Instructables
Build your own digital Anemometer by using a bike speedometer and other inexpensive parts.

Step 1: Parts

Picture of Parts
Major parts needed:

Wind cups, part #7903 from Davis Instruments (www.davisnet.com), cost around $15.
Bike speedometer from any bike shop
2 ball bearings, 1/8" ID from any hobby shop (used in radio control models)

Once you have the parts, this project should take less than three hours to construct I think.

Notes on the parts:

For the Anemometer head we use the "wind cups" from Davis Instruments. This is a significant time-saver over constructing your own wind cups! Davis sells anemometers and weather stations, their complete devices cost around $200 i think. So if you'd rather just buy a nice anemometer, i'd highly recommend theirs. The wind cups we are using are considered "replacement parts", you might need to tell them you have one of their weather station products but you broke the wind cups. I don't think they really care though. Anyway, the wind cups are a nice plastic assembly, about 6 inches in diameter with three cups attached to a central hub. The hub has a 1/8 inch hole for a shaft, and it has a magnet embedded in it for use with a magnetic reed switch for detecting rotation. They have a photo on-line of their entire wind-assembly, the wind-cups you'll be getting are just the piece at the bottom of this assembly.

In a future version of this project I hope to make my own wind cups. I've tried a couple designs out of cut-up beer cans and soda bottles, but have not gotten anything that works well enough...

It's quite convenient to use a bike speedometer here, because they detect bike speed using a magnetic reed switch. On a bike, you attach a magnet to one of your front wheel spokes and then you attach the reed switch to the front fork. Every time the magnet on the spinning wheel passes the reed switch, the switch pops closed and then open again. The speedometer detects the open-close-open of the switch, and speed is computed by how often the switch is activated.

Step 2: Construction overview

So, to make the anemometer all we need to do is put the magnetic switch from the speedometer next to the magnet on the wind cups, and "voila". The main work in this project is to construct a mounting block which will hold the wind cups and switch in the proper positions, and allow the wind cups to rotate freely.

First, make sure your speedometer is working right: move the reed switch back and forth next to the magnet in the wind cups, and you should see the speedometer register a few MPH. With the Davis wind cups, the magnet is embedded in the plastic, although you can see it on the bottom of the cups.

Step 3: Making the ball-bearing supported shaft 1

In order for the wind cups to spin freely, you will need a metal shaft/axle, and two ball bearings to support the shaft. Hobby shops that have radio-control models are the best place to get small ball bearings inexpensively in many sizes. You might find that they only have metric ones however, in which case it's probably easiest to drill the wind cups for a 4mm shaft instead of 1/8 inch. So at this point you'll have either a 1/8 inch shaft and two bearings with 1/8 inch internal-diameter (outside diameter isn't so important, 1/4, 3/8 or 1/2 inch is fine), or else a 4mm shaft and two bearings with 4mm internal diameter (outside diameter of 8, 10, or 12mm is fine). The shaft should be as long as your mounting block, 4 inches will cover it.

Step 4: Making the ball-bearing supported shaft 2

Now you will need something to hold the bearings and shaft. You might be able to find something at the hobby shop which would work. But at worst, just find a block of aluminum, plastic, or wood which is about 1" diameter and 2" long. Drill a hole all the way through the long way, larger than the shaft but smaller than the outside diameter of the bearings. for example with bearings that have a 1/8" ID and 3/8" OD, i drilled a 1/4" hole so the shaft won't touch it. Now at either end of the block drill a 3/8" hole right on top of the 1/4" hole, but only drill in about 1/4" (far enough for the bearing to fit into the block.

Step 6: Mounting the wind cups and sensor

At this point you will need some spacers/collars which fit over the shaft. These you can get at the hobby shop or at a hardware store. basically just little metal tubes with an ID of 1/8 inch and OD of maybe 3/16". You will need to put a spacer at each end of the shaft, right next to the bearings. At one end the spacer goes between the bearing and the wind cups in order to hold the wind cups away from the block about 1/2" or so. The exact distince will depend on how big the magnetic switch is. You need to attach the magnetic switch to the top of the block right under the wind cups so that it is activated by the magnet on the spinning wind cups. You'll need to play around with the distance and orientation a bit to get it right. The pole of the magnet in the wind cups is at the bottom, so the magnetic switch needs to be under the wind cup hub, not at the side of it. Two things can go wrong:

  • Magnet is too far, and never activates the switch
  • Magnet is too close and the magnet never gets far enough away to disactivate the switch.

Once you've found a setup that works, attach the switch in that position (use silicone glue because then if you botched it you can pull it apart later and re-do it). Make sure not to get glue in the bearings!

