General help and tips about LCD display and driver

Discussion in 'General Electronics Chat' started by szabikka, Aug 13, 2016.

  1. szabikka

    Thread Starter Member

    Sep 3, 2014
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    1
    Hello everyone!

    I'm currently trying to redesign one of my old projects (a 24 hour clock with LED displays). The redesign only concerns the display, as I've found LED diplays consuming a bit too much energy, so I decided to swap them for a 4 digit LCD display. I've included the datasheet as an attachment. I have never worked with an LCD display before, but from what I was able to get together, they are quite different from LED displays. I have already bought the display (it operates from 5V supply-extended temperature type) and 4 CD4056 drivers, but I don't know if they can work together flawlessly, so I've decided to ask your opinion in the following questions:
    a) The cd 4056 contains a level shifting function, so the display can have larger voltage swings when going from Vdd to Vee, than those of the BCD inputs of the driver. My question is why, if LCD displays usually work with voltages below 5 V, while the driver can tolerate as high as 15 V? The maximum voltage swings are actually smaller on the display than those on the driver.
    b) Do I need to add any resistors or other components to the design? For example, LED displays always need current limiting resistors, do I need them with LCDs?
    c) When I browsed through the CD 4055/4056 datasheet, I read something that I interpreted as the 4056 can't drive the display on its own, because it lacks the DF output that can be found on the 4055 chip (the 4056 has a strobe/latch pin instead). Can I use my display with just the 4056 or I will have to invest in some 4055s?
    d) On the first page of the 4055/4056 datasheet the voltage swings from Vdd to Vee and Vdd to Vss are given as 0 to -3 and 0 to -15. Does that mean I need a negative voltage regulator like the 79xx series?
    e) The displays datasheet marks pin 1 and 40 as BP. I know it means "backplane", is it the "common" pin of the display? Can I tie it simply to Vss or I need to connect that to the display frequency source too, like the drivers DF input?
    f) If you can add any further tips with LCD displays, I would greatly appreciate it!

    Thanks for the help in advance!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    a) If the LCD runs on 5V then power the CD4056 at 5V, the same voltage as on the LCD.

    b) The current required to run LCD is about 1μA, i.e. the resistance is very high. No external series resistor is required.

    c) DF, Display Frequency, is a square wave signal that you provide. DF-IN is the signal you provide. DF-OUT will go to the back-plane (BP) pin of the LCD.

    d) CD4056 will drive the LCD from +5V. You don't need -ve supply.

    e) BP is a square wave signal that is obtained from the DF-OUT pin of the CD4056.

    f) You will need one CD4056 for each digit. You could do the whole thing with one IC such as Intersil ICL7211 or Maxim ICM7211.
     
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  3. dannyf

    Well-Known Member

    Sep 13, 2015
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    you probably should have thought about it before buying the parts, not after.
     
  4. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Thanks MrChips! I will check out the ICs you suggested.
    About your answer for the e) part. The CD4056 doesn't have a DF-out pin. It has a strobe/latch pin instead. Can I get the square-wave out of that pin anyways? I'm asking because the datasheet shows the 4056 connected to a 4054 IC, and I'm assuming it can't drive the LCD on its own.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    A LCD works because when you put a voltage across it he modules literally twist in place changing the polarization from that with no voltage. This voltage needs to be an AC because any DC will attract any ionic molecules to one side degrading performance.

    The DF is one side of this AC voltage, and each pin is the other. The DF needs to be the same voltage and frequency. If you have an extra 4056 you could use that for DF.

    You will also need something to drive the middle colon.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Sorry, I misread the data sheet.
    The CD4056 is ok to use.
    You need to connect the DISPLAY FREQ-IN signal to the CD4056 and the back-plane of the display.
    The CD4056 does the phase inversion internally for segments that are enabled.
     
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  7. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
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    I think a CD 4054 will do the job. Thanks for pointing it out, I completely forgot the colon.
     
  8. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
    1
    Thanks for the replies guys! Just one more question, what if I don't want to use all the digits? Can I leave the segments pin floating, like with LED displays?
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    You don't need a CD4054. Use a standard CMOS inverter to drive the colon from the DF signal.

    No. You cannot leave the pins floating. Connect them to DF.

    That is:
    • connect to DF to turn OFF the segment
    • connect to /DF to turn ON the segment
     
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  10. szabikka

    Thread Starter Member

    Sep 3, 2014
    77
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    Thanks, MrChips! I appreciate your help!
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    That is true as long as Vee is connected to Vss.
     
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