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Computer interfacing problem

  1. Mar 30, 2006 #1
    well...I have 14 varisters equally distributed on the walls of my room(FCC Structure) which will produce specific resistances depending on the temperature on that place. And my program is to read those resistance values and convert them into temperatures. Then it is to apply larplace's transformation to generate a graph of the temperature distribution of my room.

    My problem is...How do I get my program to to read 12 resistor values! I can use the joystick port to read in 4!
  2. jcsd
  3. Mar 30, 2006 #2


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    You're going to need some kind of DAQ board that will accept that many voltage inputs. You could possibly work out a way to switch in groups of four and capture 4 at a time. That's basically how data buckets work by scanning each channel over a specified interval.
  4. Mar 30, 2006 #3


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    If you can do it with a joystick port, can't you just buy some more joystick port cards?
  5. Apr 2, 2006 #4
    Well, Ideally I should be reading 1 value at a time because 4 varisters at once tends to increase the values read inside.

    Anyways... Is it possible doing it through a demultiplexer...if I have the input as +5v or something and then use parallel port output bits to set a perticular channel voltage to high which will trigger a relay which will allow me to read those analog values. But then tehre are other problems :( like...relays are big and ugly and dont look good on circuit boards :( Ill try to look for smaller relays to see if i have any luck.

    And dont have that many free slots to put in joystick ports. I think they only come with sound cards now a days.
  6. Apr 3, 2006 #5


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    You could use solid state relays, much smaller and would work since each line in the parallel port can only source maybe 10mA but I think can sink 30mA, don't remember what the +5 can source but you could always tap that elsewhere.

    You can purchase external ADC cards with many inputs, or build your own with a PIC or two that can already mux the ADC across a few inputs already. I've seen some nice looking (no experience with) ADC cards now with USB ports that emulate serial ports. Pretty easy to write a few lines of code to work with that and that's pretty awesome portability.
  7. Nov 14, 2006 #6
    a joystick will only have 2 pots (for a start).

    varistors are "slow" sensors, and you can scale their value to the expected limits of your measurement. 14 varistors, might as well make it 16 sensors.

    if your joystick card can read 2 pots, you will need some kind of multiplexer, (quad bi-lateral switch), like a 4016.

    the other option is to purchase 2 8channel 8 or 10 bit ADC's (analog to digital converters), these are VERY CHEAP, and spit out their readings of your sensors, in a nicely computer readable form.

    your Printer port, can be easily used for simple digital I/O, and you can use that port to switch through the channels of your ADC's and record the values you are after.

    or you can use your printer port to control your multiplexer switch, (4 lines required for 1 of 16 selection), and use 1 of your joystick pot inputs and the measurement input, and the second joystick pot input as a reference.

    i would estimate the total cost of your measurement system (except varistors). to be mabey $30 or so. depending on your willingness for comprimise.

    i hope that help.

    there are many (and more expensive methods), plug in DAC/ADC cards are a great option.

    (i use C and LabVIEW, for my software development for instrumentation)

    or you can purchase a Microprocessor development board, like something from ATMEL, they have 8 channel ADC's onbaord, (10-bit), can operate stand along, and communicate via RS-232 to your PC. (plus they are great fun to play with !!).
  8. Nov 14, 2006 #7
    varistors are not that great as reliable temperature sensors, they have problems with self heating, if you apply too much excitation, and long wire runes will introdue errors. not enough will introduce noice and uncertainty.

    have a look at solid state temperature sensor, more accurate and easy to scale and interface.
    like the LM335Z.

    you will require 4 digitial outputs to switch 16 signals to one channel.

    or 2 times 8 channel 10-bit ADC's that you can read from your Printer port. (printer port = cheap digital I/O).
  9. Nov 18, 2006 #8
    your problem may due unnecssory progr mas
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