Photos of the insides of Fine Offset sensors.

[Ned]

Yes, nothing but a reed switch, and the cable is hard wired too. I recently replaced my cable with a longer one, using a phone extension lead with the connection cut off one end. Tried soldering the frizzy copper wire with embedded nylon(?) fibre to the board without success, and ended up wrapping and soldering it to short pieces of solid wire.
No problems subsequently with the data, only a longer wind cable is liable to induce spikes in the temp readings.

[ksangeelee]

I've attached a photo of the new DCF antenna position as well as both sides of the PCB for comparison with older units, and a labelled pinout for the solar pod cable.

These images show the inside of my Maplin N96GY / WH-1081 transmitter circuit (433MHz) The red wire soldered to the board was added by me.

I can't quite figure out where the antenna is on this circuit. My guess is it's the blue cylindrical part that looks a bit like an inductor. I've never seen an antenna that looks like this (certainly not for low-power 433MHz that's meant to range 100m), so I'd be grateful if anyone could correct me on this.

As it happens, the 17.5cm wire I added does *seem* to increase the range slightly, but not by much.

DSCF4047_Medium.JPG

DSCF4050_Medium.JPG

DSCF4049_Medium.JPG

[dpmiller]

the track that goes around the "L" cutout is the twig.

[ksangeelee]

the track that goes around the "L" cutout is the twig.

That was my initial thought, but I can't figure out why it would continue into the ground plane.

Also, that blue component that looks like an inductor actually has infinite resistance across it, whereas an inductor would have close to zero.

[sooty]

The blue part is a trimmer capacitor.


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]

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]

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]

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]

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]

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]

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.

Cheers, Alan.

[Charlie]

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...

[Philip]

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]

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]

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