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Fantastically effective as a bike headlight or hand-held floodlight Hallelujah . . . I’ve Seen The Light! By JULIAN EDGAR Features: ty construction  Durable with high quali  Easy and cheap to build power supply voltage can be selected to match lb Bu  www.siliconchip.com.au with great penetration  Excellent broad beam SS eptember eptember2003  15 2003  15 Our new DIY bike headlight is just the thing if you want to see where you’re going at night – or have others see you coming. Rather than casting an anaemic spot of light on the ground only a few metres ahead of the bike, this headlight will throw a swathe of light with at least a 50-metre range. It’s also durable, easy to build and costs little. You can power it from a conventional cycle generator, normal or rechargeable batteries, or do what we’ve done – and that is build a dedicated sealed lead acid (SLA) battery pack. I N FACT, the package of our headlight and SLA battery pack makes for a really great bike headlight system – plenty of light, excellent durability, very cheap running costs and an up-front price that’s well under many premium bike lighting systems. Or if you wish, you can place the SLA battery in a shoulder or belt pack and use it as a very powerful and light hand-held floodlight. The design So what makes this design so effective? Firstly, the light beam is tightly focused by a convex glass lens. But isn’t this expensive? Well, no – not when you use a magnifying glass! The lens used in our bike headlight is a 70mm dia­meter magnifying glass. And it is actually glass, rather than being made from plastic. Using such a large lens works very well in focusing the beam which is produced by an incandescent bulb and its dedicated reflector. Secondly, the design uses a good quality multi-faceted reflector. It’s from an Eveready torch – model E250K (and it appears that the Eveready E220, E250 and E251 torches are very similar). This is a two ‘D’ cell torch with a reflector that’s 45mm in diameter. It costs about four dollars so it’s certainly not expensive. We’d expect that any torch with a decent quality reflector would be able to be used in this application. Finally, the bulb is matched very carefully to the battery so as to give a very good output while having appropriate durability. Bulbs, bulbs, bulbs The bulb that you use in the head- 16  Silicon Chip light depends on how you intend powering it. If you are using a 6V sealed lead acid (SLA) battery, you can use a 6V 0.5A Eveready torch bulb. Most of the design and development was based around this bulb – with this bulb fitted, the headlight gives out plenty of light. This is the bulb we’d recommend. If you want more light (and a little less endurance), Mag-Lite make a very high performance krypton bulb that’s suitable for use with a 6V SLA battery. It’s designed for use with 5-cell It’s amazing what a few “odds-’n’-ends” can become: a drink container becomes a superb pushbike light! torches. At 6.2V, it draws 0.67A and is Part No. LWSA501U. If you are powering the light with a 6V lantern battery, the 4.8V 0.7A krypton bulb normally found in an Eveready Dolphin-type torch works extremely well. The reason that the 4.8V bulb cannot be used with the 6V SLA battery is that when the battery is fully charged, the SLA battery will actually have an output higher than 6V – and this causes the 4.8V bulb to have a very short life. If you want, you can even use a 2.4V krypton bulb and power the headlight via two D-cells, or a 2.2V bulb and use two rechargeable D-cells. So as you can see, the headlight is very versatile! But which ever bulb you use, make sure that it is a high-quality brand name bulb – don’t be tempted to replace it with a cheap generic one. We made this mistake during the development of the headlight and both the light output and the quality of the beam pattern suffered. Note that when built exactly as described here (ie, using this lens, reflector and the 6V 0.5A bulb), there will be a slightly darker spot in the middle of the beam. Replacing the plastic “lens” from the original torch (which has a matte-finish circle in the middle of it) back into the holder will help remove this spot but this also reduces the overall light output slightly. The housing for the torch is rustproof stainless steel – but it’s not expensive as in its former life it was actually a drinking cup! The moulding around the front of the lens is made from a U-PVC pipe cap, while the stainless steel and plastic mount was obtained from a marine shop. The reflector support inside the headlight is formed from the front part of the Eveready torch, while a weatherproof switch on the back of the headlight is from a marine or electronics shop. www.siliconchip.com.au After you’ve used a file and then fine sandpaper to clean-up the cut inner edge of the pipe cap, use silicone adhesive/sealant to glue the glass lens inside the cap. Don’t smear it all over the glass – surplus sealant can be Finding the Lens Focal Length This sounds complex but it’s actually dead easy. While inside, hold up the lens to a bright window. Behind the lens place a piece of white card (or use a light-painted wall opposite the window) and move the lens closer and further away from the card/wall. When you can see a sharp image of the distant scene outside the window on the card, accurately measure the distance between the lens and the card. That is the focal length of the lens. The completed headlight has a mass of just 300g. Making it (1). The housing The first step in making the headlight is to obtain the