Acurite Indoor/outdoor digital temp repair question (sort of)

[kingsolmn]

I have an Acu-Rite Indoor/Outdoor digital temperature thing that has been off on it's outdoor reading by more and more over time, so I decided to take a whack at fixing it. I was figuring that either the sensor is dying or some other stupid thing like a failing cap or bad solder joint. However, when I opened up the remote sensor I got confused - because I can't find anything that looks to me like a thermocouple. Looking at the traces on the board I can see what looks like where the thermocouple should go, but doesn't look like it's ever been populated! In fact, it appears that there are 3 positions for an appropriate thermocouple on the board.

I had no luck looking up the chip 'cuz I've been blobbed.

Front:

Back:


I would greatly appreciate any insights you fine folks might have on this. I thank you in advance for taking the time to read me post!

I know this is a "cheap" and "disposable" steaming pile, but I'm all about the reduce and reuse. That and I want to see if I can.

[NiHaoMike]

It's the SMD component labeled RT'.

[kingsolmn]

Thanks Mike! I didn't know that a thermocouple could be in an SMD form factor, but now I want to know more about SMD sensors! 

Hmm, now to figure out which one it is so that I can be looking at the correct datasheet! Lol I wonder how hard it would be to come up with a little circuit for a compact temp sensor... Thanks again. 

[oPossum]

I didn't know that a thermocouple could be in an SMD form factor

Thermistor

[fzabkar]

Here are the FCC filings:

https://fccid.io/RNE606TX

These are the internal photos:

https://fccid.io/RNE606TX/Internal-Photos/internal-photos-1149205.pdf

This version has a leaded thermistor.

Unfortunately the schematics are confidential.

I would start by measuring the resistance of RT1 at 25C, then at 0C.

[floobydust]

I have worked with these 433MHz wireless temperature transmitters, various brands.
La Crosse are absolute garbage for accuracy (bad firmware), Oregon Scientific are the best so far but hard to find since they were bought out. I think only a few contract manufacturers are making these.

They are not using an A/D converter, instead it's an RC circuit and measuring the ramp times using a reference resistor and cap, compared to the thermistor. Those reference parts might be out of spec - instead of blaming the thermistor. Or, I found out my neighbor has a transmitter in a bad location and I was displaying that instead of mine. I used SDR with RTL433 to monitor the local devices to figure it out.

You can remove the SMT thermistor, connect a known resistor or potentiometer in place and see what values are transmitted, to reverse engineer the thermistor specs that are used.
Example with the crappy La Crosse transmitters, anything less than 11kΩ gave +60°C, 50kΩ +22.0°C, 100kΩ +9.2°C, 221kΩ -7.00°C, 500kΩ -22.3°C which all works out to a common 50kΩ/25°C thermistor beta 3950. Two La Crosse transmitters never gave better than +/-8°C accuracy at low temps so I tossed them in the garbage.

For the Oregon Scientific/Radio Shack I just added a series trimpot and calibrated it, so far it's within 0.1°C and works very good. I think it uses a 10kΩ/25°C thermistor.

[TimFox]

I have a La Crosse indoor/outdoor thermometer with an RF link.  I mainly bought it for the clock, but also wanted an outside temperature measurement.
The indoor reading seems reliable (to 1^o F), but the outside temperature reading is way too sensitive to sunlight, despite being on a north wall of my house.
The "Oregon Scientific Store" (not the manufacturer) has no stock of any outside temperature thermometers, says the manufacturer has no contact, and thinks the manufacturer has no operations in US.

[floobydust]

I found the temperature readings inaccurate and wander with the LaCrosse. I got a warranty claim with a replacement TX141 transmitter (which was an older date-code for some reason) and it was a little better. At first I thought it was sunlight, wind etc but right next to it the Oregon Scientific unit was rock solid in readings, and there was no sun, no wind.

The La Crosse does some weird averaging - at the display end I think, over many readings, as well to CYA. Just a correction, the TX141 transmits a wide temperature range but the display clips it. A shorted (100Ω) thermistor packet sent is +89.2°C but display shows +60°C.

An outdoor 50k thermistor is useless in Canada at -30°C is 828kΩ, the La Crosse cannot read this high a resistance, the TX was up to 10°C in error when cold say -30°C outside and wandering. I think they forgot to check/allow deadtime when switching the RC network around. The units performed terribly for me. Even with lithium battery power.

[TimFox]

I found the temperature readings inaccurate and wander with the LaCrosse. I got a warranty claim with a replacement TX141 transmitter (which was an older date-code for some reason) and it was a little better. At first I thought it was sunlight, wind etc but right next to it the Oregon Scientific unit was rock solid in readings, and there was no sun, no wind.

The La Crosse does some weird averaging - at the display end I think, over many readings, as well to CYA. Just a correction, the TX141 transmits a wide temperature range but the display clips it. A shorted (100Ω) thermistor packet sent is +89.2°C but display shows +60°C.

An outdoor 50k thermistor is useless in Canada at -30°C is 828kΩ, the La Crosse cannot read this high a resistance, the TX was up to 10°C in error when cold say -30°C outside and wandering. I think they forgot to check/allow deadtime when switching the RC network around. The units performed terribly for me. Even with lithium battery power.

I went back to the LaCrosse website and noticed they sell plastic "radiation shield" enclosures for the outdoor sensors.  Do you have any experience with such items?
Here in Chicago (as opposed to Duluth, MN where I grew up almost Canadian), I can live with a range of -20^o F to +110^o F = -29^o C to +43^o C.

[james_s]

I would bet almost all of these that use a thermistor use a common 10k NTC part as those are the most common by far, and thus typically the cheapest. You can probably improve the accuracy substantially by adding a pot into the thermistor circuit with which to calibrate it.

[floobydust]

Convection air currents, i.e. sunlight on a nearby fence/lawn, then that heated air blows on the sensor. Radiation from nearby reflections i.e. the same fence can also cause unexpected solar IR mirrored over to the sensor, can also cause unexpected warming, despite the sensor "being in the shade".

If the sensor is out in the sun, the solar radiation shields keep them cool - but not the ground under them. So hot air rises anyway, from the ground, building etc. and you get a high reading.

[kingsolmn]

Thanks to all for the great info on the topic!    Great points on the indirect heating sources, I had considered that and moved the outdoor sensor a few times over the course of a few weeks for this reason to help eliminate that. When that didn't make much of a difference I moved both parts of the pair to my bench with a thermometer right next to them for comparison and recorded the reading over the span of a few days.

And thanks for correcting me on the thermistor/thermocouple thing, I think I have a weird form of dyslexia where I mix up nomenclature. Haha

Great idea with putting a trim pot in the circuit, I'll put one in after I check the rest of the components in that chain. I guess I kind of was on the right track with my thinking of the possible fault when I started out! 

Thanks again all, after I get some time at the bench I'll share what I found.