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Interfacing the Standard multimeter

Discussion in 'General Electronics Discussion' started by eptheta, Apr 13, 2010.

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  1. eptheta

    eptheta

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    Dec 20, 2009
    I have a standard multimeter like the picture below. Nothing fancy, no USB support...just your everyday multimeter...
    [​IMG]
    My goal is to use this multimeter to connect to my laptop so i can log the readings it takes.
    Please read the following list to see what exactly i am interested in doing:
    1. I do not want to buy a 400$ multimeter with USB support.
    2. I want to use my parallel port (with 8 bit input) to log the data, but if that will just not do then I'm open to alternatives.
    3. From what i know, the multimeter circuit is way too complex for me to fiddle with. However, at the end of the day, all the output goes on to an LCD panel, whose inputs are accessible to me. If i can somehow re-route the inputs to my computer's port, then i could construct a program to interpret the data.

    I would like some help on this matter as I need it for a physics project that is coming soon.
    Thanks !
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    How about a $99 meter?

    If your time costs you nothing then you may be able to build something cheaper, but I'm not sure. If your task is to build one, then fair enough. If it's just a tool and they're commercially available, then...

    But in any case maybe there's something in this design that you could borrow (say the totally isolated link to the computer).

    One amusing solution would be to use a webcam, capture the image and send it to some OCR software. The cost (and the speed) would be quite low. You would certainly have isolation :)
     
  3. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    99$.....400$...it's all the same to me="too costly"

    Ha ha ! The irony ! I happened to use this exact same idea for my last chemistry project (calculating luminous intensity of a chemical reaction in terms of resistance), but the reaction time was so low, i got only 100 points on my graph instead of 500 !

    That idea aside, is there anything else you can suggest ?
    I want to just utilize the outputs already provided and input them into my computer...somehow..
    I can easily adapt a program to interpret the input signals and give the needed output, but to get that input from the Multimeter is my problem...
     
  4. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    What you are trying to do can not be done easily. Likely, it can not be done at all. I highly doubt that there any accessible point in the miltimeter where you can grab the value being sent to the screen.

    You also need to be really careful about grounding. If you just connect the multimeter to the PC, your negative lead is now grounded to the earth ground. There are many measurements that you will not be able to do and unless you are careful about it all the time, you will soon damage the multimeter and the circuit you are measuring.

    ---55p
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Given that the multimeter display is a very simple 7 segment display, and given that you could set things up to have great control over lighting and camera position, *I* (personally) with a budget of $0 would go the camera route. With experience in writing software, you *should* be able to process the images about as fast as they come off the camera. They don't have to be high resolution. If you can't get several good readings per second (conservatively) then I'd be surprised.

    How fast do you need to read the results?

    p.s. just a day or so ago I uncovered the multimeter attachment for my HP41 calculators -- which is one of those really weird coincidences.
     
  6. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    Darn...That sure was discouraging.
    The thing is, my multimeter response time is in any case slow (about 300ms).. What i am assuming is that the LCD screen takes a long time to process those signals and then put them on screen. I believe that the signals are sent to the screen at a much larger frequency(about 1 every 20ms...) so if i can tap into those inputs, depending on how fast my parallel port can handle inputs, i may be able to get a better frequency.

    Judging from the comments, am i to assume that i just cannot go forward with this project? Or should i wait a bit longer and hope for inspiration(or yours).?

    If i can only find a USB multimeter for dirt cheap(5-10$) :rolleyes:
     
  7. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    What is your basis for that belief? Hint: You believe wrong.
    Unfortunately, wanting it to be so does not make it so.

    ---55p
     
  8. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    Alright then, whats the std frequency of the digital multimeter anyway ?
    Even if it is something like 1 per 200ms, i still do want to interface it to my computer for data logging. My camera probably has sad FPS anyway.......
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I doubt there is one.

    probably at least 25 fps at lowish resolution. It will take some clever programming, but that's just up to your effort.
     
  10. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    Alright, thank you both for your help...
    Some one can kill this topic now, I'm done here... I guess I'll move on to something else and maybe just buy a USB multimeter...
     
  11. eight08

    eight08

    14
    0
    Apr 16, 2010
    this could be a great project! maybe a possible iea to put them into one, test leads out of the laptop. :)
     
  12. NickS

    NickS

    367
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    Apr 6, 2010
    If you have no love for your audio card then I have seen some DIY projects for turning one into a weak oscilloscope.

