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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!
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