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Temperature measurements — Parallax Forums

Temperature measurements

Chris23Chris23 Posts: 16
edited 2008-07-09 12:28 in BASIC Stamp
Hello everyone,
·
· I am a student and i am at the·begin of building a circuit for a "smart home" application. I need 8-10 (as much as possible) different·temperature readings, so i can measure the temperature at every room.·I intend to use the DS120 or AD592 probe, but i don't know what would be better for the application. I took a look at the "Applied sensors tutorial", but i got a little confused.
· One nice thing about·DS1620 is that it returns readings directly as digital numbers. But one disadvantage it has is that it is a chip with 8 pins, hard to turn into a probe that can be used apart from a circuit board. An other disadvantage for this application is that i can use only 4 DS1620·temperature sensors,·because a DS1620 needs a three pin connection with the BS-II.·That could eliminate my project's intensions.··
· On the other hand for·every AD592 probe you need one pin connection to the BSII,·but·you use the rctime command, in order to do the necessary calibration. Would that be effective? Would it be better to use a A/D converter to read the AD592 probe ? But, if i do that, 3 pins would be required for every·A/D converter.That could also·eliminate my project's intensions.·
·
Please, any suggestions??
·
Thank you in advance,
·
Chris

Comments

  • ercoerco Posts: 20,259
    edited 2008-07-08 22:33
    Thermistors are dirt cheap (25-50 cents at Hosfelt) and can be used very simply with the RCtime command. At that price, buy twice the quantity you need and just use the best matched ones so you don't have to have individual calibrations for each sensor. Then the only variable is the length of wire (and associated resistance) per sensor, but you could also make them all the same (maximum) length.

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    ·"If you build it, they will come."
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2008-07-08 23:51
    The number of pins taken by multiple DS1620 is not so bad as you think. They can all share the same clock CLK and data DQ signal lines, and only the chip RST needs to be separate for each one. Thus if you want to use 8 DS1620, you need 10 pins on the Stamp, one RST for each DS1620 plus shared DQ and CLK. The number of Stamp pins required for the 8 RST lines could be condensed further by using multiplexer. It is true that the DS1620 is an 8 pin chip, but does also come in a surface mount package that can fit along with a bypass capacitor on a small rider pcb. I'd worry more about all the wiring. The DS1620 is relatively expensive, but it is nice to have the data directly in digital form.

    If you have a BS2p or 'pe or 'px, another option is the one-wire DS18B20. A one-wire network is made by stringing all the sensors out on one pair or triplet of wires. Each sensor has its own unique digital address. The wiring for an extended system is much simplified.

    The AD592 with RCtime will not be as accurate as the AD592 with an ADC. That is due largely to drift and similar issues with the RCtime command. There are analog to digital converters that have multiple channels but still only a few wires required from the Stamp. For example, I use the TLC2543, which has 11 input channels and has a 4 wire connection to the Stamp. The ADC chip takes a command to select a channel and then returns data from that channel.

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    Tracy Allen
    www.emesystems.com
  • jeffjohnvoljeffjohnvol Posts: 197
    edited 2008-07-09 00:55
    Chris, check out the finished project by Oldspring at http://forums.parallax.com/forums/default.aspx?f=21&m=276243

    It uses a SHT15 chip, only uses one signal pin AND gives you the option of measuring relative humidity as well. It also includes the code. I haven't tried that chip or code, but I'm looking forward to it someday.
  • FranklinFranklin Posts: 4,747
    edited 2008-07-09 03:16
    Why no this guy? www.maxim-ic.com/quick_view2.cfm?qv_pk=2795

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    - Stephen
  • stephenwagnerstephenwagner Posts: 147
    edited 2008-07-09 11:54
    Chris,

    Chapter 7 of the PARALLAX BS1 app notes sescribes a method using RCTime a thermister and the math to straighten the curve. You can use MSExcel instead of GASFIT to work the coefficients. http://www.parallax.com/Portals/0/Downloads/appnt/stamps/bs1Appnotes.pdf·I am note sure if GASFIT is still supported by PARALLAX.

    SJW
  • Chris23Chris23 Posts: 16
    edited 2008-07-09 12:28
    Thank you a lot for your suggestions and ideas... They have been very helpful for me. I will make some experiments and inform you for the results.

    Chris
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