WXforum.net
Weather Station Hardware => Davis Instruments Weather Stations => Topic started by: dedon on June 12, 2019, 03:39:30 PM
-
Found Davis Aerometer at ham swap meet. No part number or brand name. Was able to determine it is a Davis device from pictures on the web. It has a wind direction vane along with wind speed cups. It has a 20' cable with stereo plug on end. I cut the plug off and stripped wires. A black and red wire plus shield. I want to use it with an Arduino board. An Arduino sketch shows another wire used. So perhaps I can't use the device. An ohmeter shows changing resistance between shield and either wire when I spin the cups or move the vane. What model do I have here?
Thanks
-
Having only a red and black wire using a stereo plug does not sound like a Davis anemometer?
-
You may be correct. But, it sure looks like one. Even the 12.5 inch black rod and bracket that screws onto vertical side of a structure. If not a Davis, it does have 2 working "pots". One for the cups and one for the vane. I think one input of the the Davis unit is connected to Arduino as analog. The other is digital.Maybe I can get the output of other side to go in as analog too.
-
It could be from an old Digitar station, which predates the Davis name.
Does it look like this?
(http://www.sloweather.com/blog/2019/190614digitar.jpg)
-
It could be from an old Digitar station, which predates the Davis name.
Does it look like this?
(https://thumbs.worthpoint.com/zoom/images1/1/0213/14/vtg-digitar-weather-master-station_1_c7aa027524e6d4ce9f287702fad403b5.jpg)
Looks like your img url isn't working, says 403 Forbidden when I go to the url in my browser.
-
How about now? I saved the image to my own server and relinked it.
(https://thumbs.worthpoint.com/zoom/images1/1/0213/14/vtg-digitar-weather-master-station_1_c7aa027524e6d4ce9f287702fad403b5.jpg)
Looks like your img url isn't working, says 403 Forbidden when I go to the url in my browser.
[/quote]
-
How about now? I saved the image to my own server and relinked it.
(https://thumbs.worthpoint.com/zoom/images1/1/0213/14/vtg-digitar-weather-master-station_1_c7aa027524e6d4ce9f287702fad403b5.jpg)
Looks like your img url isn't working, says 403 Forbidden when I go to the url in my browser.
Looks good now
-
Yes it sure does! All I have is just the main unit that has cups and a vane plus a long red/wht/shield cable attached, Looks just like in your picture. So, what's the best way to use with Arduino? I found this, a very long white paper.
https://shareok.org/bitstream/handle/11244/48947/Pinkerman_okstate_0664D_14519.pdf?sequence=1&isAllowed=y
Page 62 starts showing how the instrumentation was set up. Page 81 is very interesting. Page 191 show Arduino sketch. Tried it, but neither sensor showed any change on the Arduino Serial Monitor. I think its wired wrong, I need send in external voltage above 9 volts into wind speed side?
Thanks ,,, Don
-
The wind vane should be a potentiometer. The anemometer (the cups) should be a reed switch. See if you can use a n appropriate allen wrench to loosen the set screw and pull the cups off. There should be a small magnet in the cup hub, and maybe the reed switch in the housing hub.
-
I found the trouble for the cups part of the anemometer. I needed a pull-up resistor on digital pin 3.
I now see mph on the Serial Monitor as I spin the cups. The vane is still not working. Only 2 wires, red, white and shield (ground) are available from the unit. The white is from the cups part. The red wire goes to the vane.
Using ohmmeter across red and ground, I see 0 to 20K as I turn vane CW. Continuing to turn, meter goes to zero K. Then immediately starts going towards 20 K as I keep turning CW. So this is a potentiometer type vane. Not sure what to do next. As shown in your picture there is a "control unit" with all the parts. I do not have this unit. I want to use what I have.
-
You will also need a pull up resistor of some sort on the direction pot. Ground the shield, red to the pull up resistor, and the other side of the resistor to plus of Arduino power. Then the red/resistor connection goes to an analog input on the Arduino.
-
You are absolutely correct! What I used for both was the Pull_Up option. So no external resistors needed. Now I see both speed and direction on Serial Monitor. However new problem for direction side. As I turn vane slowly just going past 255 degrees, the reading jumps to -20 degrees moving more and more negative as I turn vane. Around 0 degrees it starts going positive again, never do I see like for example 270 degrees. I notice using an ohmmeter I see interaction between speed and direction. The meter is connected across the red and white wires. When vane is pointing North and a certain position of the cups, I see zero resistance. Turning cup a little further, resistance goes high. Now turning the vane, I see gradual change in resistance. Anyway, bottom line is at around 255 degrees and continuing to turn vane reading jumps to - 20 degrees and continues downward. Thanks for your help.
-
Keep in mind that, if this is from a Digitar, it is well over 25 years old. All 360° pots like this exhibit some discontinuity at the max R/0 spot, and this one may be well worn.
(That picture is one a borrowed from another site. My Digitar is long gone, but I do recall the phone jack connections, and that the cables felt like silicone rubber insulation, rather than plastic.)
