Build your own digital Anemometer by using a bike speedometer and other inexpensive parts.
Step 1: 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 5: Making the ball-bearing supported shaft 3
Now you can assemble the free-spinning shaft: put a bearing in each end of the block, put the wind cups on the shaft, and put the shaft through the bearings. it should spin completely freely. if your drilling wasn't quite straight, you should be able to get it right in a couple of tries.
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).