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Thread: Sensors, Interface Kit, wire lenght, power supply?

  1. Sensors, Interface Kit, wire lenght, power supply?

    I am looking at purchasing the Interface Kit 0/16/16, and likely 4 temp/humidity, 3 pressure, and 4 temp sensors. My main concern is two things: How far can I run the wires to these sensors, and can I provide power to all of them via at a single point (where the board will be). I would prefer to have the system on continuous power, rather than battery.

    Suggestions?

    Thanks,
    Roger

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    Re: Sensors, Interface Kit, wire lenght, power supply?

    Welcome to the forum! How far do you need to run them?

    Analog signal travel is largely going to be dictated by wire gauge - but calibration for really long runs becomes a nightmare as you start doing the math to try to get as close an approximation as possible.

    Generally I would use a microcontroller as close to the sensors as practical, take my readings, armor them in some kind of error-tolerant format, then ship them out over serial - or RS485 serial for even longer distances.
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  3. Re: Sensors, Interface Kit, wire lenght, power supply?

    I could have runs as much as 100 feet. The application is an attic monitoring system. In case you are wondering, I am tweaking an attic for ventalation, temperature, and moisture control. I live in North Texas, where poor attic temperatures can cost you an extra $200 to $400 a month in cooling/electricity bills.
    Last edited by RogerGuess; 06-21-2009 at 02:05 PM.

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    Re: Sensors, Interface Kit, wire lenght, power supply?

    Interesting project!

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    Re: Sensors, Interface Kit, wire lenght, power supply?

    I think I would put the controller in the attic close to the sensors - probably in an "alarm box" kinda enclosure. I would use USB over CAT5 extenders, and just run my USB to the computer over CAT5E.

    Simpler wiring, more accurate, easier maintenance - does that make sense?

    [Actually - if it were _me_ more specifically, I'd use something like a little Arduino to capture the data, then I would would send it to my computer over XBee wireless. But I'm an XBee fanatic.]
    Last edited by Adrenalynn; 06-21-2009 at 02:48 PM.
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  6. Re: Sensors, Interface Kit, wire lenght, power supply?

    Ok, that makes sense. That will likely drop my longest run to the board down to about 20'. So, what would I do for power to the board and sensors in the alarm box? Where should I read about powered usb over cat5? I can easily run cat 5 to a location in the attic.

    Thanks,
    Roger

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    Re: Sensors, Interface Kit, wire lenght, power supply?

    20ft is still pretty long for cheap sensors.

    What's your skill level? Programming, hardware, software, firmware, .... ?

    I would toss together a little 5v/1A regulator or two (depending on the number of devices) for my sensors (working on a tutorial for that right now, actually), and power my board and sensors off of a wall-wort, using the regulator to get me a really clean reference to my analog devices.

    Wiring to the sensors should be as heavy a gauge as practical. If you ran 12/2 / 12/3 (depending on whether it's a two wire or three wire sensor) romex out to each of them, *I* wouldn't laugh at you.

    I've used a bunch of these: http://www.svideo.com/usbcat5extend.html professionally, and have a couple sets of the 2pt personally.
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  8. Re: Sensors, Interface Kit, wire lenght, power supply?

    Ouch, the powered usb will nearly double the $$ on what I planned to do. Is the problem with long runs on the sensors the accuracy? Isn't it just about calibrating them to known values?

    .. I am a c# developer, but do not have much electronics knowledge.

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    Re: Sensors, Interface Kit, wire lenght, power supply?

    It's about voltage drop over distance. If you have a 5v sensor and a 0-5v analog input, and you were to run 18G bell wire for 150', your voltage drop would be something along the lines of half a volt per 100' typically, or 0.75v. So now we'll never see more than 4.25v on our 5v input, which would lower our peak read ability by 15%, and would also lower our precision by 15%. If we don't care about the peak (because the sensor peaks higher than we need), and we're willing to accept the error, than we're probably ok. Lowering that error in software is probably generally doable with something like regression analysis. You'd want a handheld calibrated measuring device, and take a whole bunch of samples over a range with both devices, do the math, and build an algorithm to get the best fit possible. The more samples you have, the closer it's going to be. We still lose our precision and our peak though.

    No sensor is perfectly accurate, so you need to at least multiply its error by our error in precision to get an idea of what you're looking at. Typically, I'd guess we'll see ~25% for a run-of-the-mill thermister.
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    Re: Sensors, Interface Kit, wire lenght, power supply?

    Ok.. so attic monitoring, what are we doing with the data collected though? Are we opening vents? Just trying to help gauge how important accuracy really is...

    -Fergs

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