Quote:
Originally Posted by opiesche
I'd like to put additional sensors on it as well (at least one upstairs, one downstairs, and one outside), and will have to experiment a little with the maximum wire length. I'm using the DS18B20 sensor (also Dallas 1-wire) - do you have any idea how long the wires can be before you start getting problems with signal degradation?
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I'm using 4-strand bell wire that I had laying around just because I had it. I'm using a three-wire system because for me it's much easier to use an extra wire strand than to take all the precautions that a 2-wire setup require. I'm hearing that the 2-wire setup can be finicky.
Pure hear-say on this one, but the guy I got my Arduino data logger from said that he was able to deploy at least 70 sensors, and that runs of almost 100 feet are possible.
From the experiences of other, it seems that if you put branches in your system, you can risk the reliability of your data.
1-wire boards, front & back. ("COW" should read "COM")
I am using some tiny printed circuit boards that were designed by a local guy to mount my sensors onto and to connect the leads to, also. There's nothing particularly worth noting about the boards, with the exception that there are pads for soldering a termination resistor onto for the last sensor (sensor most distant from the micro-processor board). The idea is that a signal pulse that travels down the wire, will 'echo back' if it hits a drastic change in impedance, like if it hits an open (zero impedance) at the end of the line. The board also has pads for tiny caps to go from V+ to ground, if required. No one around here has needed to use the caps.
My house is small, two-story with a basement, and I'm running my sensor string up along the chimney, which is pretty much in the center of the house.
So far on my system, I am doing everything wrong... I have two single-sensor branches (short ones) coming off the main line, and no termination resistors and no power line caps anywhere. I'm getting good, reliable data. My total length is about 30 feet, excluding the short branches. I figure I have another 25 feet, and three sensors to go.
I have read horror stories about people who have used CAT-6 cable, and have gotten horrible results, I have also read reports about the same setup and people who get great results.
I had one apparent problem with one mounted sensor, so I replaced it with another, problem solved. Closer inspection revealed that my bad mounted sensor had a bad solder joint. I re-soldered the joint and it is now reliably in use.
Aside from violating all the rules, I have been careful in the construction and deployment of my sensors... I check for good data, at each step. So far, I have not entered into the
ZONE OF UNRELIABILITY. If things begin to get sporadic, I'll start with a termination resistor... If that doesn't do it, I'll go with the tiny caps... if that doesn't do it, I'll replace the branching topography with a linear one.
That's what I can tell you at this point, hope it is of some use to you.
Best,
-AC