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DIY Wind vane
#1

Hi,
I want to build a home-made wind vane. Pic of my test-prototype is attached. But there are some unclear / suspicious points.
1) Are the resistor values shown at Weather Sensor Assembly p/n 80422 correct? ( http://www.meteocercal.info/forum/Thread...nstruments )
2) Can the TX circuit read wind vane value if there is no wind speed signal? Or can I test wind vane circuit wihtout anemometer?
3) How can I adjust wind vane correctly? Just adjust R8 as10 k is adequate?
Regards.


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#2

Hi,

1) Yes, those resistor values appear to be correct. Most people just buy a standard Vane Sensor so are "stuck" with the values chosen by Fine Offset. But IMHO FO made a poor choice, so I can suggest some better values (below) if you are building your own station/sensor.

FO have basically used a sequence of 1k, 2k, 4k, 8k ....128k ohms for the 8 "primary" directions, but these are not all "preferred" (E12 or E24) values so some approximations are required. Also, such a sequence would be appropriate if any permutation of 4 switches determined the 16 possible dirrections (i.e. 0, 1 ,2, 3 or 4 closed, as might be the case with a Gray Code wheel) but is not the "best" when a maximum of only two "adjacent" reed switches will (should) ever operate at the same time. This gives a "dynamic range" of from 0.69k (1k in parallel with 2k2) up to 120k and requires ADC resolution and accuracy of at least 8 bits. Note that the FO transmitter does not use a normal "successive approximation" type of ADC (as implemented in most micros) but a 16-bit "Current to Time" converter.

My alternative set of resistor values is as follows, where the values in brackets are the calculated resistances of the parallel connection of the eight primary directions:

N=12k (3.82k) NE=5.6k (1.58k) E=2.2k (0.69k) SE=1k (0.825k) S=4.7k
(2.99k) SW=8.2k (6.78k) W=39k (22.99k) NW=56k (9.88k) N=12k

This particular set of E12 values has the same lowest value as the FO design, but the highest is only 56k (compared with FO's 120k), yet no values are closer than 19% to each other, so 5% resistor accuracy should be satisfactory, even with an 8-bit ADC. Also, as they are all standard E-series values, they can all be scaled by factors of 10, or by smaller differentials (e.g. 1k to 12k, 5k6 to 68k, etc.). Two pairs of resistance values in the FO design have differentials of only 12% and 13%, so some additional trimming or calibration of the lookup table may be required.

2) Yes, the Wind Vane sensor can report a direction with absolutely no wind speed (it must be the direction the last time that the wind was fast enough to overcome the "static friction" of the vane bearing). But I believe that the Cumulus software "ignores" the direction (sets to N) if zero windpeed is measured. I don't know how the WeatherDuino software handles this situation.

3) Personally, I don't believe the FO resistance values are sufficiently well toleranced to use 5% values (I don't know what they use) or if they have any test/calibration method. Adjusting the 10k load resistance may give an acceptable method of physically "trimming" the direction detected, but you may need to modify the lookup table. Normally it's hardly relevant because the magnetic circuit is so badly designed that the intermediate directions (with two reeds closed) hardly occur at all. Sad

Cheers, Alan.
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#3

Hi,
Thank you very much for the info and explanations.
I 've tested every resistor (1% tolerance). They correctly works.
An other "important" result is Weatherduino software does not display (on LCD) wind direction info if there is no wind.
And finaly, I've adjusted R8 just 10k. That's all.

Regards.
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#4

Hi, I am interested in making a a DIY wind vane, Having seen hamix's picture of the component side of his board is there any chance of a picture of the track side or do you have a schematic available for perusal.

regards

tyntop.
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#5

Hi,

Yes, werk has posted a .PDF of a data sheet for the Fine Offset sensors here.

Cheers, Alan.
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