The wind cups have a very tiny set-screw in the side, tighten up the set screw to lock the cups to the shaft, and push the cups and spacer snug against the top bearing. now put the spacer on the other end of the shaft, and put a bit of glue on to glue the spacer to the shaft (no glue near bearing!). now the shaft should be locked onto the assembly, and spinning freely, and the speedometer should work when you spin the cups! almost done!

attach the speedemeter to the block, or else attach the block and speedometer to a handle or rod of your choice, and tie down the wires.

Step 7: Calibration

All you need to do now is calibrate! What I found is that most bike speedometers won't have the correct range to display actual MPH / knots / KPH. Instead you'll have a reading which is a multiple of the actual speed. Mine is three times the actual speed. So if my meter says "60", then it's really 20 MPH wind. The bike speedometers have an adjustment in them for different sized bike wheels, and this will give you enough flexibility to get a convenient multiple like 3 or 4 or 5. They just won't go as low as 1 or 2. The easiest way to calibrate is to get a friend and go driving in a car on a day with no wind. A big parking lot is good. Have your friend drive the car at exactly 20 MPH in one direction, and record the reading on the meter. Then go the opposite way and check the meter. If there was really no wind, it should be the same reading, but if there was 1 or 2 MPH wind, you should get readings a little apart. average the two numbers, and this is the real number. say you got 55. This is close to 60, which would be a convenient multiplier to have. So adjust the speedometer's wheel size down by 9% and try driving again. This time it should read 60 when the car is going 20. to make sure there's no crosswind you can drive in all four directions and take the average. You can make sure it's working by driving 10 MPH and 30 MPH and verifying your multiplier. I've found that most bike speedometer's have a maximum speed of around 120, so keep your multiplier low if possible. Once you reach the speedometer's top speed it won't give an accurate reading anymore. Drive the car faster and faster until the reading stops going up with the car speed, and remember this number, it's your maximum. Most likely you'll max out at about 40 MPH (actual).
 
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rgerber13 years ago
while it looks fun to build I bought one from Ebay and it was 20 bucks. I love to tinker but sometimes you just cannot beat the Chinese for price...

ckgreenman4 years ago
I had another thought.  I read elsewhere that after a while the reed switches will wear out especially if you leave the sensor up all the time and live in a windy area.  Seems to me it might be possible to use a magnet and a hall effect sensor. 
Thoughts?
dsnevin4 years ago
Good project!
The bearings are called: Sealed Bearings -- consisting of numerous ball bearings, encased in a metal cartridge.
trisampson5 years ago
You could also... duh... use another bike computer and ride around the block and compare the values. GPS is also a possibility in that regard. On the other hand to average out any values to account for any wind, using cruise control in the car might be helpful, but using GPS to nail your speed would probably be best.
Hello, I am interested in making this anemometer, but i was looking on the Davis site, and they have 4 differnt wind cups. 2 different sizes for the part #07903, but small size or large?
static7 years ago
Out here where the owls get the chickens. Some build whirligigs out of bike wheels a cream separator plates. I have toyed with the idea of using a bike wheel and axle for the rotor. That way the digital read out could be used as is, using no math.
Marcos8 years ago
When counter-boring holes, if you drill the larger holes first, it's a snap to have the smaller one be concentric, as you'll still have the bottom of the hole, and it will have the center point ready-marked and "center punched." But you knew that already, yes?
dan (author)  Marcos8 years ago
you need to drill the small hole all the way through first. if you drill the two large holes part way, then try to drill the small one from one end, it is never going to end up at the exact center of the big hole on the other end.
Marcos dan7 years ago
Aha, looks like I was not paying enough attention. I was thinking of drilling from one side only. I figured you knew better. ;-)
bubbagump8 years ago
Ummm, "Ball" bearing? You sure about that?
Very cool, elegant in it's function & simplicity. This would go well with a digital weather vane a friend built using a nifty bcd encoder (to support the vane & read it's position) feeding a bcd-to-octal or -hex decoder which lit LEDs set in the compass rosette pattern. (Sadly, I've been living vicariously through other peoples projects, that's why I was so happy to stumble on this site.)
Hammer71378 years ago
An idea for home-made wind cups...those plastic easter eggs that break in half have a nice parabolic shape that could work. Their strength and durability might be questionable, and I haven't come up with a mounting design yet but it might be a start. Actually, it would probably be easier to use a more durable plastic ball. I plan on doing a quick test using plastic "k-nex" toys or legos.
Pauwcarn8 years ago
This is a great idea! I have found that a wealth of miniature ball bearings can be had from salvaging discarded VCR's and hard disk drives.