Thursday, July 17, 2008

Noisy 1Wire Readings

Of the 8 sensors I installed, the last one on the chain fluctuates a little between readings. It's not bad, just not as accurate as the other 7 sensors. What's different about this one?

It's at the end of the 1Wire line - could there be signal issues? I originally wanted to create my 1Wire network in a star pattern as that would be the easiest to wire from the patch panel, but the 1Wire docs advised against it. So I ran the network as 1 giant wire with each sensor as a stub. I actually used 4 wires to run them (voltage and ground since I decided against powering them parasitically and a wire for network in and network out). I chose this method to control the length of the stubs - otherwise the stub length would be the wire distance from the patch panel to wherever the sensor is located, which would vary for each location as each Cat-5 run is some different, unknown length. This, of course, increases the network length, but should be well within the 700 meter limit.

The sensor is also the only one outside. It's installed sticking out of the wall of our enclosed porch. It was exposed, but after seeing the fluctuations, I covered it up with electrical tape. It still fluctuates. I tucked it behind the porch speaker. It still fluctuates. I'm not sure what else to try at this point. Maybe that's the way it is outside?



Here's an RRD graph of the master bedroom temperature vs. the outside temperature. Starting Wednesday evening, after I got all the graphing and data gathering correctly set up, you can see the very smooth curve for the master bedroom sensor. The outdoor sensor, however, has fluctuations, on the order of a degree or more at times. The readings are accurate - they just don't make for a pretty picture.

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