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Photos of the insides of Fine Offset sensors.

Discussion specific to Fine Offset and similar rebadged weather stations
sooty
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Re: Photos of the insides of Fine Offset sensors.

Postby sooty » Sat Nov 26, 2011 3:01 pm

The blue part is a trimmer capacitor.
Image

The L copper track forms an inductance. It doesn't have much inductance but this is operating at a very high frequency and it doesn't take much inductance to form a series tuned resonant circuit with that trimmer cap - and there's your tuned antenna.

AllyCat
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Re: Photos of the insides of Fine Offset sensors.

Postby AllyCat » Sat Nov 26, 2011 3:25 pm

Hi,

Yes, the antenna is the track beside the cutout, but I wouldn't call it a "twig" as it's probably a "loop" antenna which radiates the magnetic component of the electromagnetic radiation rather than a monopole (half a dipole) whip antenna. The antenna looks quite like that on the 868 MHz version, but an advantage of loop antannas is that they are not as critical on physical dimensions. If it were a monople the mystery component would be a "loading coil" (inductor) at the driven end, but it's probably a variable (trimming) capacitor of about 10 - 30 pF to tune the loop to 434 MHz.

Take a look at the RFM data sheet linked by Orion in this thread.

Cheers, Alan.

ksangeelee
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Re: Photos of the insides of Fine Offset sensors.

Postby ksangeelee » Sun Nov 27, 2011 2:07 pm

Thanks all, that starts to make sense.

So, to change to a monopole whip, I could remove the trimmer cap and connect a trimmer inductor in series with my 1/4 wave wire and tune by trial and error?

AllyCat
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Re: Photos of the insides of Fine Offset sensors.

Postby AllyCat » Sun Nov 27, 2011 2:57 pm

Hi,

Well, strictly you shouldn't make ANY changes to the transmitter (you might alter the Effective Radiated Power) thus invalidating the type approval license and CE marking, etc. and so are operating radio transmitting apparatus illegally. ;)

A loading coil is intended to correct for antennas shorter than the nominal quarter-wave and antennas even slightly longer than quarter-wave very rapidly fall off in gain (a definite example of when bigger is not better). So either don't use a loading coil and gradually cut down the length of the monopole from lambda/4 or use a variable inductor and start with a monopole considerably shorter.

There is still the issue of impedance matching, you certainly don't want both the loop and monopole connected at the same time. The transmitter was presumably designed to inject maximum current into the loop antenna and I can't say how effective it will be in feeding voltage onto the monople.

So in general, it would be much better to modify the receiver antenna.

Cheers, Alan.

ksangeelee
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Re: Photos of the insides of Fine Offset sensors.

Postby ksangeelee » Sun Nov 27, 2011 9:57 pm

AllyCat wrote:So in general, it would be much better to modify the receiver antenna.

Cheers, Alan.


That's earmarked for part 2, and I've got some 50ohm coax lying around that I can strip back and take vertically out of the receiver unit.

Doing it in this order means I can tinker with the transmitter without having to walk so far to test the range.

Thanks again,

Kevin

Charlie
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Re: Photos of the insides of Fine Offset sensors.

Postby Charlie » Sat Dec 10, 2011 5:30 pm

Gina wrote:The way this sensor works is that a magnet attached to the rotating vane turns on one or two of the reed switches. It is meant to detect wind directions between the 8 compass points by relying on the magnet turning on two adjacent reed switches. That is thought to explain the rather weird sequence of resistor values associated with the corresponding switches. The chip in the main transmitter unit converts the resulting resistance into a digital value which is then turned into a number between 0 and 15 in the firmware.

I've been trying to puzzle out how this works with a spreadsheet. When adjacent switches close, the resistor value between the terminals varies in some cases by as little as 200 ohms, ans in other cases by as much as 42K! By my calculations, as the vane turns, the value at the terminals jumps up and down, sometimes a little, sometimes a lot. I honestly don't see how you can acurately measure it, at least not within a couple ms.

Mine is way up on the roof, so I can't double check the diagram Gina posted, although I'm sure she has it right. Any guesses how this works in real life?

AllyCat
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Re: Photos of the insides of Fine Offset sensors.

Postby AllyCat » Sun Dec 11, 2011 2:42 pm

Hi Charlie,

The resistor (or two in parallel) has one end switched onto the supply rail (at the start of each A/D conversion cycle) which charges up a capacitor (4.7uF). Then the microcontroller measures the time it takes for the voltage to reach half the supply rail. This takes between a few and about 500 milliseconds. There is a "timeout" if this voltage is not reached and the sensor/console reports a disconnected/faulty vane (no wind direction shown).

Presumably the micro uses a lookup table to relate wind directions to A/D time delays. Note that there are also some intermediate "dead spaces" between certain resistances (or time delays) when again the console displays "no direction". The precise value of the electrolytic capacitor and voltage threshold are not important because the micro pre-calibrates them against a (more accurately) known resitance value.

IMHO the FO resistor values may well have been "designed" by trial and error/guesswork. But it's an interesting challenge (because the values must wrap around in a circle) to design the most efficient or tolerant set of resitor values for any particular number of directions and resistor tolerances. :D

Cheers, Alan.

Charlie
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Re: Photos of the insides of Fine Offset sensors.

Postby Charlie » Mon Dec 12, 2011 10:38 am

Hi Alan,
I understand THAT part of how it works - my issue is with the resistor values and the possible acuracy of the sampling.
For example, the difference between E and W is 1K to 120K, so very easy to detect. However, ENE=891, E=1000, ESE=688. Distinguishing between these is a LOT harder, especially if you need a range that also goes to 120,000!
Maybe some of the Fine Offset direction stability does not have to do with the vane at all, but rather the ability to discern which way it's pointing...

