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[on:]

Hi,

I've got a problem getting an SHT15 temperature/humidity sensor getting to work properly. I've soldered short wires to the little breakout board (sparkfun) and am using the library from http://www.arduino.cc/playground/Code/Sensirion. I've checked all connections and as far as I can tell they're allright. However when using the non-blocking example from the page above I get:

Error: CRC mismatch!
Status reg = 0xFF
Error: No response (ACK) received from sensor!
Error: CRC mismatch!
Status reg = 0xFF
Error: CRC mismatch!
Error: CRC mismatch!
Temperature = 2581.30 C, Humidity = 0.10 %, Dewpoint = 265.37 C
Error: CRC mismatch!
Error: CRC mismatch!
Temperature = 2581.30 C, Humidity = 0.10 %, Dewpoint = 265.37 C

Most of the time at least. Sometimes I do get a sensible reading (the 'error: crc mismatch' is gone then) and once I got a whole list of readings without any errors. But this happened only once and I cannot reproduce it. Sometimes I also get the message: 'Error: No response (ACK) received from sensor!"

I've already replace the wires, used different pins, fiddles with the contacts and read all I could find on the forums. However, I wasn't able to find any pattern yet.

Does any one have any idea what might be the problem or what I could try further? At this point I'm tempted to buy another sensor to see if that makes a difference, but they are a bit expensive to buy them just for that purpose alone.

Any help is appreciated! Thanks!

Olaf

[Reply #1 on:]

The first thing the example program does is:

Code:

  sht.readSR(&stat);                     // Read sensor status register
  Serial.print("Status reg = 0x");

That is failing:

Error: CRC mismatch!
Status reg = 0xFF

I think this means that your wiring is incorrect. The breakout board has four wires:

VCC <- 3.3v or 5v (probably 5v if you want to use it with a 5v Arduino)
DATA <->  Pin 2
SCK <-  Pin 3
GND <-> GND

[Reply #2 on:]

Did you have any success with the non-blocking example?  It's a little simpler.
I've occasionally found problems with these sensors.  You can have problems if you query them too quickly.  I usually try to wait at least 5-10 seconds or more between readings.

I think Vcc needs to be 5 volts for the sht15.

-transfinite

[Reply #3 on:]

I think Vcc needs to be 5 volts for the sht15.

The data sheet (http://www.sensirion.com/en/pdf/product_information/Datasheet-humidity-sensor-SHT1x.pdf) gives VCC as:
Minimum: 2.4v, Typical: 3.3v, Maximum: 5.5v with a footnote saying that it was calibrated at 3.3v so that is the best voltage for accuracy.

[Reply #4 on:]

Thanks for your reactions! I checked all connections once more, but they seem to be fine. I even checked the connection from the board to the sensor itself with a multimeter (voltage on sensor 5.0V, resistance from sensor to board on data and clock 6ohm, but I don't have probes that small so I suppose it means they're connected).

Before the 3.3v <-> 5.0v remark I was using 3.3v as I read on the specs that 3.3v is best for the sensor, but I didn't check whether the arduino can handle these lower signals. Anyway, switching to 5.0v didn't help. As a matter of fact I now get:

Error: No response (ACK) received from sensor! (followed by bogus readings)

As if the wires aren't connected at all 

@transfinite: yes, I also tried the other sketch. It returns all values zero, so that's why I used the non-blocking in the first place: it has the error codes. BTW the sensor is polled every 5 seconds, so that should be fine.

So unfortunately no working sensor yet...

Thanks anyway, Olaf

[Reply #5 on:]

Well, I learned something from this thread.  I need to try the sht15 at 3.3 Vcc and see if it the results are different than at 5VDC.  I guess I should read the data sheet more carefully.

I can attest that the Sensirion library works well.  If you are confident in your wiring, then it seems like you have a defective part.

-transfinite

[Reply #6 on:]

so the library is ok too, hm...

that adds up to:
the library is ok
the code is ok
I replaced the wires once, the chance that they're bad twice is very small
I checked and rechecked the connections, and measured them, so unless I really need more (less?) coffee they're ok too
I've tried multiple pins on my Arduino (including ones that I know are ok from other hardware), so they're not the problem
I've used both 3.3 and 5.0 V on the sensor, didn't matter either

I guess the only option left is order another sensor and see (hope...) that helps  (And next time I'll check if the 5v arduino can cope with a 3.3V sensor before connecting...) It may take a while before I can get a new sensor, but I'll post an update by then.

Thanks again! Olaf

[Reply #7 on:]

Were you able to get your sensor working?  I've used a number of these sensors (10+) and not run across a bad one yet.  The longest wiring I've tried is about 2 feet / 60 cm.  You are certain the pin numbering in the sketch matched your wiring?

Regarding 5V vs. 3.3V:  The "D1" constant the library uses for temperature calculations is specifically for 5V operation.  You'd need to change that if you use a different voltage (see the datasheet).  I haven't experimented with this but doubt it is worth running the sensor at anything but 5V with a 5V Arduino.

[Reply #8 on:]

No, unfortunately I wasn't. I rechecked everything once more and then concluded that my best option is buying another sensor. But considering you're mileage it might be worthwhile to have another go at it. I did go on a short holiday in the mean time so I might be able to have a fresh look at the project anyway 

I'll keep this topic updated!

Olaf

ps. thanks for the voltage explanation!

[Reply #9 on:]

You've included the required pull-up resistor on the data line?

[Reply #10 on:]

  required pull-up?

I had no idea I had to use one on the data line... I'll definitely try that, thanks! 10k/20k will do?

[Reply #11 on:]

The Sparkfun breakout appears to already have a pullup on data (and a pulldown on clock for some reason).  It also has a bypass cap on Vcc so it should be in good shape given reasonably short wires.

[Reply #12 on:]

Agree with cjands40 (I had not looked at the SparkFun breakout board to see that it included pull-ups). I see a lot of questions regarding various humidity sensors, most of which required a pull-up on the data line in their spec sheet.