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back lit LCD questions

Discussion in 'Hobby Electronics' started by Qwerty, May 25, 2005.

  1. Qwerty

    Qwerty Guest

    This is the first time I really started to mess around with one of these, but I have a display that is not
    working correctly and have some questions as to how this is 'suppose' to work.

    I am measuring 4.15v on all lines for the display, even on the pin that the segment is 'clear'.
    What kind of signal is used to activate the segment as it does not seem to be DC controlled.

    The driver is ok, I swapped in another display and it work fine. Is it possible that the display is not good (no physical damage, however),
    or maybe there is something just 'goofy' with the display? Is it possible that the connections inside the
    display have been servered on the non-working segments?

    Not sure if back lit LCD's work the same as standard one (have no experience with either), but I kind of
    assumed that you applied a DC voltage to each segment to activate it (like a LED), but this does not
    seem to be the case in this situation.

    I hope that is clear...
     
  2. KLR

    KLR Guest


    It was explained to me about 10 years ago, but you certainly dont
    drive them anything like an LED or Vacuum fluorescent device.

    As I recall there is a backplane voltage, and the segment/digit
    drives are somehow driven in some relationship to that, and in a
    certain phase relationship too - to make things worse :)



    A backlit LCD is the same device electrically but has simply got a
    light behind it, (whether it be a small light bulb, LED, or
    electroluminescent material) where a non-backlit just relies on
    ambient light, and would have a reflective backing on it.
     
  3. Qwerty

    Qwerty Guest

    It is a simple display similar to a digital clock. I know the pcb outputs the correct
    signal as another display works fine, yet not all the segments will 'light' up on
    the defective display.

    Is it possible that the signal used to drive the display could be weak and not
    able to drive the 'defective' display?

    I would like to know what signals are need to drive the display, that way I can
    maybe bench test each segment.

    thanks,
    Tim
     
  4. Mark Harriss

    Mark Harriss Guest


    What kind of LCD is it? character ?
    some types need a negative contrast
    voltage to function drive them.

    There used to be a LCD faq once that
    was good for info.
     
  5. Ken Taylor

    Ken Taylor Guest

    LCD's aren't driven by a DC voltage, as someone else said. They get fed with
    a clocked waveform which has to have a zero DC voltage bias on it. You can
    take a look at something like
    http://www.pacificdisplay.com/ics_app notes/hitachi/HD44100.pdf
    if you're interested in seeing how a driver works.

    For your problem it would appear more likely that you have a connector fault
    or a display fault. Check that the pins are mating properly and that there's
    no cracks in tracks. The LCD module itself will sit in a connector by way of
    a strip of conductive (I think) rubber - it's all pretty hairy IMO and you
    need to make sure that it's mating properly. It may have been jolted and not
    connecting properly. What make/model is the display?

    Cheers.

    Ken
     
  6. Qwerty

    Qwerty Guest

    It's a custom display and it does not use the rubber strips for connections, it has physical
    pins coming out of the display the push into through connectors on the driver PCB. I have
    connected another display to the PCB and it works ok, so that bascially eliminates everything
    to the display itself. The display in question has no physical damage to it (that is visable), it is
    just missing some segments. Actually in the area that is the problem, on the segments that do light
    up, the ajoining ones also seem to light up faintly. Does this indicate some sort of short in the interior
    connections??

    ps>> thanks for the link
     
  7. dmm

    dmm Guest

    Not necessarily, there may be some signal getting through to the segments within the lcd
    by capactive coupling. I wouldn't be concerned with this unless the segments are activated
    when they shouldn't.

    Each segment of a LCD is XOR'd with the backplane signal, but driving LCDs with
    more than one backplane is more complicated. If you have a datasheet of the display,
    you could see if the damaged segments are driven with a common backplane signal.

    The pins attached to a LCD are fragile and damage can occur that may not be
    immediately visible. If you have a magnifier, check around the pins/glass/track
    interface of the damaged segments, as well as the backplane pin.
     
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