor in the
middle of the
PC board. It’s the
only one which isn’t
33W!
LED
LEDS: 48 x ULTRABRIGHT WHITE, 5mm
λ
L39
λ
L33
λ
L30
λ
L27
λ
L24
λ
L21
λ
L18
λ
L15
λ
L12
λ
L9
λ
L6
λ
L3
E
Q17
B
C
λ
L36
λ
L38
L35
λ
λ
L32
λ
L29
λ
L26
L23
λ
λ
L20
λ
L17
λ
L14
L11
λ
λ
L8
λ
L5
L2
λ
K
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
L31
L28
L25
L22
L19
L16
L13
L10
As mentioned before, the Prawnlite is designed to fit inside a glass jar. The particular glass jar we used originally
contained Dorito’s salsa dip – a semi-liquid, so we knew
that the jar would be waterproof. It’s made that way by a
gasket incorporated into the screw lid.
The jar measures about 87mm (OD) x 75mm (outside
height). Of course, you could use other similar-sized jars
but make sure that (a) the PC board assembly will fit (the PC
board is 65mm diameter) and (b) that it can actually hold
water; that is, a gasket of some sort must be incorporated
into the lid to seal against the glass jar.
What complicates the issue a little is that we need to
supply power to the PC board, which necessitates drilling a hole in the lid. We also wanted to fit a handle to the
jar so it could be used like a wand. This also necessitated
drilling a couple of holes for bolts and nuts to hold the
handle in place.
We overcame both these problems through the liberal
use of silicone sealant,
L7
Waterproofing
L4
that fitted inside the handle. Now that would be clever!
A L1
Straight-on view of
the completed PC
board mounted on
the glass jar lid. As
you can see, it works!
L34
L37
K
λ
L42
L47
λ
λ
L44
L41
λ
λ
λ
λ
L40
L43
L46
Start by checking the PC board for any defects. They’re very
rare these days but a quick check now can save a lot of head
scratching when something doesn’t work as it should.
January 2005 69
GLU
Parts list – Prawnlite
Semiconductors
48 ultrabright white 5mm LEDs (L1-L48)
17 C8050 NPN transistors (Q1-Q17)
3 1N4148 silicon diodes (D1-D3)
Capacitors
2 22nF MKT
Resistors (0.25W, 5%)
1 1kW
16 33W
K217
FOAM
LID
FOAM
GLASS JAR
DLE”
Making the “container”
This is probably the most difficult section because you
have to be 100% sure that the final thing is waterproof.
The handle is made from a short length of 20mm PVC
electrical conduit (25mm would also be fine). The end of
the conduit is cut on a shallow angle to allow it to mount
flush with the lid (see diagram at left).
Two holes are drilled through the conduit, with matching
holes in the lid, for the stainless steel nuts and bolts which
hold the conduit to the lid. A separate hole is drilled in the
lid for the power leads. These then progess up the conduit
handle, to emerge at the top end, thence to the battery via
a pair of crocodile clips.
SILICONE
Finally, liberal amounts of silicone sealant are used to
SEALANT cover the conduit-to-lid join and over the tops of the stainAS GLUE less steel bolts. The same sealant is used at the top of the
(OR HOT
handle, not only making the conduit top waterproof (it will,
MELT
at some time, receive a dunking no matter how careful you
GLUE)
are!) but also making the power leads captive.
It’s probably best to work on the lid assembly first as
ENSURE this must be right!
Cut the conduit to a suitable length (ours was a tad over
SEAL IS
INTACT
IN LID
You’ll find it easiest to populate and solder the PC board
by working from the middle out – so start with the four
diodes, transistor Q17, the two capacitors and then all the
resistors. All components mount as hard down onto the
PC board as they will go. By the way, by some quirk the
three diodes on the prototype PC board were labelled D6,
D5 and D7 instead of D1, D2 and D3. We’re assured that
production PC boards will be labelled correctly.
To make it easy for you, we’ve coloured green the only
1kW resistor (labelled R17) on the component overlay. Put
this one in first, then you can’t make a mistake with the
rest – they’re all 33W!
Next, fit all the remaining transistors. All face the same
direction around the ring – the flat side is clockwise.
Solder these in place, taking care with bridges between
close pads.
Finally, it’s time for the LEDs. Unlike the transistors,
70 Silicon Chip
there is no consistency to LED direction. Take
careful note of the overlay and the overlay also
printed on the top side of the PC board. If you
get any LED wrong, the other two in its string
will also not work.
Give your finished board the once-over and if
you’re happy with the component placement and
soldering, temporarily connect the two power leads.
These solder underneath the board – take care with
the polarity!
Connect the red and black to your 12V battery or
power source, obviously red to +ve and black to –ve.
Assuming it works – ie, all LEDs light up – you can
disconnect and unsolder them. Incidentally, don’t look
directly into the LEDs because they are so bright, they
will dazzle you.
