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I just got my thermometer together tonight. All the nixies and leds light up. The IR led works and I can switch through all the different modes. So what's my problem?

The displayed temp is quite a bit higher then actual. After I let it run for a few minutes it settled on 79.7.℉ but I know my house is 74 or lower. Any idea what would cause this? How accurate should it be?

Reading the other threads I stumbled on this:
viewtopic.php?p=285#p285 and Tony's reply...

Just to clarify, I did attach the heat pad, paying close attention so that it would be square under the IC. I plugged it back in this morning and the temp started at 71.5, which is probably lower than actual. Every time the display cycled to humidity and back it would raise the temp a few points: 71.5 to 71.8 to 72.0, etc. So it was a pretty quick rise in temp, not an hour like the other post.

The temperature at startup will always be 20 degrees C or the equivalent, the software integrates samples over time so it can take up to 60 seconds to reach the actual temperature. If it's in the case and the heatsink pad is fitted make sure the soldering round the TCN75 is OK - if the controller can't read valid data from the IIC bus it falls back to the DHT11 reading, which will be at the internal case temperature. (I may remove that if I update the code, it was intended as a backup option for anyone who didn't want to solder a SMD device but that hasn't really been an issue)

Inevitably though it is sensing the temperature of the case bottom which will be heated by the 2W or so dissipated inside. An external sensor is the best way to get an accurate reading.

Is there any way to tell/test to determine if the TCN75 is soldered and working correctly? I did have some trouble soldering this in the correct place...

Short, sad story: I first tried to solder the chip to the SMD pads on the far right of the PCB. I was cussing up a storm because the pads were too close together for the little chip. It wasn't until I tried to solder and remove it twice that I realized I must be doing it wrong, checked the instructions again, and slapped myself in the forehead. It's the 7 pads more towards the center of the PCB!

Anyway, I've visually checked the TCN75 solder points and I can't see anything amiss. Anything more scientific I can do?

Update on temp: I'm brought the thermo to work with me and will let it run all day. So far, it is up to 82.8F and I think the building is kept pretty constant at 75-ish. I will update at the end of the day.

That is a long way out. Probably the easiest way to test will be to heat it up - apply some heat to the TCN75 from under the board while trying to avoid warming the DHT11 sensor. If the reading changes quickly (before it warms up the PCB) it's probably working.

Yeah...I'll check it when I get home.

Just as an update, looks like the temp is holding steady at 83.3. The bottom of the case is fairly warm, primarily at the back, near the T12, which makes sense I think. Looking over the case assembly instructions one more time and I can't tell if the silicone pad should be under the DHT11 or the SMD. I think I might have it more under the DHT11, which is a little further to the left. I will check that too when I get home.

What does the SMD do? I figured the DHT11 was the temp and humidity sensor, but an earlier comment made it sound like the SMD drove the temp.

The DHT11 is only used for humidity with the temperature being supplied by the TCN75 as it should be accurate to +- 0.5 degrees C or about 1F though the case heating causes some error. The silicone pad should be sandwiched between the IC body and the base, under the PCB.

Well I'm at a loss...the SMD definitely is driving the temp. Testing my finger against the chip causes the temp. to start increasing pretty obviously and an ice pack brings it back down. When I try the same thing on the DHT11 nothing happens. When the board was out of the case it was still running a degree or two hot, but much better than before. Moving the silicone pad seemed to help a bit too. But once it's back in the case the temp. quickly climbs back up into the 80s. Any other ideas for what I can check?

Could you measure the drop across R28 just in case there's some problem causing it to use more power than expected?. There will be some self hesting but it shouldn't be as bad as this.

I touch either line of my multimeter to the two sides of R28. When the nixies are lit the voltage reading is 150mV. If it's just the LEDs or all off then the reading is only 10mV. I guess that's 0.14V to 0.01V. Is this normal? Anything else I should test?