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I have a spare DMM (Digital MultiMeter) that I would like to use to log the displayed data at 1 sample/sec.

What would be the least expensive way to mod the DMM to log the data that appears on the display?

I know that the meter data goes though an AC/DC converter, ADC, decoder, & then to the display.

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    \$\begingroup\$ Without knowing more about the multimeter, it is pretty difficult to suggest anything. \$\endgroup\$ – Tom Carpenter Oct 14 '15 at 1:45
  • \$\begingroup\$ It's just a cheapo Cen-tek from Harbor Freight. It has a blob of epoxy covering the main chip, so there's no way to get the chip id info. But I did find a url on Instructables to mod the DMM--so today is a good day! Thanks for your feedback! \$\endgroup\$ – zeffur Oct 14 '15 at 6:00
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Actually many low cost DMMs are based on ASICs that do have the ability to send their output through an interface to a computer. Yours may be one that has that capability.

See this web site Unlock Serial Communication on a DMM

or here

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  • \$\begingroup\$ Excellent response! Neither of your links was to the cheapo meter that I have, but your wording was enough for me to find the exact mod that I am looking for here! Thanks for your help! \$\endgroup\$ – zeffur Oct 14 '15 at 6:01
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Webcam and OCR is probably your best bet. The meter could be too integrated to extract the signal between the ADC and the display (the ADC and display controller could be integrated on the same chip, so you would either have to interpret the LCD segment drive signals or provide your own ADC). Also, you could run in to serious issues with isolation. Some meters support transmitting readings over RF or IR, but this is uncommon. Might as well just read the display directly.

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  • \$\begingroup\$ Sorry, I meant display controller \$\endgroup\$ – alex.forencich Oct 14 '15 at 1:55
  • \$\begingroup\$ Agreed -- I plan to use an external optoisolator. I guess the output to the LCD segments will probably binary on multiple pins. Any thoughts on how that could be read & stored digitally outside the DMM? \$\endgroup\$ – zeffur Oct 14 '15 at 5:58
  • \$\begingroup\$ Lcd segments are generally driven with low voltage ac. Also, there will probably be some sort of multiplexing going on. There are also going to be a good handful of electrodes as well, not just the digits but also battery and unit indicators, analog style bargraph, etc. Recreating the display from looking at the segment drives is not a particularly trivial option. \$\endgroup\$ – alex.forencich Oct 14 '15 at 6:15
  • \$\begingroup\$ forencich - Is there a defined protocol for such a data stream? \$\endgroup\$ – zeffur Oct 14 '15 at 9:47
  • \$\begingroup\$ There is no defined protocol. It's the display drive for the physical LCD segments, not the interface to a display controller chip. It will depend on the construction of the display itself. Most LCD drivers use low voltage ac to drive the segments, so you would need some reliable way to detect this as well as figure out the multiplexing scheme. \$\endgroup\$ – alex.forencich Oct 15 '15 at 3:56

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