HAN
CON
DUIT
“
PVC
1 PC board, 65mm diameter, code
K217 (Oatley Electronics)
1 small glass waterproof jar with lid
(with sealing gasket to make
waterproof) – around 85mm diameter
x 75mm high
1 12V SLA battery
OR
1 12V NiCad or NiMH battery
3m length polarised Figure-8 cable
OR
2 lengths 3m red and black hookup
wire, twisted together
1 red crocodile (alligator) battery clip
1 black crocodile (alligator) battery clip
Silicone sealant
E
TO 12V SLA
BATTERY
(BELT PACK)
The completed assembly immediately before it goes into its
glass jar “case”. The “HOT” salsa is entirely optional . . .
we wonder how it would taste with prawns?
siliconchip.com.au
A further two shots of the top of the Prawnlite showing
how the conduit is shaped, then screwed and glued to the
top of the lid. Make sure it’s thoroughly waterproofed with
silicone sealant or hot melt glue . . . or both!
1m which is about right for an average-height adult. For a
child it would want to be somewhere about 650-750mm)
and then cut one end at a suitable angle (the drawing at
left should give you a good idea of this).
Drill two 3mm holes through the conduit at about 12mm
and 72mm from the end, with matching holes in the top
of the lid. The stainless steel screws we used had countersunk heads – if yours do, carefully countersink the holes
in the conduit. Drill an additional hole in the lid right in
the centre – this one should be larger, say about 5-6mm,
to suit the power leads you use. The power leads should
be able to pass through easily.
Pass your power lead through the conduit from the square
(top) end, leaving about 100mm protruding from the anglecut end. That means the bulk of the power leads emerge
from the top end. Solder a pair of crocodile clips (red and
black) to the far ends of the red and black power leads.
Now pass the power lead through the hole you drilled
in the lid and fasten the angle-cut end of the conduit to the
lid with the two screws. Tighten the nuts and then apply
generous dollops of silicone sealant or hot melt glue onto
the lid, right around the edges of the conduit. Make sure
all parts of the join are well sealed.
Also apply some silicone to the underside of the lid, over
the tops of the two nuts and bolts and, if you wish, to the
point where the power leads come through.
Also fill in the top end of the conduit with silicone sealant or hot melt and leave to dry.
Mounting the PC board
While that’s happening, let’s look at mounting the PC
board.
Naturally, it needs to sit so that the LEDs shine out
through the bottom of the jar. This means you’ll need a
support cylinder of foam rubber, styrene foam, etc, just a
bit smaller than the inside diameter of the jar. The depth
needs to be just a bit less than the internal height of the jar
less the height of the PC board assembly. Assuming you’ve
soldered all LEDs hard down on the board, that’s around
12mm or so – call it 20mm to be safe.
The cylinder will need a hole up the middle for the
power leads to not only pass through but squash back up
into somewhat, when the PC board is brought down onto it.
siliconchip.com.au
It can be secured to the jar lid using silicone sealant (as
a glue) or you could use contact adhesive.
When the handle/lid assembly is dry, cut the power leads
to say 50mm (or even less if you have a fine soldering iron).
Strip off 5mm of insulation and solder the leads, again
watching out for polarity, to their appropriate positions
on the underside of the PC board.
As the PC board is brought down on the foam/styrene
cylinder, the power lead has to squash back into the middle
(aha! that’s the reason for the hole!) The PC board itself can
be glued or taped to the cylinder so that it sits square on it.
(We used ordinary clear sticky tape). The final result should
look something like the photo on the opposite page.
Before screwing the glass jar onto the assembly, check
to make sure the gasket (normally glued to the inside of
the lid) is intact – you don’t want any path for water to get
in – because if it can, it will!
Also, check again that the LEDs still light when you connect power. If so, screw the jar onto the lid, ensuring that
the LEDs miss the bottom of the jar by a few millimetres or
so. That’s it: your Prawnlite is now completed. Connect it
to a 12V battery and test that it works as intended.
Good luck with your prawning – can’t you just taste
those scrumptious little crustaceans already?
And if you discover a really good prawning spot – please,
SC
let us know? Promise we won’t tell anyone else . . .
Where from, how much?
The Prawnlite comes from Oatley Electronics, who hold
the copyright on both the design and the PC board.
Oatley have available two kits of parts:
K217 sells for $22.00 and contains 24 white LEDs plus 9
transistors, 9 resistors, 3 diodes, 2 capacitors and the PC
board. This makes the “half power” version.
K217e, when added to the first, makes the full power version. It contains another 24 white leds, 8 transistors and
8 resistors and retails for $15.00
You need to supply the glass jar, the length of conduit,
screws & nuts, silicone sealant/hot melt glue, etc.
Contact Oatley Electronics on (02) 9584 3561 or via their
website, www.oatleye.com
January 2005 71
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