Need an idea

Discussion in 'The Projects Forum' started by wayneh, Sep 15, 2010.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    OK, here's a clean a sheet of paper. I need a way to LOG a series of on on/off pulses, so that I have a record of time a circuit is "on" versus total time. Even better, I'd like to also log a DC voltage along with that. Essentially I need a strip chart recorder.

    Here's the scenario: I've gotten a nice, thermostatically controlled cooler circuit working nicely. It gives me temperature control to within about 1°C by toggling a TEC on and off every few minutes. Now I need to start collecting data using my cooler, looking at how it responds to things going on inside of it. (Yes, it's being used as a calorimiter.) An ideal solution would be a strip chart recorder to record the sawtoothing temperature and also the square wave of voltage applied to the TEC. But I don't have a dual pen strip chart recorder and need to rig up a poor man's alternative. I DO have old computers lying around that could be put to the task, but they're not good with DC voltages. Sound yes, DC no.

    Any cheap, simple ideas? Controlled stopwatches?

    I should add that an observer with a clock and a multimeter is also a perfect solution. They could record temperature and on/off times by hand. But we're talking about experiments that will take hours, maybe 6-12 hours or more, so I need an automated solution.
     
    Last edited: Sep 15, 2010
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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    If your PC is a desktop model, you can get plug-in interface boards. And even if it's not, you can get USB-based interfaces--example found randomly on the net, and I have no financial interest:

    http://www.dataq.com/products/startkit/di148.htm

    Actually that looks like a pretty good system. $25, and you get recording software with it.
     
  3. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    I was hoping for something even cheaper, but I've been wanting data acquisition for a long time. Might be time to bite the (small caliber) bullet. Really would save a lot of time in the long run. I'll see if I can find a Mac version that cheap.
     
  4. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    If you dont mind mucking about with it a bit you could remove the decoupling caps from your soundcard input and use the two ADC's that are at its heart ...
    Google - sound card oscilloscope -

    Plenty of software and examples ... mostly free.
    (probably not as good as an interface board mind you, it depends on your budget)
    Al
     
  5. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Interesting! As much as it scares me to remove the safety of the decoupling caps, it's intriguing to think this might give me a DC voltmeter. Could be the perfect poor man's solution. Identifying and bypassing the caps might be a challenge. These things are pretty small these days.
     
  6. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    bypassing just involvs a small wite loop, usually un-insulated and arranged not to toutch anything.
    You will probably find it easier to atach to the track rather than the cap.
    I would recomend a cheep plug in sound card ... dont do this on the mother board.
    if you measure the DC resistance, with a mulitmeter, from the input pin back through any serease componants the offending cap probably the first to block the DC completly and settle to an infinite reading.

    Alternatives...
    Picotech do several small ADC products designed for low end data aquisition.
    http://www.picotech.com/data-acquisition.html

    And back to the sound card ....
    Most have game controler ports which use a timed pulse to determine a resistance.
    Its not fast ot particulaly accurate but given your application may just do what you need without butchering or buying anything. Heres a primer ... (There ar many more)
    http://pinouts.ru/Inputs/GameportPC_pinout.shtml

    Al
     
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    25 dollars is pretty cheap, what is your time worth? An arduino could also be used, but again is in the 25 dollar range.

    By using a voltage controlled oscillator you can go direct to your sound card and record signal as a wave. Decoding would involve some software. The VCO could be based on a 555 circuit
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    If your PC has a old style printer port there are (what?) 10-14 bytes you could use with a simple A/D converter. Same basic idea.
     
  9. wayneh

    Thread Starter Expert

    Sep 9, 2010
    12,094
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    Another follow-up.

    Well, I decided to go large and got myself a LabJack U3-HV for $114. It's an awesome device with many, many capabililties: http://labjack.com/u3
    It appears well built and I'm very impressed after playing with it now for a few weeks.

    I was attracted by the high versatility of the device, its programming options and the support level they offer, including Mac support. With their help I've been able to write some Visual Basic macros to build an interface to the device within Excel. So now it'll be easy to watch ~8 voltages all at once and log them all into a virtual stripchart.

    That's all I need for now but it's barely touching the capabilities of this thing. One thing I plan to exploit is its ability to output controlled PWM. All in all, having this thing at my disposal will completely change what I need - and don't need - to build in the future. I just wish now that I would have bought it sooner.
     
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