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I have a custom segmented LCD I took out of an old device that I'm now driving with an Arduino connected to LCD Driver AY0438. These are old custom displays so I don't have a datasheet or anything, I reverse engineered and figured out what pins control what segments (luckily it isn't multiplexed or anything). It has 14 pins and 12 segments. 2 of the segments show a Low Battery indicator that I don't need displayed.

I have the LCD, Arduino, and Driver on a PCB with traces to all LCD pins EXCEPT for the two that show low battery. For some reason, on a few of my displays (maybe about 2/7) the Low Battery indicator seems to be tied to one of the other segment's pin. When I display that segment, both of the Low Bat symbols come up and when I turn that segment off both turn off. This doesn't happen on a majority of them so I believe its not by design.

This isn't a terrible issue but I was wondering what could cause this. The LCD driver generates an AC waveform so I know that part is fine. Also, I did my reverse engineering on another display so its not like I damaged them during testing. Does this just happen from long term use of this type of display?

Do I just have to live with this or are there some things I could try to get rid of the issue? I tried desoldering those specific LCD pins and verified they weren't contacting the PCB and yet I could measure a voltage on them and saw them display. Any ideas would be greatly appreciated.

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    \$\begingroup\$ Is there one common contact for all segments? A schematic would help. Unconnected segments can receive signals by capacitive coupling to adjacent pins. \$\endgroup\$
    – Jens
    Commented Oct 13, 2023 at 13:24
  • \$\begingroup\$ Can you add a photo of the unit, in particular how the panel connects to the external contacts? Some systems use conductive fingers that push onto the display, and this can get misaligned, bridging two conductors. \$\endgroup\$
    – colintd
    Commented Oct 13, 2023 at 13:31
  • \$\begingroup\$ @JRE That suggestion would likely make things worse, as the common backplane is driven with square wave, so that LCD segment would always see a square wave drive signal. \$\endgroup\$
    – Justme
    Commented Oct 13, 2023 at 13:52

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As there is a single common backplane for all segments and each segment is driven individually in respect to the common, there is likely capacitive coupling between the unused segment pins to nearby segment wire or ground.

The most correct thing would be to drive also the unused segments and just drive them always as off. A secondary solution would be to tie them to the backplane signal so they will also are undriven.

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