stainless steel drinking cup. will it scratch easily, it will also discolour over time and won’t have the light transmission or other optical properties required. The 75mm magnifying glass used here was bought from a newsagent for $4. It had a focal length of about 18cm and was originally mounted in a plastic holder. (3). Front moulding Once you have the lens sorted, you’ll need to buy a plastic pipe cap from a hardware store. The cap needs to be a tight fit over the end of the cup and in our case, a 75mm pipe cap was perfect. Using a hole-saw and/or a sharp knife, cut out the centre of the cap so that you’re left with just the rim and a The one shown here has a front dia­meter of 75mm, a rear diameter of 50mm and a height of 100mm. These dimensions aren’t critical – so long as you adjust the other parts requirements to suit. So, the glass lens will need to have a diameter that matches the opening size of the cup, for example. Stainless steel has a huge advantage in this application – it’s rust-proof. Aluminium cups can also be used (they’re also rust-proof) but they’re not quite as strong. (2). The lens Once the cup has been acquired, buy a glass magnifying glass to suit the cup’s mouth diameter. Don’t buy a plastic magnifying glass – not only www.siliconchip.com.au small width of front face around the edge. The glass lens should fit inside the cap and the cap should then in turn fit tightly over the end of the stainless steel housing. removed using a rag moistened with mineral turps. The silicone should form a watertight seal around the lens. You should then be able to trial mount the front lens in place. (4). Reflector support As mentioned earlier, the reflector and its support are obtained from an Eveready torch. Unscrew the reflector and lens end of the Eveready torch and then very carefully remove the reflector and the plastic “lens”. This lens won’t be used though, because it reduces the overall light output. A hacksaw can then be used to cut off the front end of the torch – the black collar and its threaded section. You should be left with a highly September 2003  17 • • • • • • • • • • Parts List – Lamp Reflector and lampholder from suitable torch (see text) Bulb to suit battery used (see text) 70mm (approx) glass magnifying glass Stainless steel drink cup 75mm (approx.) pipe cap Weatherproof toggle switch (5A DC) Mounting bracket Silicone sealant Medium/heavy-duty fig-8 cable Stainless steel self-tapping screws polished multi-faceted reflector, a black collar and its associated male thread (not shown), and the cap that screws down over that collar. Because the lens is now removed, when the reflector is re-inserted into its holder and the cap screwed down over it, the reflector can rattle. To cure this, place a rubber ring inside the holder (we used an old drive-belt from a VCR). No LEDs? You may be wondering why we’re using a relatively power-hungry incandescent bulb for this bike headlamp, rather than much more efficient high-intensity white LEDs. Well, we wanted to use LEDs and spent a long time working with different LED prototype headlights. We tried multiple LEDs bunched together, we tried total internal reflection (TIR) optical guides directing the light from lots of LEDs to the one focal point (and then focusing that beam), and we tried multiple LEDs – each in its own reflector. But none of these headlight designs produced enough light: while a LED works in a small torch, for a bike headlight where a much broader bright beam is needed, LEDs can’t (yet) cut it. To get a broad, high intensity beam, the only way was to use a traditional (albeit high-quality) bulb. However, we’ve made the very best of that light by using a good quality reflector and then a giant focusing lens. The resulting output rivals 12V dichroic halogen reflector lights using up to seven times as much power. 18  Silicon Chip The reflector will now be held firmly in place when the cap is screwed down. The assembled reflector should look like the one shown below. (The plastic ‘lens’ is left out because it will absorb some of the light and the completed headlight is weatherproof anyway.) This complete assembly should now slide down inside the cup, with the front face ending up about 40mm down from the mouth of the stainless steel housing. The taper of the reflector housing is a good match for the taper of the stainless steel cup, so it sits in place neatly. But don’t do it quite yet – there are lots of steps to come first! (5). Power for the bulb Power to the bulb comes via two wires that are soldered into place. One is soldered to the back of the bulb holder (where once the positive terminal of the battery nestled home) and the other to the metal rim around the bulb holder. You can now solder these wires into place, using reasonably heavy-duty figure-8 wire. Remove the 2.4V bulb that was supplied with the torch and replace it with a bulb to suit your power supply (6V for the 6V SLA battery or, as shown here, 4.8V for 6V dry batteries). Apply power to the bulb and make sure that it shines brightly. Next, drill a hole in the lower part of the stainless steel housing where you want to the cable to come out, then insert a grommet and slide the cable So that the bulb can be replaced when it blows, the bulb/reflector assembly needs to be able to be removed when necessary – so you can’t just glue the whole holder in place inside the cup. Instead, two self-tapping stainless steel screws are inserted from outside the cup, so that they screw into the plastic part of the reflector holder. Drill pilot holes for these and place a dob of sealant