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Cheap & effective unit uses a $12 LCD bicycle speedometer! Digital Anemometer If you’re a sailor, fly a kite or model aeroplane, or just like knowing what the weather’s doing, this anemometer project will be of interest. The design would also be very useful in the geography or science departments of a high school or perhaps you could build it for a science project. By JULIAN EDGAR For those who don’t know what it is, an anemometer is a device that measures wind speed. Our battery-operated anemometer has a digital screen that shows wind speeds up to 99km/h (higher if you wish to spend a little more). It can display wind speed in either km/h or mph, has an inbuilt service indicator (more on this later) and is very durable. Best of all, the complete anemometer should cost you well under $50! The design uses a spinning cup-type 14  Silicon Chip assembly that’s rotated by the wind. A magnet positioned on one of the four cup arms triggers a fixed-position reed switch during each revolution, with the output of the reed switch monitored by a combined LCD/processor unit. Unbelievably, the LCD/processor unit is available from Woolworths for just $12. They call it the Acme Cyclocomputer but basically it’s just a digital bicycle speedometer. While that’s the electronics out of the way in one fell swoop, the mechanical design is very important if the anemometer is to be both durable and reliable. A lot of effort was put into devising a rotor that would last a long time, despite being constantly exposed to the elements. Our final design uses stainless steel cups, polypropylene arms and a dual ball-bearing axle. Does that all sound expensive and difficult to source? Not really – the axle assembly is the front hub of a bicycle, the stainless steel cups are from soup ladles and the poly­propy­lene is cut from a plastic kitchen chopping board! All it takes is a little initiative and you can scrounge the parts for just about anything! Building it The first step in the construction is to select the bicycle hub. A visit to any bike shop will reveal a multitude of front hubs – including some very nice alloy ones! Often the shop will have secondhand hubs available and for our anemometer, we selected what appeared to be a brand new steel hub from the secondhand selection offered to us. It cost just $6.95. When picking a hub, make sure that the axle spins freely but without end-float. If it turns with a “cogging” motion or if the grease in the ball bearing area is old and coagulated, don’t buy it. If you live in an area that’s very prone to corrosion (for example, near to the beach), you may wish to splash out and buy an anodised alloy hub. Either way, make sure that you also get the nuts that go on the axle. Next, you need to cut out the plastic rotating arm assembly. This must be done very carefully so that the rotor retains good balance – more on this later. The first step is to select a polypropylene cutting board. This should be at least 285 x 285mm and must be at least 10mm thick. We purchased a board a little larger than this for $6.95 from a discount store. The board should be cut to the shape shown in Fig.1. The plastic material “works” beautifully and can be cut with an electric jigsaw or even a coping saw. When cutting out the rotor, don’t be tempted to replace the curved corners shown on the drawing with 90° cuts – the curves reduce the Fig.1: the rotor can be cut from a plastic chopping board. The dimensions of your design don’t really have to follow this drawing exactly but make sure that the rotor is symmetrical about the centre mounting hole. MARCH 1999  15 The display can either be mounted on the mast as shown here (because it’s designed to be used outdoors on a bicycle) or located remotely (eg, inside the house). chances of the arms fracturing later on. Once cut, the edges can be filed and/or sandpapered smooth. Next carefully mark and drill the centre hole, starting with a small drill and then increasing the hole diameter until it matches that of the axle. You can then place the arm assembly on the axle and temporarily tighten the nut. Spin the assembly to check how good the balance and run-out are. If you have made a mistake and the assembly is way off balance (perhaps because you drilled the hole in the wrong place), buy another chopping board and start again. If the assembly is only a little out of balance or is perfect, keep going! The next step is to detach the soup ladle cups from their handles. When buying the ladles make sure of two things – that the cups are actually stainless steel (it’s usually stamped on the ladle) and that the cups can be easily detached. The ones we used were spot welded to the handles and they broke off with just some wriggling. 16  Silicon Chip Rivets (or stronger spot welds) can be drilled out. Our ladles cost $2.95 each from a discount store but note that you can pay much, much more than this if you buy branded, fashionable ladles. The trick is to look in bargain stores – not trendy kitchen­ware places! Attaching the cups The cups are attached to the rotating arm assembly by self-tapping screws about 20mm long. These pass through the cups near their edges and then screw into the ends of the arms. If you first hold a cup next to the end of an arm, you’ll see that the end needs to be slightly curved so that the cup will nestle comfortably into position. Use a hacksaw and a half-round file to make this curved end for each of the four arms. This done, the holes can be drilled through the cups to allow the screws to pass through. On the units we selected, the spot welds used when the cups had a previous life as soup ladles were still clearly visible. We drilled through one remnant spot weld on each cup. The cup is then held against the end of the arm, the hole position marked and a small diameter pilot hole drilled into the arm to take the self-tapping screw. Before selecting the size of drill bit for the pilot hole, experiment with different drill sizes on a scrap offcut from the plastic cutting board. The size of pilot hole that works well in plastic is not the same as you would use in other materials and depends very much on the coarseness of the thread on the screw. Experiment until you find the hole size that best suits the self-tapping screws you are using. Note that over-tightening the screws will cause the plastic to “strip”, so be careful. For durability, the best bet is to use stainless steel for all of the fasteners used on the anemometer. If you live in a very windy area and want the rotating assembly to be super-heavy duty, you could make the rotor out of thick marine-grade ply. In this case, mount the cups using nuts and bolts, with the bolt passing through the centre of the cup and then through a hole drilled tangentially into the arm. This heavier assembly will be less sensitive to light