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Philip
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Re: Photos of the insides of Fine Offset sensors.

Postby Philip » Mon Dec 12, 2011 11:00 pm

Here is a quick table of values I did when I was looking at this subject,
values are in ohms.


SW1 R1 33K N 33000 N NE 6567.96
SW2 R2 8K2 NE 8200 E NE 891.30
SW3 R3 1K E 1000 E SE 687.50
SW4 R4 2K2 SE 2200 S SE 1406.56
SW5 R5 3K9 S 3900 S SW 3135.68
SW6 R6 16K SW 16000 W SW 14117.65
SW7 R7 120K W 120000 W NW 42120.06
SW8 R8 64K9 NW 64900 N NW 21876.40


Philip

AllyCat
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Re: Photos of the insides of Fine Offset sensors.

Postby AllyCat » Tue Dec 13, 2011 12:02 am

Charlie wrote:ENE=891, E=1000, ESE=688. Distinguishing between these is a LOT harder, especially if you need a range that also goes to 120,000
Hi Charlie,

The closest difference is about 12%, most resistors now are better than +/- 5% tolerance and +/- 1% are readily available if needed. The difference is about 0.1% of the full-scale value so it is going to need at least a 10 bit timer/counter. All micros have at least an 8-bit counter, often with an optional pre-scaler and some (as I suspect here) support 16-bits (65k or about 0.0015% resolution) or more.

The A/D converter needs to achieve this resolution anyway for the temperature sensing. The resolution is 0.1 degree C and the range over 100 degrees (-40 to +65), so better than 0.1%. But thermistors normally have a logarithmic resistance characteristic, so better than 0.01% resolution may well be needed.

When I was testing my magnetic modification to "equalise" the 16 wind direction sectors, I did notice that one sensor occasionally confused NW for W. That's the highest resistance, so I suspect that there was some current leakage or maybe the additional capacitance of my long extension cable was upsetting the timing. Otherwise the 16 directions on several units which I've tested have been decoded perfectly.

Cheers, Alan.

aussiewmr
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Re: Photos of the insides of Fine Offset sensors.

Postby aussiewmr » Sun Jan 01, 2012 12:28 am

For those of you that have WH3081 Weather Stations as used most probably in Australia, NZ, UK (EG Non USA) here are some photos of the Outdoor and Solar / Lux Sensors.

From what I can see the Wind Sensors are the same as those posted earlier in this thread.

The solar sensor photo here is not the one I modified as per a post here (Solar and UV Sensors going offline - fix?). There is little point posting that because the mod did not fix the issue I was having.

Cheers
Phil
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AllyCat
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Re: Photos of the insides of Fine Offset sensors.

Postby AllyCat » Wed Jan 04, 2012 3:13 pm

Hi Phil,

Thanks for those very detailed pictures; from another thread it seems that you're aware of most of the components, but perhaps it's worth adding the details here.

The Lux sensor is under the white diffuser dome on the right, the microcontroller under the black blob nearby and the UV sensor is a tiny chip at lower left. The 8-pin package near the centre is a small serial flash memory, presumably for Lux/UV lookup tables and perhaps calibration for the specific components.

Personally, I suspect that the UV sensor is a standard UVA+UVB type (the "UV index" is fundamentally UVB) and should be mounted under a UVB-pass filter (which may explain the poor accuracy). Recently, I saw a patent which claimed that UVB could be inferred (calculated) from individual UVA and Visible light measurements but I don't know if this is correct or if FO attempt it.

Just as this solar module first "saw the light of day" purely as a PV cell to charge the transmitter batteries, FO appear have plans to extend its functionality further. The unpopulated area beside the L-shaped slot is obviously for a radio transmitter and I guess some of the pads on the right will accept a daughter-board, perhaps for temperature and humidity sensors. This, together with the pads for two tag-ended batteries (or a holder), could make a complete "external unit" (which might suffer less problems than the present "two-processor" arrangement). A socket for the rain sensor is already fitted so the main unknown is whether the wind vane and/or anemometer would be omitted, or perhaps attached by a "captive" cable from some of the other "spare" pads on the PCB.

Generally, the overall quality of the PCB assembly (soldering, etc.) seems far higher than previous FO units and it must be disappointing (for all concerned) that the current performance and reliability is so poor.

Cheers, Alan.

aussiewmr
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Re: Photos of the insides of Fine Offset sensors.

Postby aussiewmr » Thu Jan 05, 2012 9:59 am

Hi Alan,

Here's the detail of the light sensor with the diffuser removed. I think it's some sort of Photo Diode rather than the LDR I expected.

Sorry about the focus - its a bit hard to get closeups with the equipment I have.

Phil
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Andyman
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Re: Photos of the insides of Fine Offset sensors.

Postby Andyman » Wed Feb 01, 2012 8:42 pm

Hi all. The wires in my wind direction sensor have come off. Does anyone have any info on which colour wire goes where?

regards
Andy

AllyCat
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Re: Photos of the insides of Fine Offset sensors.

Postby AllyCat » Wed Feb 01, 2012 9:23 pm

Hi Andy,

Yes, I encountered the same problem, so I'm not sure of the original connections either. However, as the cable is a ribbon style there are really only two (logical) ways to reconnect the wires. Electrically, it shouldn't matter which way they are connected because the two inner wires go to the anemometer and the two outer ones to the vane. However, the A/D converter driven by the vane resistance is rather sensitive so there might be a subtle difference. Perhaps somebody else knows?

Sometime I'm going to dismantle another vane and will photograph it, but as you (and Gina in her photo earlier in the thread) have discovered the wires are quite fragile (and the adhesion of the tracks on the PCB not too good either) so I'm not going to hurry.

Cheers, Alan.


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