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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 siliconchip.com.au (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