Weather Station Hardware > DIY Wunder Weather Station

DIY Wunder Weather Station -- WiFi -- Rapid Fire -- Standalone

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GaryStofer:
Just a quick note on progress:

I added a  metric display option for the local display, so that the folks in Canada and Europe have an easier time.  Also added a Density Altitude readout, handy for aviation related use.

On the firmware side I added the option to upload to CWOP, WX-Bug and PWSweather/WeatherForYou in additiotion to WeatherUnderground. The upload protocols are complete and working, I just need to add a couple more fields to the station GUI for ease of configuration.

On the HW side I switched to the phone/FAX style cable for the Air-data sensor modules. This makes it easier to move the sensor away from the main station controller. The FAX cables are available at Fry's and on E-bay for cheap, I have a couple of default lengths available as well.

And on the Air-data  module I introduced a new Silicon Labs hygrometer chip that has a better filter than the Honeywell part.

Check out the blog pages at http://wws.us.to/blog and sign up for the email or RSS feed.


Gary
   

GaryStofer:
In addition to Wunderground options for CWOP/APRS, WX-Bug , PWS/WeatherForYou  and the UK Meteorological Office service WOW  have been added to the firmware now.

G

Naviguesser:
Hello Gary,

A lot of rain gauges use reed switches to count incremental amounts of rain fall.  I recently came across a new IC by Honeywell that apparently uses very little power and is triggered by a change in magnetic fields.  The output is in micro Amps.

Here are a couple of links and pieces of data:

Digikey Part#: 48-5908-1-ND
Honeywell Part#: SM351LT
http://sensing.honeywell.com/SM351LT-Magnetoresistive-Sensors

This appears to look like a possible replacement for the trouble some reed switch in tipping bucket rain gauges.  While I have no problems assembling a Heathkit, I'm not Electrical Engineer.  I have a Davis VP1 that I am refurbishing and I thought that this might be a solution to replacing the reed switch.

GaryStofer:
This chip might work, but there are some complications with it.

These "nano power" hall effect sensor get their minuscule operating current from a very low duty cycle, or in other words from  being asleep for most of the time. For example the device you mentioned is asleep for 0.1 second and then wakes up for a mere 0.000015 second. So if the tipping bucket magnet flies by the sensor when the device is asleep it will not register anything.   

However there are other hall effect sensors/switches that will work just fine in a tipping bucket setup. You would be looking a for a bipolar hall effect switch with an open collector output.  These devices are permanently on and consume between 3 and 5 mA, too much current for battery operation, but just fine as long as you have a power supply. Also the open collector output acts more like a reed relay switch and will not inject 5V into the measuring circuit. Bipolar so that it doesn't matter which magnetic pole triggers the device. These come in transistor housings and are much easier to mount then the tiny SMD ones.
G

Naviguesser:
 Hi Gary,

Thanks for the quick reply.  It is much appreciated.

To me, a tenth of a second is not that long.  I measured two quick snaps of my fingers at approximately 1/2 second or 1/5th of the time to produce a tenth of a second.  I suppose we would have to actually measure the "tick-tock" of the bucket mechanism hitting its stops during a heavy downpour to get an idea of actual travel times during fast cycling periods.  Then figure the strength of magnet needed, maybe the position of a couple of sensors, etc.  Looks like a challenge for a rainy day.

I noticed that one of the chips was sensitive to 7 Gauss and the other to 14 Gauss; a tenth or a fifth of a refrigerator magnet (~50 Gauss) which seems to be in the ball park of the magnet Davis uses.

I took a look at the output amperage and thought it was rather small; 5 mA.  I guess the bottom line question is if it could detect a magnetic field swinging by, would the output be enough to trigger a count? or would the signal have to be amplified?  All this is out of my realm and into the realm of at least a tech if not a EE.

I understand the idea of using other devices but at the expense of battery life.  I look at the compasses in cell phone and realize it might not be much longer before these reed switches are replaced with ICs.

Thank you for your time.

Peter

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