winds, though. With the cups mounted and the rotating assembly temporarily on the axle, you can blow on it and make it go round and round. Once you get bored with doing this, hold the axle horizontally and check that the assembly stops in a different position each time, indicating that it is perfectly balanced. However, if one arm always points downwards, indicating that it is heavier than the others, mark it with a Texta pen. This information will be useful in a moment. Water shield To prevent water flowing into the bearing from above, a shield needs to be mounted above the hub, just below the rotor. This extends down over the hub without fouling it. A plastic screw-on cap from an old oil container (or similar) can be used to form the shield (see photo). When the right diameter cap is found, drill a hole through the middle of it and mount it on the axle under the rotor. Remove the rotor from the axle before performing the next step. Incidentally, note that dropping the rotor can dent the cups, so care should be taken during the rest of the manufacturing process. Once the rotor has been removed, the axle/hub assembly can be mounted on a polypropylene mast bracket, using either saddle clamps or a clamp fashioned from scrap aluminium (as in the prototype). We made a mast bracket using an offcut from the chopping board, again selecting this material to prevent corrosion. Alternatively, you could use an aluminium bracket. The magnet and its pick-up need to be mounted next. Remember how you marked the heaviest cup? To help balance the rotor, mount the magnet on the arm directly opposite. The magnet can be attached to the arm using two small self-tapping screws and should be placed with its centre about 55mm from the rotor axle. This done, replace the rotor assembly and mount the sensor at the top of the mast bracket so that the magnet passes directly over it. One again, use a self-tapping screw to secure it in position. Be sure to leave a gap of a few millimetres between the This close-up shows the mast bracket, the aluminium clamp which holds the bearing assembly in place, the rain shield over the upper end of the bearing and the sensor location. magnet and the sensor. You should now be able to spin the rotor and read a speed on the bicycle speedometer – after you’ve connected the sensor leads, of course! Of course, the speed will be wrong but the instructions in the next section will fix that! If the assembly is out of balance once this stage has been reached, balance it by screwing small weights to the outer edge of the arm that’s opposite the heavy one. Using a cable tie to hold the weight in place can be useful while doing the balancing but make sure it doesn’t fly off when the rotor is being test spun! Calibration The digital display can show the wind speed in km/h (as shown here) or in mph. The odometer reading (here 12.5km) can be used as a service indicator. The km/h symbol flashes when the anemometer is rotating but the wind speed is too slow for measurement. The anemometer can be calibrated by checking it against the speedo of a car driven at a fixed speed on a still day. Be sure to choose a still day, otherwise the calibration will be inaccurate. The anemometer should be mounted on a short (60cm) mast which is firmly clamped to the roof rack, with the lead to the display run through a side window. You will need a willing assistant to drive the car along a quiet backstreet while you read the wind speed on the digital display and compare it with the car’s speedometer. The Cyclocomputer bicycle speedo can be programmed for different wheel diameters and this facility is used to calibrate the anemometer. If the speed shown by the instrument is low, you need to set the wheel diameter to a higher number. Conversely, if the speed shown by the instrument is high, set the wheel diameter to a smaller number. With the prototype, MARCH 1999  17 For best results, the anemometer should be placed high on a mast, away from trees, house roofs and the like. Note here how the arm-mounted magnet is about to pass over the reed switch sensor. setting the wheel diameter to its maximum (2999) gave the correct measurements. If you find that you run out of calibration settings at the “large wheel” end, add a second magnet to the rotor assembly directly opposite the first. The LCD module will then “think” that the rotor is spinning twice as fast as it actually is! As a result, you will be able to use a reduced calibration number to set the instrument accurately. You will need to re-balance the rotor with the extra magnet in place though. Note that you should be careful when carrying out this calibration procedure. At 100km/h, for example, the anemometer is spinning very quickly indeed – fast enough to cause injury if your arm was to come into contact with it. Don’t drive at 100km/h with the unit attached, though – a speed of 20-60km/h is the most practical for calibration and avoids the risk of a mechanical failure. Again, don’t touch the unit until it stops rotating. Note also that you should mount the 18  Silicon Chip anemometer far enough away from the car so that the vehicle’s aerodynamics don’t affect the measured wind reading – 50cm should be enough. Final setting up The prototype was mounted on a 1-metre length of square aluminium tube. Incidentally, if you’re wondering how expensive materials such as aluminium can be used on a budget project like this, I’ll let you into a secret. If you go along to a large non-ferrous scrap metal dealer you’ll find that you can buy (by the kilogram) offcuts of aluminium angle, plate and tube for next to nothing. The metre of tube used here cost about 30 cents! The figure-8 cable that connects the sensor to the display can be lengthened beyond the metre or so provided. Quite how long you can go with this cable we’re not quite sure but certainly 10 metres doesn’t cause a problem. If a very long battery life is required, the 3V button cell in the display can be easily replaced by an external pair of AA cells and the new power supply leads soldered to the original battery clips. If you want to read higher wind speeds than the 99.9 km/h available on the Cyclocomputer, select another brand of bicycle speedo. Some can measure speeds of up to 200km/h, which should be sufficient for all but tropical cyclone conditions. Incidentally, the prototype was tested at speeds of up to 120km/h without any mechanical problems. For absolute maximum durability, paint the complete anemometer. Even some stainless steels will rust if they are of low grade and all plastics will last better if protected from UV radiation. Finally, what about that “service indicator” mentioned in the first paragraph? That’s the odometer part of the display. When it gets to 5000km (or whatever figure you decide is appropriate), it’s time to re-grease the bearings in the axle and check their SC clearances!