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Monitoring 15 thermistors by computer

Discussion in 'Electronic Design' started by amdx, Nov 28, 2008.

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  1. amdx

    amdx Guest

    Hi All,
    I have freezers that I'd like to monitor by computer display.
    I can place a thermistor in each freezer and cable these to my computer.
    I'm looking for how to get the data into the computer and a program to
    display the temperatures
    on the monitor. High temperature alarms would be a bonus.

    Thanks, Mike
     
  2. Joerg

    Joerg Guest

    Or single channel ADC plus two 74HC4051. Possibly you can also use the
    sound card with an excitation frequency of a few kHz and measure the
    amplitude with its input. Saves the ADC. Other than the 74HC4051 and
    some jelly bean parts that would be almost free. But someone has to
    write the software ...

    Depending on what accuracy you need you might need to use four 74HC4051
    in order to calculate out its Rdson, since sound cards have a fairly low
    input impedance.
     
  3. K Ludger

    K Ludger Guest

    What about dallas 1 wire temp sensors, no analog interfacing required.

    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2815
    google around there are quite a few examples on how to use them
     
  4. Hi Mike,
    I use this program with Dallas/Maxim hardware. It will do everything
    you want including alarms.

    http://www.elektroniikka.org/thermometer/

    Dave
     
  5. hrh1818

    hrh1818 Guest

    One thing the other two replies haven't mention is how to measure the
    change in resistance with temperature of your thermistor. One method
    is supply a constant current to the thermistor and then measure the
    voltage drop across the thermistor. This method requires a constant
    current supply for each thermistor. Depending upon your accuracy
    requirements you may be able to measure the voltage drop at the power
    supply. But if your accuracy requirements are high you will need to
    run 4 wires to each thermistor,measure the voltage drop at the
    thermistor and use an analog to digital converter with a differential
    input. A second method is to connect the thermistor in Wheatstone
    bridge configuration. With this method you need one constant voltage
    power supply, 3 precision resistors for each thermistor and a
    differnetial input analog to, digital converter.

    I hope I have scared you away from using thermistors to measure
    temperature. A much better approach is to use Maxim one wire
    temperature sensors. See http://www.maxim-ic.com/products/1-wire/
    The temperature range of one wire temperature sensors is -40 to 85
    degrees Centigrade.
     
  6. Joerg

    Joerg Guest


    Often you can get by with a simple trick: Use a known resistors in
    series with each thermistor and either supply them all with a constant
    voltage or measure that voltage. 15 thermistors leaves one mux channel
    free to do that so Mike could save the 50c or so for a regulator circuit ;-)

    Done it many times, works. Of course this is not going to yield super
    scientific precision.
     
  7. amdx

    amdx Guest

     
  8. amdx

    amdx Guest

    Thanks guys, I'll mull over these ideas and see if I can get something put
    together.
    Mike
     
  9. neon

    neon

    1,325
    0
    Oct 21, 2006
    why thermistor not thermocouple. well anyhow if you finaly figure out how to read one then all you need is multiplexing for how many ch. you want.
     
  10. gravpoet

    gravpoet Guest

  11. amdx

    amdx Guest

  12. hrh1818

    hrh1818 Guest

    The following Web page has some good information that can help you
    evaluate if the one wire route is suitable for your application.
    http://www.maxim-ic.com/products/ibutton/software/tmex/index.cfm

    Howard
     
  13. hrh1818

    hrh1818 Guest

    Another approach is to use the Arduino open-source electronics
    prototyping platform. See http://www.arduino.cc/ for an overview of
    the Arduino platform. The item we are interested in is the Arduino
    Duemilanove. See
    http://arduino.cc/en/Main/ArduinoBoardDuemilanove This prototyping
    platform has a microprocessor, 14 digital I/O pins, 6 analog input
    pins, a 10 bit analog to digital converter, and connects to a PC with
    a USB cable. The cable is used to download programs to the
    microprocessor, send data to a PC and to supply DC power to the board.
    Here is a link to a page that demonstrates how the Arduino platform
    can be used to measure temperature with a thermistor.
    http://www.reprap.org/bin/view/Main/Temperature_Sensor_1_1 Or for
    one wire temperature sensors see:
    http://www.arduino.cc/playground/Learning/OneWire
    If you need more than 6 analog input channels analog multiplexer ICs
    like the 4051 or MC14067can be used to increase the number of analog
    inputs. One person used the MC14067 to increase the number of analog
    input channels to 96.

    But the biggest selling point is the Arduino Duemilanove. only costs
    $35 plus shipping.

    Howard
     
  14. amdx

    amdx Guest

    Hi Mike,
    I use this program with Dallas/Maxim hardware. It will do everything
    you want including alarms.

    http://www.elektroniikka.org/thermometer/

    Dave

    Hi Dave,
    I've dowloaded that program and the update and language pack.
    I can't open the .lng file exgtension. What do I use?
    I couldn't locate help file in the program.
    Can you give me some guidance?
    I want to see if I can set it up to do what I want, before I order hardware.
    Thanks, Mike
     
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