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Thermistor into XP computer excel

Discussion in 'Electronic Basics' started by rob, Nov 27, 2004.

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  1. rob

    rob Guest

    I am recording 7 locations for temperatrure with thermistors and a
    digital Multimeter and manually entering into an Excel spreadsheet. I
    would like to automate. I only need to take sample temps once every
    15 minutes (or so) 12 hours a day.

    i saw a site where 2 temps can easily be logged into excel via game
    port, using Visual Basic in XP.....soooo....I figure it shoouldn't be
    a big stretch to do what I want.

    Can anyone point me in right direction to build hardware to do
    this?..or if more than moderatly difficult an off the shelf card...I
    am aware of Data logging hardware out there but again I would like to
    try to rig myself.

    Thanks in advance...
    Rob
     
  2. JeB

    JeB Guest

    My experience has been that the game port is sloppy at best. I don't
    know what kind of accuracy you want but I don't think you can expect
    much from the game port. I'd imagine there are many A>D boards (eBay?)
    to pick from which could handle the thermistors for you.
     
  3. Rich Webb

    Rich Webb Guest

    Sounds like a perfect application for a microcontroller. Quite a few
    include 8 A/D ports that could easily read the drop across a thermistor,
    do some digital filtering, and send the result out to a PC via a
    built-in UART.
     
  4. rob

    rob Guest

    Is this something a keen DIYer could handle?

    Rob
     
  5. Rich Webb

    Rich Webb Guest

    Absolutely. You could do it with, for example, one of Atmel's STK-500
    "starter kits" (about $80 from Digikey) and a handful of thermistors
    plus resistors for the "other half" of the voltage dividers. The STK
    already has the RS-232 level shifters and a DB-9 connector that can go
    to the PC. You'd need to setup the A/D channels, the UART, and a timer
    plus the usual glue code. All pretty easy to do with assembler,
    following the examples in the product datasheet.

    They used to include (IIRC) an AT90S8515 with the STK. Not sure which
    one they include with it nowadays.

    An ATmega8535 would be good for this application. It's available in a
    40-pin DIP; AFAIK, the smaller form-factor DIPs (e.g., 28 and fewer)
    don't have all eight A/D channels pinned-out.

    The only "gotcha" with these is that the ATmega(series) usually ship
    with the system clock set to the internal oscillator. It's simply a
    matter of using the programming software (free from Atmel) to set a fuse
    to tell it to use an external oscillator or crystal but it's <cough>
    possible to forget to do that.

    An alternate route would be something like the PMD-1208LS from
    Measurement Computing. The total cost is in the same range.
     
  6. JeffM

    JeffM Guest


  7. Rob,

    You can expand to four channels as a game port contains two sets joystick
    connections. But without extra electronics that's the limit. You can look
    for analog multiplexers to serve more thermistors but to control them you
    need some output from the computer. The game port has no outputs so you'll
    need the serial - or parallel port for it. When you come so far you can as
    well go all the way and use the serial port to do whole the job. A PIC or
    AVR micro with build in AD convertors and a UART will perform much better
    then a game port both in reliability and accuracy. You sure will have to
    invest in some hardware and have a learning curve but IMHO the alternative
    will take the same cost with worse results.

    petrus bitbyter
     
  8. Jag Man

    Jag Man Guest

    Rich,

    Is the programming software an assembler, a compiler for a well-known
    high level language like C, or what? In what hardware/software environment
    does the software run, and how do you get the machine code over to the
    chip?

    Just trying to get my feet on the ground...

    TIA

    Ed
     
  9. Rich Webb

    Rich Webb Guest

    This would be pretty quick in assembly, assuming that you did the heavy
    lifting of converting from sensed thermistor voltage -> resistance ->
    temperature over on the PC side; that is, send the A/D result over to
    the PC. All of the conversion could be done in assembly, of course;
    doesn't seem to be a good investment of time when it's simpler on the PC
    side.

    C would work as well, of course. There is a cross-compiler, gcc port
    (i.e., free GPL software) available for the AVR family. For downloads
    (of this and other goodies) as well as app notes and general info, click
    over to www.avrfreaks.net.

    There is a range of commercial C compilers, as well. I use ICC-AVR from
    ImageCraft www.imagecraft.com/software. There is a mailing list archive
    at www.dragonsgate.net/mailman/listinfo.

    As far as loading the assembled or compiled code, most AVR device
    programmers accept Intel-hex input files. There is a free assembler /
    debugger / programmer-driver package, "AVR Studio 4," available from
    Atmel at www.atmel.com/dyn/products/tools.asp?family_id=607. The PC
    serial port plugs into the STK-500 (or AVRISP) to load the device.
     
  10. Jag Man

    Jag Man Guest

    I'm beginning to get the picture. You work on the PC using a development
    environment,
    finishing up with generation of a file of hex codes. That is downloaded to
    the
    microcontroller currently plugged into the STK-500. Then you move that chip
    to a project board with the ancillary electronics to make it all work?

    TIA

    Ed
     
  11. Rich Webb

    Rich Webb Guest

    Correct. Alternately, you can use the STK-500 to do in-system
    programming to a chip on a project board (or soldered to a PCB) by
    connecting a ribbon cable between the STK-500 and the target system.
    The manual www.atmel.com/dyn/resources/prod_documents/doc1925.pdf
    shows the required connections in Section 6.

    For your app -- A/D converters to the outside, serial port to a PC --
    everything could be done on the STK-500 without needing a separate
    board. So if you're just monitoring, say, the home thermostat over a
    24 hour period then you don't need anything else: the regulated power,
    clock oscillator, RS-232 level converter, ... are already provided.
     
  12. John

    John Guest

    Dataq has a 4 channel, 10-bit A/D converter that connects to a serial
    port for $24.95US (http://www.dataq.com/194.htm)

    It's probably the cheapest intro kit available - and includes
    software!

    John
     
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