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I built this using the anemometer design originally posted by LBOY349 ( http://mindstorms.lego.com/nxtlog/ProjectList.aspx?MemberId=a5a27f87-584... ) on NxtLog. I started by improving the resolution of the light sensor and redesigning the light/dark rotation area. I set it to count dark spots at 4 per rotation. This gives the meter a resolution of 90 degrees.
After that, I wanted to calibrate it to a weather kit I purchased a while back from a local store. However, the resolution of the sensor wasn't sufficient. I decided to switch to using an RCX rotation sensor which measures 16 positions around a circle. This was better, but I thought it could be improved. So I built a gear train to step up the rotations 15x using 5:1 and 3:1 ratios in a two stage train. This would have given me 240 steps per circle or 1.5 degrees per increment. Unfortunately, the friction was too much and I think it wouldn't have spun in any wind less than 6-7 mph. I dropped back and implemented a single gear pair using an 40 tooth driving an 8 tooth gear to improve the rotation sensor resolution to 80x or about 4.5 degrees per tick.
The first picture shows the profile so you can see the supporting structure. The second picture is a closeup of the light sensor assembly, the gearing, and the rotation sensor. To make the light and dark areas, I used 8 black technic pins and put 4 1x2 light bley bricks over every other pin so they made a square around the axle. The counter works by detecting the transition to black. It doesn't loop until it sees the lighter color again. This ensures it works at any speed.
The third picture is the top view. I used the same vane design as LBOY349 and added some octagonal windscreens to the ends as my cups.
The last pic shows the screen with the program running. I display the raw light sensor data, rotation count, and MPH.
I calibrated the program with my storebought anemometer and a box fan.
If you don't have the rotation sensor, you could put the gearing on the main rotation axle and improve the resolution from 90 degrees to 18 degrees (20 increments per rotation).