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Weather Station Hardware => Davis Instruments Weather Stations => Topic started by: miraculon on May 05, 2020, 10:33:15 AM

Title: Lexington Wx Davis Anemometer and WXTech Docs
Post by: miraculon on May 05, 2020, 10:33:15 AM
I find myself regularly sending users to the excellent Davis anemometer guide at https://www.wxforum.net/wxtech/Anemometer-Vane.htm (https://www.wxforum.net/wxtech/Anemometer-Vane.htm)

Could the admins please make this sticky under the Davis Instruments Weather Stations (https://www.wxforum.net/index.php?board=59.0)?

Possibly the wxtech documentation archive, with the soldering and technical library archive as well. The anemometer section is a linked page from the documentation archive (https://www.wxforum.net/wxtech/), so that might serve the purpose. (and yes I finally put a bookmark in for this...)

Greg H.



Title: Re: Lexington Wx Davis Anemometer and WXTech Docs
Post by: Plainwind on October 02, 2020, 07:52:43 AM
Many thanks for the link to the excellent anemometer Guide.

I have question about the 909 Kohms resistor across the yellow and green wind-vane potentiometer terminals.
To quote from the guide:

"A 909K Ohm fixed resistor is mounted across the pot from the yellow to the green wires. This resistor prevents the slider voltage from ‘floating’ while it is crossing the open end points of the pot. This resistor causes the indicated direction to remain at north while the circuit is briefly open.  If not for this resistor, the direction indication may briefly go to a false direction."

I am using a 6410 anemometer (speed and direction) as the input to a Raspberry Pi based weather station.  So the voltage from the Pi to the yellow pot terminal is 3.3v.  As expected, the pot slider voltage rises nicely (i.e. linearly) from 0 to 3.3v as the pot is turned through 360 deg.  However when the slider is crossing the open ends of the pot, the measured voltage is 1.74v. I would have expected the 909k resistor to have kept the slider voltage in this situation at 3.3v.

Can anyone figure out what's going wrong?
Title: Re: Lexington Wx Davis Anemometer and WXTech Docs
Post by: miraculon on October 02, 2020, 09:04:19 AM
Many thanks for the link to the excellent anemometer Guide.

I have question about the 909 Kohms resistor across the yellow and green wind-vane potentiometer terminals.
To quote from the guide:

"A 909K Ohm fixed resistor is mounted across the pot from the yellow to the green wires. This resistor prevents the slider voltage from ‘floating’ while it is crossing the open end points of the pot. This resistor causes the indicated direction to remain at north while the circuit is briefly open.  If not for this resistor, the direction indication may briefly go to a false direction."

I am using a 6410 anemometer (speed and direction) as the input to a Raspberry Pi based weather station.  So the voltage from the Pi to the yellow pot terminal is 3.3v.  As expected, the pot slider voltage rises nicely (i.e. linearly) from 0 to 3.3v as the pot is turned through 360 deg.  However when the slider is crossing the open ends of the pot, the measured voltage is 1.74v. I would have expected the 909k resistor to have kept the slider voltage in this situation at 3.3v.

Can anyone figure out what's going wrong?

What is your ADC (analog-digital converter) input resistance?

My guess is that the input resistance (impedance) is around 1M Ohm.  The voltage divider equation predicts that a 909K resistor feeding into 1M with 3.3V will yield 1.728V. Close enough in my estimation.

You want a low enough value that the input doesn't load this pullup resistor when the pot goes through "north" (open), yet doesn't affect normal operation against the 20K pot resistance. This might require some experimentation to get the right value. Good luck.

Greg H.
Title: Re: Lexington Wx Davis Anemometer and WXTech Docs
Post by: Plainwind on October 02, 2020, 10:02:32 AM
Hi Miraculon
Thanks for your reply
I hadn't considered that the input impedance of the ADC needs to be factored in.  Good point!
As you suggest, I will experiment with various resistor values.

Seems odd to me that Davis didn't put the 909k resistor in the console rather than directly onto the pot terminals?