on each of them before screwing them home. through it. Place the reflector and its holder in the cup, put the glass lens in place and then check out how good the beam is. When shone at right-angles against a wall, the prototype headlight had a circular spot diameter of 60cm at a distance of three metres. While this sounds very narrow, when you consider that the beam range is about 50 metres, it spreads out nicely. In fact, one of our design aims was to have www.siliconchip.com.au How Long Will The Battery Last? a headlight that was wide enough in beam spread to attract the attention of motorists (ie, to allow the bike to be spotted) while at the same time illuminating plenty of road. If your beam is too narrow (or too broad), you’ll need to look at changing the reflector-to-lens distance, or the focal length of the magnifying glass. Experimentation is the simplest way. (6). Power switch The switch needs to be weatherproof. Marine stores sell 12V weatherproof switches, while some electronics stores sell weatherproof rubber boots, or caps, that fit over normal toggle switches. So that we could use a small switch, we took the latter approach here. The switch is mounted on top of the lamp housing towards the rear, where it clears the internal reflector support but is easy to get at. Drill the hole for the switch and mount it now. So with a fully charged battery, how long will the light last? That’s a much harder question to answer than it first appears – but in short, a good length of time. But isn’t it easy? Don’t you just divide how many amp-hours the battery is rated at by the current the bulb takes? So, with a 0.5A bulb and a 4.2Ah battery, won’t the bulb last 8.4 hours? Well, yes and no. The manufacturers of batteries provide curves showing discharge versus voltage – but so much depends on the starting voltage (ie, how fully charged the battery is), whether the current draw is continuous or in short spurts and, of course, the load. The 4.2Ah battery shown here is actually rated by the manufacturer as a 3.4Ah battery when supplying a current draw of 0.68A for 5 hours. However, we did some careful testing to make sure that the battery wouldn’t fall over in 30 minutes or something terrible like that. With a starting (under load) voltage of 6.2V (6.44 without the load), the battery had the following actual performance when continuously powering a 6V, 0.5A bulb. It took just under an hour before the battery voltage dropped to 6V. By the end of the second hour, the voltage was over 5.8V, and by the end of the third hour it was 5.6V. However, when left switched off overnight, the next day under load the battery had jumped back up to 5.8V – see what we mean about the difficulty of getting a clear picture? In discontinuous use, we’d expect no problems with at least four hours of light – and in continuous use, three hours should not be a problem. And then, of course, you just plug in the charger and for a few more cents you have another 3-4 hours of light. This is much cheaper than buying more batteries – and also much simpler than pulling out rechargeable double-As and inserting them into a charger – and then remembering to put them back into the headlight. Mounting it The headlight is mounted using a polycarbonate and stainless steel ‘adjustable rail clamp’, bought from a marine supplies shop (we used clamps from Whitworths Discount Marine Supplies, Cat. 70482 at $6.95). A hole was drilled through one arm of the clamp and a large diameter screw inserted through it. A washer www.siliconchip.com.au and a nut were placed against the clamp, then the screw attached to the lamp housing. Washers and nuts were used either side of the stainless steel of the cup. (Note that the back of the reflector plastic mount may have to be cut away a little to give clearance to the nut). All the hardware is stainless steel – the bolt, nuts and washers. Stainless steel fasteners can be obtained from marine stores – again we bought them from Whitworths. The clamp was set up in this way so that easy adjustment of the side-toside aiming is possible (just loosen the nut against the clamp), and easy up/ down aiming can also be carried out (just loosen the clamp). The slight ‘stand-off’ also gives room for the clamp screws to protrude past the clamp, as will happen when September 2003  19 Parts List & Sources: 6V Battery Pack • • • • • • 6V SLA plug-pack battery charger 6V 4.2Ah rechargeable SLA battery Alloy box Fuseholder and fuse Waterproof plug and socket Frame clamps and stainless steel nuts and bolts The first four items were purchased from electronics supply stores, and the last two from a marine supplies shop. After much searching to find a matching box and battery, the battery was purchased from Jaycar Electronics (Cat. SB-2496) and the box from Dick Smith Electronics (Cat. H-2206). Unfortunately, the box is a fraction (like about 1mm!) too small in height and so the lid stands a little proud when it is screwed down. However, this holds the battery very firmly in place and a watertight seal is still retained by the use of some silicone sealant around the lid. Any size 6V SLA battery can be used – the one shown here was chosen on the basis of its compact size and good capacity. If you go smaller you’ll have less hours of light; bigger capacity equals more hours of light. So if you’ll never want more than (say) an hour of light in one stretch, you could use a smaller SLA 6V battery. The plugpack charger is from Jaycar, Cat. MB-3516, designed specifically to charge 6V SLA batteries. It charges at 0.5A and then when the battery is fully