-
You are absolutely correct! What I used for both was the Pull_Up option. So no external resistors needed. Now I see both speed and direction on Serial Monitor. However new problem for direction side. As I turn vane slowly just going past 255 degrees, the reading jumps to -20 degrees moving more and more negative as I turn vane. Around 0 degrees it starts going positive again, never do I see like for example 270 degrees. I notice using an ohmmeter I see interaction between speed and direction. The meter is connected across the red and white wires. When vane is pointing North and a certain position of the cups, I see zero resistance. Turning cup a little further, resistance goes high. Now turning the vane, I see gradual change in resistance. Anyway, bottom line is at around 255 degrees and continuing to turn vane reading jumps to - 20 degrees and continues downward. Thanks for your help.
So, you're not using external resistors - you are using the Arduino "Pull_Up Option". And you are seeing interaction between the two devices, and sometimes -20 degrees wind direction.
I think there's a clue there.
-
If I had to guess, I'd say that was a late 90's era (conceivably 2000ish) Davis 7911 anemometer. The earlier ones had a straight arm IIRC. This one has the slightly kinked arm (but less kinked than later designs) and small cups.
-
OK very good info. The arm on my device is straight and did have a stereo plug I cut off to see the wires.
Next, I'll use individual external pull_up resistors and remove the lines for internal pull up. Be back.
-
I just happen to have saved a PDF of the Digitar Weather Pro Product #7203.
The manual does show what appears to be an inline plug akin to a ring-tip-sleeve mini phone plug. (not much detail other than the diagram)
I'll attach the diagram.
Also, I believe that the pull up resistor value for newer Davis anemometers is a quite high (90K? 901K?). This is to give a solid value when the anemometer pot wiper goes High-Z at North. I also suspect that the pull-up values are too low and are "fighting" the pot resistance. It just needs to be low enough that the input Z of the monitoring circuit doesn't reduce the voltage. This might be a bit of a challenge finding the right value, so that the pot indicates correctly during non-North resistance values.
By the way, I attempted to look up the excellent Anemometer reference on Lexington Weather's site (by wxtech), but alas the domain is showing a GoDaddy expiration page. I was looking there to find the pull-up value.
I thought that this document was hosted on WxForum somewhere, but it isn't "sticky". My search didn't find it. Is it here on WxForum?
Greg H.
-
Thanks Greg, I'll keep the higher values of pull-up resistor in mind. Been working on my 3D printer for a couple of days But will get back to anemometer problem on weekend. Too much to do, too little time!
Don
Well, I see now the potentiometer method has lost favor. I added a 4,7K pull-up to both the cups and vane sensors. Some improvement but no real success. I suppose if I had the control unit, maybe a lot better. Right now I can register about 310 degrees, then continuing to 360, no good. At about 1 degree and continuing reading looks valid. So about 50 degrees is dead band. Thanks all for reading this. Guess I'll archive it.
-
Well, I see now the potentiometer method has lost favor. I added a 4,7K pull-up to both the cups and vane sensors. Some improvement but no real success. I suppose if I had the control unit, maybe a lot better. Right now I can register about 310 degrees, then continuing to 360, no good. At about 1 degree and continuing reading looks valid. So about 50 degrees is dead band. Thanks all for reading this. Guess I'll archive it.
Does the dead band vary with the use of different values of pull-up resistor?
-
Yes it does. And near 270 degrees and higher, it looks like its not linear at all. If I move the vane a bit, I see the same reading for a few degrees, while lower degrees, moving the vane the same amount shows definite change. The wind speed sensor has worked fine, with pull-up or external resistor of 10K. So I think I'll put it up outside and read speed inside and the direction by going outside and looking at it. Or maybe find a wind sock!
-
Did you try the direction potentiometer without any pullup resistor at all? (i.e., pot line hooked to vcc)
-
One thing to try might be a pull-up pot to more easily determiner the optimum pull-up value experimentally.
-
Just to clarify, is the pull-up resistor on the ADC input circuit? (I hope that this is an analog input and not a "digital" one...)
There needs to be an "excitation voltage" for the pot. One end of the pot needs to go to this voltage (VDD of the interface). The other end of the pot needs to go to the circuit common (ground/earth).
The "wiper" of the pot is what is fed into the input A/D port. (WRT circuit common)
Davis dynamically creates a pulsed excitation voltage for their transmitter stations. This is to conserve battery power, but there isn't any reason this couldn't be steady state for your application.
If the "pull-up" resistor is on the wiper, it should actually be disconnected. Actually, I made a mistake here. You do need the high value pull-up so that the open-circuit "North" indication is correct. The wiper goes "high Z" at North and the input would float otherwise giving indeterminate values.
An excitation voltage should be applied to one side of the pot opposite of the common side.
Greg H.
-
You may be correct. But, it sure looks like one. Even the 12.5 inch black rod and bracket that screws onto vertical side of a structure. If not a Davis, it does have 2 working "pots". One for the cups and one for the vane. I think one input of the the Davis unit is connected to Arduino as analog. The other is digital.Maybe I can get the output of other side to go in as analog too.
I'm confused by this whole description. Please clarify. Are you absolutely sure there are only 3 connections? (red, black, shield)? Two pots?