    *HOWEVER* I would like to emphasize that you will likely destroy the audio card and it may well damage your mother board as well.

    But If you could offer proper isolation then there are quite a few low frequency MM type tasks that you could set up.
     
  13. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    Sorry for reviving an ancient thread but for some reason I want to give this project a try again....

    Here's my logic(probably very flawed, but it's worth a shot):
    The multimeter works 'God-knows-how' but at the end sends signals to 3 7-seg LCD displays.

    Each of those displays has X terminals, which I can see clearly. Which means that somewhere along the way, the multimeter sends inputs to the X terminals of the LCD screen.

    Instead of the LCD screen, if i had a magical amplifier and LEDs attached to each of those inputs, they should glow as if they were each segment of the LCD.

    With a program that checks properly (if seg1 & seg2 & seg3 are glowing, output 7 or something like that), an output can be generated on a computer.

    The problems:
    - Assuming that I am going to check the segs simultaneously, it would mean i need many more than 8 ports.
    Otherwise I need a serial port or something...

    -The multimeter circuitry may actually just be slow and be sending inputs at the same frequency as it is generating
    Otherwise the LCD response time is high and several readings are vanishing into this air.

    NickS, I used to love my sound card. Then i found out that she was seeing someone else. It was all over then. After I stuck some random circuitry into her, I converted her into an oscilloscope but I think I destroyed her lower cognitive functions and all she can see these days is AC.

    Please do suggest ways to implement this project and If it is not possible, just say so.

    Thanks
     
  14. NickS

    NickS

    367
    0
    Apr 6, 2010
    I sort of eluded to this but never outright said so, sorry. Your sound card scope has a very narrow frequency window for operation. Part of that is no DC because you are certainly going to be AC coupled.

    Sorry to hear about your cards fidelity issue, You may be able to remove the coupling caps and replace them with 0ohm resistors....BUT without knowing the details on the input amplifier it will be easy to do more permanent damage. That however would buy you the ability to see DC.
     
  15. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    So tempting....... But I need someone who likes to break things to try this out first. I asked a lot of people I know and no one wants to sacrifice their sound cards or motherboards.

    The only problem is finding out which capacitor to rip out......
    [​IMG]
    Seriously ! Too many caps !

    Assuming I do decide to buy a second hand Pentium3 with about 10MB of RAM, I guess I'll fiddle around with it.

    I still don't get why the digital multimeter thing won't work. I tried it a long time ago, but i don't think the voltage was enough to log on a parallel port. I didn't feel like amplifying it so i let the idea die.

    Any idea ?
     
  16. NickS

    NickS

    367
    0
    Apr 6, 2010
    The cap/caps you are interested in will be fairly large in value(the higher C the lower frequency they pass). And you may be able to trace it in from the input.

    You are using the microphone input right? So I think that will be mono but check it to make sure. If so you can follow the input trace coming on board from the connector. I would expect the cap to be the first component it sees but judging by the pic you posted, there may be an amplifier first. This is a risky thing to try if you are unsure of which cap to remove. For instance if you pulled a power decoup cap and replaced it with a short that would short the power rails which is usually a smoke inducing event with a 90% chance of vaporized trace.

    Sorry I don't know why it wasn't working for you. Perhaps trying to inject audio tones of the same level first to make sure something is getting through the chain.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Unless an amplifier has been designed to be DC coupled, removing DC blocking capacitors is likely to do really bad things.
     
  18. NickS

    NickS

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    0
    Apr 6, 2010
    Assuming he stays in range what concerns have come to mind?
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If there are multiple stages of amplification, removing capacitors between the stages will screw up the biasing of the following stages.

    Mostly applies to amplifiers made from discrete components, but depending on the gain, you're going to start having to consider DC offset issues.
     
  20. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    If the DC offsets are constant or follow an easily traceable equation then that shouldn't be a problem that software can't fix. However if it is sporadic and unpredictable then God help us all.............................Sir. (Sorry, couldn't resist because i have no idea why it's there)

    Yes, i am using the microphone input. Anyone here wants to try something like this before i go around buying eighth-hand computers ?
     
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