charged, automatically switches to trickle charge. This change in charging state is indicated by the LED on the charger starting to flash. This means that the charger can be left plugged into the battery pack for long periods without any problems – and that the battery will always be ready to go but not overcharged. The waterproof plug and socket was bought from marine suppliers Whit­ worths. It is much heavier duty than is really needed but we couldn’t find any smaller weatherproof designs. The plug doesn’t need to be weatherproof but the socket needs to be able to be sealed off when the bike is out and about. Also, you don’t want the socket to rust or otherwise corrode. One advantage of this socket is that it has a weatherproof cap on a captive chain – always screw it on whenever the battery isn’t being charged, as the terminals are always ‘live’. The stainless steel and polycarbonate frame clamps and hardware are the same as used in the headlight design. The completed battery pack has a mass of 1.4kg. has good endurance, and is convenient – you simply plug a pre-built battery charger into the battery pack whenever the bike isn’t being used and unplug it when you take the bike out. It’s an ideal match with the 6V 0.5A bulb. It will cost mere cents to charge the battery this way – so low, in fact, that it may not even turn the electricity meter! Another possible alternative: if your bike is left outside during daylight hours (eg, after riding it to school or work), you could even place a small solar cell or two somewhere on the bike and charge the battery with free electricity from sunlight during the day. We haven’t tried this but it’s certainly an option. Building it they’re finally tightened. Final Assembly The final assembly process involves using sealant – around the self-tapping stainless steel screws that hold the reflector holder in place and around the plastic rim at the front of the headlight. You’ll need to break this seal and undo the screws to change the headlight bulb. Another approach is to use 20  Silicon Chip a large O-ring around the underside of the rim. The O-ring will prevent leakage of water into the headlight without any sealant needing to be applied. The clamp can be used to mount the headlight on the bike handlebars. An SLA battery pack This 6V rechargeable battery pack is easy and relatively cheap to build, The first step is to drill the box to take the charger socket. This requires three holes for the mounting screws and a larger central hole for the cable access. The screw cap is normally retained on a chain but here it has been removed to facilitate the mounting of the socket. The mounting clamps are next, and – as with the headlight – these clamps are spaced away from the box using stainless steel nuts and screws. This gives enough room for the adjustment screws to be tightened so that the clamp can grip the bike frame. Washers are used on the inside and outside of the box to help distribute the load – remember that the battery is quite heavy and the forces applied by the bike as it rides over bumps can be quite large. www.siliconchip.com.au The wiring is very simple – the two socket terminals are connected to the battery terminals via a fuse and the power supply for the headlight is taken off after the fuse. For safety, the fuse should be located as close to the battery as possible. Here, a blade (ie, The three parts of the project: top left is the battery pack, top right is the pushbike headlamp itself, and at right is the very slightly modified commercial battery charger. Fig.1: how to wire the SLA battery pack. automotive type) fuse and holder were used. A hole needs to be drilled for the headlight supply wiring to escape and that’s about all there is to it! Fig.1 shows what the circuit looks like. Make sure that the fuse is located as close to the positive terminal as possible and remember that the charging socket terminals are always ‘live’. A 5A fuse is quite sufficient, however I didn’t have a lower value than 10A lying around so I used that. The SLA battery charger comes with female spade terminals attached. These need to be removed and replaced with the plug to match the already-installed socket. Make sure that you get the polarity right – ie, that the positive terminal from the charger (the one with the red connector on it originally) goes to the positive of the battery! After that, it’s just a case of putting the lid on the box, sealing around it with some silicone for waterproofing and finally checking that it all works. Conclusion The SLA battery pack is easy to use, safe in an accident (it would be nearly impossible to get an acid spill) and is pretty cheap to put together. Even if the headlight is used frequently, it www.siliconchip.com.au Headlamp Durability? This headlamp should be very durable. The stainless-steel housing will stay rigid and corrosion-free, the polycarbonate mount with stainless-steel nuts and bolts is marine grade, and the glass lens won’t go milky or soften. The reflector – while being used with a higher powered bulb than intended – doesn’t get excessively warm, while the bulb itself is being used strictly as designed. The rubber-booted switch should be fine, and the cable grommet should weather wind and sun and rain without problems. The front plastic rim is UV-stabilised PVC – in short, this headlight should work well for many years. However, as with any component exposed to sunlight, painting the headlight body will give it even better longevity. should provide years of service, with running costs that can be measured SC in cents. September 2003  21