Fine Offset thermometer - location of sensor?

[h2005__uk__]

Hi

Does anyone know where the temperature sensor (or thermistor?) is located in the transmitter module for the Fine Offset WH1080 weather station?

See attached an image of its insides. I thought it may be the white square component but that's actually for humidity.

Thanks!

[AllyCat]

Hi,

Welcome to the forum.

AFAIK, the temperature is (now) supplied by the Humidity sensor, which communicates via a serial data bus (or maybe just a simple serial data stream). In the "older" FO transmitters, the temperature was supplied by a (blue bead) thermistor mounted on the main board (near to a different humidity sensor), as shown in the third post in the "Photos of...sensors" sticky thread.

The "NTC" at the top right of the daughter board indicates a thermistor, but if that is not populated (on the underside) then the temperature data is presumably coming from inside the "white box" (the temperature is of course an essential value required for calculating relative humidity).

Cheers, Alan.

[h2005__uk__]

Ah that's interesting!

I did an experiment whereby I removed the white box (presumably the humidity sensor) and placed the transmitter in the fridge... the humidity is reading 10% - 20% and the temperature, which had been in the 20s, has shot down to the teens (and will probably go further down if I left it there!).

So that must mean the temperature sensor is somewhere else which is intriguing.

[AllyCat]

Hi,

Yes that is interesting. Is there no sign of a thermistor wired to the pads marked NTC? But AFAIK yours is the first photo posted that labels those pads NTC (Negative Temperature Coefficient); previously (e.g at the foot of the third page of the photos link above) they were called CON2. Many microcontrollers have an internal temperature sensor (or can use the voltage drop across an external diode), but it's not very accurate or calibrated. Also, there are now "semiconductor" sensors sometimes called "linear thermistors".

But my guess is that one of the SMD components on the underside of the PCB may be a thermistor. They're quite sensitive, so a "warm finger" on the relevant component might be enough to identify it. But of course you have to wait (up to) 48 seconds for the Console to show the change. However, it would probably have a resistance of 10 kohms at 25 degC and (typically) 7k5 at 30 degC, so it might be fairly easy to find, if you have a multimeter.

Cheers, Alan.

[h2005__uk__]

Hi, thanks for your reply!

Attached are a couple more photos - one of the underside of the whole thing and one of the underside of the PCB that the humidity sensor is mounted on.

[AllyCat]

Hi,

I was surprised to see another inetgrated circuit on the underside of the daughter board. I guess that whole board substitutes for the "normal" temperature/humidity sensor modules such as the DHT11, DHT22*, etc.. Those modules have three pins, whilst the humidity sensor on the daughterboard has only two, so the temperature sensor may be elsewhere on this board. My guess would be the unmarked SMD component, i.e. not the (beige) C1 or C2, nor the three (black) numbered resistors.

* The DHT11 appears to report temperatures only down to 0 degC, but the DHT 22 is rated down to - 40 degC.

Cheers, Alan.