FEZ Panda II Connecting to 2 DHT11

NETMF & Gadgeteer
1

Hi,

I am new to putting hardware together.

I am trying to connect 2 DHT11 sensors from DFRobot to my FEZ Panda II. (Please see attached image)

Although I am getting readings from both DHT11, the readings are inconsistent. (Sometimes up to 5 degrees C apart). If I pull out one of the data pins, it would affect the reading of the other DHT11 that is still connected. (Yikes!)

I think I’m doing something wrong with the circuit. Can anyone kindly point me to the right directions?

Thanking you in advance!

Jacob

2

Hi Jacob, and welcome to the Forums !!!

DHT11, what code are you using? Did you use the “two-pin” variant on http://www.tinyclr.com/codeshare/entry/228 or did you use the RLP variant on http://www.tinyclr.com/codeshare/entry/278 ?

To troubleshoot this, I personally would only connect one at a time, and roughly measure the temperature and average it over say 20 samples with the two sensors individually. So take a set of readings from one that tell you it’s now say 25 degrees C, and then replace the data pin connection to the other sensor and repeat the sample set and see if the value still changes so dramatically.

3

Hi Brett,

Thank you very much and thanks for responding.

I followed your suggestions and tested the sensors. Good thing I bought a few extras to test. The results aren’t very encouraging:

I tested 4 sensors. 3 of them have about the same temperature reading and one was about 3 degrees C off.

But of the 3 sensors, only 2 agree on humidity reading while the other was off by about 20%.

Assuming I wired everything correctly, I think I may have some faulty sensors or that these sensors (especially the humidity reading) is awfully unreliable.

I got the sensors from an online shop that was listed in GHI Electronics website. I’m not making any accusations, but you know, my suspicions were raised when the sensors arrived in their packaging with the serial numbers blacked out. Maybe it’s normal. I’ll try get some more from another store.

I used the RLP variant with a small change (the one in the code share only remembers the last pin that was called in the Setup so will need tweaking to work with more than 1 sensors) and also some bits from DHT11_Temperature_and_Humidity_Sensor__SKU__DFR0067_-DFRobot.

Would you happen to know a sensor with better accuracy on humidity?

Thanks again.

Jacob

4

@ jmojiwat - You may want to take a look at the SHTxx (SHT11 etc) devices, but they are expensive compared to the device you are using.

EDIT: When comparing the outputs of these types of devices straight from the post it is best to keep them in a warm and dry place for an hour or so, to remove any traces of moisture.

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