Author Topic: EEVblog #1045 - How To Drive an LCD  (Read 546 times)

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Offline EEVblog

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EEVblog #1045 - How To Drive an LCD
« on: December 08, 2017, 12:21:46 PM »
How to drive a static LCD display with digital logic and an Arduino / microcontroller.

Code: https://github.com/EEVblog/Arduino-Seven-Segment-LCD

 
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Offline Mighty Burger

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #1 on: December 08, 2017, 12:59:31 PM »
Lots of useful information! Brilliant use of the XOR gate!!
 

Online NiHaoMike

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #2 on: December 08, 2017, 03:47:04 PM »
I once took a junk LCD and left it connected to a 9V battery for months on end to see what effect it will have. To my surprise, the most I got was a temporary ghosting effect. Did I just get really lucky?
Cryptocurrency has taught me to love math and at the same time be baffled by it.
 

Offline Z80

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #3 on: December 09, 2017, 12:47:25 AM »
DC can cause ion breakdown and migration of contaminants over time, it's a common problem and not unique to LCDs.
More info here: https://books.google.co.uk/books?id=n_epgHWbpwkC&pg=PA35
 

Offline giosif

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #4 on: December 10, 2017, 09:01:27 AM »
Interesting (and lucky for me) synchronization with these videos, Dave! :-+
I've just got a bit of a fight with a broken PM6303 LCR meter which is currently having problems with what it shows on the LCD (you can check it in the Repair section, if you so feel inclined).
Watching your videos, I was able to understand more than I would have otherwise.
However, this particular LCD is a multiplexed one, and I am struggling to understand how it would normally work and what is wrong with it now.
So, I'm looking forward to your upcoming video on multiplexed LCD's (you hinted that may come in the near future).
Maybe I could run some experiments or do some captures for your video, if that helps (please let me know).

Thanks!
 

Offline amyk

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #5 on: Yesterday at 04:36:43 AM »
I once took a junk LCD and left it connected to a 9V battery for months on end to see what effect it will have. To my surprise, the most I got was a temporary ghosting effect. Did I just get really lucky?
It depends on the LC material used. Some are more sensitive to electrolytic effects than others.
 

Offline Brumby

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #6 on: Yesterday at 10:16:24 AM »
I once took a junk LCD and left it connected to a 9V battery for months on end to see what effect it will have. To my surprise, the most I got was a temporary ghosting effect. Did I just get really lucky?

It's not a good zone to get comfortable in.  Over time, things will very likely go irrecoverably downhill.  How long that takes is generally the question.  However, it is one you shouldn't really be asking if you are considering driving a panel for any long term application - but for an experiment or a MacGyver situation, it can work.

 

Online Mr. Scram

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #7 on: Yesterday at 10:30:54 AM »
I'm obviously missing a vital bit of the explanation here. I get the part with the phases and inverting them, but I don't quite see how the second signal is biased in relation to the other. That's the interesting bit that eliminates a -5V rail.

It seems to have to do with the LCD being a discrete unit with no relation to the common ground, but I'm obviously missing something.
 

Offline Brumby

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #8 on: Yesterday at 11:52:47 AM »
There is no bias.

If driving the LCD from an MCU or logic chip giving 5V output, then measuring with respect to the chip ground:
Signal 1 goes +5V
0V
+5V
0V
+5V
0V
Signal 2 goes
0V
+5V
0V
+5V
0V
+5V

But if the LCD is not connected to this 0V reference point, then what is the reference point for voltage measurement?  Answer - whichever connection to the LCD you care to choose.  The one for signal 1 or the one for signal 2.  It doesn't matter which one you choose - but for the sake of the discussion, let's pick signal 1.  Put your black lead on that point and leave it there.

The result is that this reference point will follow the voltage excursions of the output signal and will always measure 0V with respect to it (stick your red lead on that point.  Yes, you will have both the red and the black leads on the same point - but that's typically what a 0V measurement entails!).  It doesn't matter what the voltage is with respect to anything else or how that changes over time - it will always be 0V.

Now move the red lead to the connection for the signal 2.  What voltage does it measure now?

In DC terms, it will change over time.  When signal 1 is low, signal 2 will be high - and since signal 2 is 5V higher than signal 1, the (instantaneous) voltage measurement will be +5V

Now, when signal 1 is high, signal 2 will be low - and since signal 2 is 5V lower than signal 1, the (instantaneous) voltage measurement will be -5V.

Thus, the voltage change on one pin will be either +5V or -5V with respect to the other pin.

This is, in effect, exactly the same as an AC square wave with +5V and -5V levels.


As others have said, this is working identically to how an H-bridge operates ... so if you are still lost, it will be worth finding a reference on that.
« Last Edit: Yesterday at 11:54:26 AM by Brumby »
 
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Online Mr. Scram

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #9 on: Yesterday at 12:06:23 PM »
I just should have watched the follow up video. It looks like I wasn't the only one who didn't quite get it the first time round. Your explanation is also helpful and appreciated.

 

Online NiHaoMike

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #10 on: Yesterday at 02:59:51 PM »
It depends on the LC material used. Some are more sensitive to electrolytic effects than others.
It was a common black/white segment LCD. I presume something like a color graphics LCD would be far more sensitive.
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Offline Brumby

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #11 on: Yesterday at 03:30:18 PM »
All LCDs are black/white.  They work on polarization of light - either preventing light from passing through or allowing it.  The colour comes from coloured filters.

Having said that, though, there are a couple of different types of liquid crystal.
 

Offline SeanB

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Re: EEVblog #1045 - How To Drive an LCD
« Reply #12 on: Yesterday at 03:37:13 PM »
Still depends on the actual LCD itself. Some have different formulas that are very low in any ionic salts that break down, while others do contain a lot of ionic materials and will break down with time as water vapour diffuses through the side seals, all part of the special mix the particular manufacturer uses in the panels. The breakdown depends on the external temperature, humidity and pressure changes, and this does take time. The common failure mode of the segments going dark, losing contrast or getting a ghost outline of the electrode patterns is a symptom of water ingress, and this really is part of how the panel was manufactured and sealed, the DC will aggravate this. Those who have had Fluke and other manufacturer LCD displays fail with time are aware of this, and it is just some particular batches are affected short term, while all will eventually have it over the decades.
 


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