Seeking 3-1/2 digit LCD Volt Meter

Discussion in 'General Electronics Chat' started by MrSoftware, May 17, 2016.

  1. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    I'm in need of a 3-1/2 digit (0.123) DC volt meter and LCD all-in-one package, can anyone recommend one? I'm building a prototype device that needs to monitor voltages in the range 0.500 to 0.900 VDC. Panel mount, through hole or SMD is OK. I need the cost to be under about $10/ea in low volume. Basically I'm looking for something like this, but already combined with an LCD: http://www.intersil.com/content/dam/Intersil/documents/an05/an052.pdf

    Can anyone recommend one?
     
  2. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I think I got just the thing
    Lemme check my stuff and I'll be right back
     
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  3. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    5 years ago I placed a purchase order with this guys: www.martelmeters.com, but their web page seems to be unresponsive.

    I've attached the user's manual of the display units that I acquired from them.

    I also googled their stuff, and this is what came up:
    http://technical-sys.com/electrical-martel-electronics-corporation/
    http://technical-sys.com/wp-content/uploads/2014/09/Martel-Meters-catalogue.pdf
    https://www.testequipmentconnection.com/manufacturer/Martel/meters/
    http://www.martelelectronics.com/
     
  4. Nykolas

    Member

    Aug 27, 2013
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  5. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    http://www.murata-ps.com/en/products/digital-panel-meters.html Generally nice meters when I used them. Please read some of their application notes.

    Your DPM solution has lots of "gotcha's". Look at their application notes.

    There are issues with the common mode range and generally you need an isolated supply. Even the ebay voltage/current meters have gotcha's. The put the shunt in the ground leg.

    Be careful!
     
  6. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    Thanks guys. The term "panel meter" was a big help! That Martel stuff looked good, but were all $30+/ea.

    The catch is I need very low voltage readings, under 1v. Everything at digikey was very expensive, but I found some cheapies on ebay that might be good enough for a proof of concept, then I'll have to find something better if the concept proves to have legs. I'm still open to suggestions, but I'm going to give these two a shot and see what happens.

    http://www.ebay.com/itm/131138599834

    http://www.ebay.com/itm/261215844907
     
  7. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    Thanks for the tips. My plan is to power the panel meter from a 9v battery. The same 9v battery will power a voltage divider that will provide 5v to an external sensor. The panel meter will be used to both adjust the pot on the voltage divider to get to 5.00v, and to read the voltage returned by the sensor. I'll use a double pole switch to switch the meter back and forth between the voltage divider and the sensor output. Does it sound like there would be any gotchas with this setup?
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I bought some of those cheap panel meters on eBay and they tend to drift terribly when first turned on - taking 10 to 20 minutes to stabilize. Some never stablize. Some go though batteries in a hurry. Also, (in most cases) you need to add a divider resistor to an empty spot on the panel meter circuit board to set the range (0-0.2V, 0-2V, 0- 20V, 0-200V). A few of the meters I have did not come with instructions. They are all sitting in a box under the workbench.

    They also take 0.2 to 1 second to stabilize on a new voltage once you make the switch between your two sources.

    Finally, a voltage divider on a 9V battery does not make a good 5V reference. A new 9V (unloaded or lightly loaded) will put out 9.1 to 9.6V depending on chemistry. Voltage will continually drop as the load on the battery changes and as the battery is consumed. The meter will likely stop working as the battery drops below 7.5V. Use a 5V regulator (7805) or other dedicated 5V reference - that will also come out of 5V compliance if supply is below 7V but at least it will be a steady 5V until then.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would think putting together one of the many picmicro versions out there on the web, you could also customize it, also if portable power is used you could use a one/two line display and have a power saver option that will turn display power off after a limited time.
    Max.
     
  10. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    More about my use case: this will be a tool used for adjusting a sensor. It only needs to keep steady voltage for a couple of minutes. The user will turn the tool on, connect to the sensor, adjust the tool to 5v output, flip the switch to read the sensor feedback and adjust the sensor, then turn the tool off. Total duty cycle is probably 2-3 minutes. The sensor draws just over 1mA, the display and volt meter portion will probably draw a few mA, but I'm pretty confident it will hold relatively steady voltage for a minute or two. It's not ideal, but it's inexpensive and should work "good enough" for this case.

    At the same time, there will also be a version using this proper voltage reference IC, which works fantastic. I'm going to compare the performance of the two and see if there's any real difference in results for this particular use case.

    My goal is to keep the cost to make these at $20/ea or less, including the box and the 9v battery. This is why it's important for me to find a volt meter and LCD combo for a reasonable price, and why the voltage divider method is attractive. I'm OK buying my own ADC chip and LCD separately, as long as I can keep the price reasonable.
     
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  11. cmartinez

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    Jan 17, 2007
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    I was going to suggest a voltage reference for your application. It would be far cheaper if you were to build a comparator circuit with three LEDs. One for overvoltage (red) one for within range (green) and a third for undervoltage (yellow)

    I don't think you really need an LCD for what you want to accomplish.
     
    Last edited: May 18, 2016
  12. tcmtech

    Well-Known Member

    Nov 4, 2013
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    eBay and Amazon are loaded with mini LED and LCD panel meters for under $10 that could easily be made to fit in a pannel.

    I don't know about .123" display height though. .250" are the smallest ones I ever went look for and bought.
     
  13. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    @cmartinez - The sensor will be set to different values based on the performance of other parts. i.e. sometimes the user will set the sensor to 0.58v, sometimes to 0.64, 0.65, etc..

    @tcmtech - by 0.123, I didn't mean .123", I meant like 3 points past the demimal: v.xyz, I didn't explain that well. :)
     
  14. cmartinez

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    How many different settings will there be?
     
  15. tcmtech

    Well-Known Member

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    The standard 200 mV or 2 volt ones would do what you want. +- .1999 or 1.999 reading ranges.
     
  16. MrSoftware

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    It's infinitely variable within that range. The bounds will be about .56v and .80v. I wouldn't expect anything outside of that range.
     
  17. WBahn

    Moderator

    Mar 31, 2012
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    Do you really need three digits? I think you might find that the third digit is purely an illusion -- if you were to set it to 0.500 V, is the rest of the circuitry in your device really good enough for that to be meaningful compared to, say, 0.504 V or 0.495 V?
     
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  18. MrSoftware

    Thread Starter Active Member

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    I really need 2 digits, 3 would be nice. There is definitely a difference between say .64v and .65v.
     
  19. cmartinez

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    I was thinking about digits too... with only two digits, 0.80 - 0.56 = 0.24 = 24 different settings, fits in a 5-bit number. So using dip switches is a possibility.
     
  20. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    I see what you mean, and while technically that would work, the people who will be using the tool (basically the general public) are not that technical and wouldn't understand it, and wouldn't be willing to figure it out. I'm currently selling a product into this space, and creating this new tool would 1-up my competition. So while technically it needs to be accurate, politically it also needs to be as simple as possible and non-intimidating. People like knobs and LCD displays. ;)
     
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