OT: LCD - Just the glass plate

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Ok, I give up. Google isn't being much help.

Anybody know how to drive a 7 segment LCD panel? No electronics, just the glass plate. (Yea, I know there's 2 plates and some goop and electrodes....)

I saw a page about driving the backplane with a 3state output and 2 resistors to get 3 levels and using a plain output to get 2 levels for the foreplane, but his timing diagrams weren't a lot of help knowing just how to disturb those little molecules.

BTW: Didn't it take a high voltage, like 21 volts to run these? It's been too long since I fiddled with one. AllElectronics has a cute panel for 25 cents if you buy in quantity 4. It seems to be 4 1/2 7 segment digits, 3 lines of writing on the lower left and some alpha characters in the lower right. There's pins on the terminals for connecting.

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IF you inspect the LCD glassworks you may notice a slight sheen in certain areas.. this is the residual electrode made of tin oxide a conductive surface.the images wee see in LCD are the outlines of these conductive surface electrodes. On the other glass plate is the other electrode. When apotential is applied to the two opposing electrodes voila the crystals do the magick.

Ok so with a 41/2 dispaly you will probably find all visible segments on one glass plate and the other plate contains a flat non segmented electrode minus the image area corresponding to tracks leading into the segments on the other plate so that only the wanted segments will become activated.

So a very simple structure requires a simple drive signal.

consider a square wave ( of a few volts say 5 v peak to peak) applied to the back plane. The same square wave appled through individual two input EXOR gates to the individual electrodes on the other glass plate.

By holding the spare input to the EXOR gate LOW the the output of the EXOR will be in phase with the backplane signal no potential difference will exist between the electrode and backplane and no visible symbol will "lightup". By holding the spare input to the EXOR hight the output of the EXOR will be inverted with respect to the backplane and the electrodes will now be at a potential difference and a symboll will "lightup"

The reason why we are driving this with a square wave is to provide the crystals with a suitable NON DC drive signal. If driven with a DC signal the display will work..but for for a whileand then degrade.

This can be implemented witha micro readily,by connecting a port pin to a segment pin ( huge number of pins but ...easy to do)

Alternatively some external hardware decoding scheme ( to achieve 41/2 segment drive capability).

Intersill used to make driver chips for LCD displays to cover this requirement

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This is where a mega169 comes into play. The m169 has the integrated drive electronics to use the display.

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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To excite one of the segments, you just drive one "side" to +5 and the other to ground with a square wave.. What kind of frequency?

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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The LCD is an electromechanical device in the sense that the mechanical moement due to the electric field is converted to a visible phenomenon.

As such ( mechanical) its frequency response is on the low side. Namely the molecules will twist but due to viscosity, stiffness mass etc may callfor elevated higher frequency voltages.

So lets look at the lower end of frequencies.. anything below about 20 Hz will have a visible component of flickeer.

OIn the past tv half frame rates were employd to createa quality display.

So 50 -> 60 Hz should be the target frequency. In anbsence of a mains refference a irocontroller timer /divider can be usedtogenerate the LCD drive signal.

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Cool. I'll give it a try this afternoon. Oops, have council meeting this afternoon. Ok, try this evening, or maybe Monday. :shock:

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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Quote:
Oops, have council meeting this afternoon.

Well good luck and I'm sure I speak for all of us when I say we hope you don't get voted off the island! :shock:

Please note - this post may not present all information available on a subject.

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Actually, getting voted off the island is a possibility :roll:

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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LCD glass is a little more complicated than the above posts seem to imply.

The comments about the electrodes and the construction are OK, but the electrical
description needs more detail.

The liquid crystal that is between the 2 glass plates changes it physical structure
with applied voltage that is above a certain threshold. IF you apply 5V to one side
and 0V to the other side the liquid crystal will change states and block or pass
light. You can reverse the voltages and it will still block or pass light.

What you can't do is to keep the voltage applied to the liquid crystal very long or
you permanently damage the crystal structure and your device will be destroyed.

So how do these things work? The trick is to NEVER apply a DC voltage to the
device. So when one terminal goes to ground the other terminal must go positive.
By alternating this reversal at a high rate the human eye cannot see the flicker.

OK, so that's how you turn it on, so how do you turn it off?

Remember those tri-level drivers talked about before? They allow the outputs
to toggle high and low, but not all the way to V+ or ground. The outputs in this
mode switch between levels that are below the threshold of the liquid crystal
to change state. They too are allways changing state to not apply a DC to the
device.

The LCD's that I have worked with recently had turn on swing levels about 1.3V,
and turn off levels about 0.6v. Too high of an applied voltage can harm the LCD
also, so read the datasheet.

I hope this helps.

Will

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You can pick up some LCD fundamentals from the ATmega169 data sheet.

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Thanks, Wil.

If I had a datasheet for these LCD's, I wouldn't have needed to ask :oops: but then, I wouldn't have gotten 4 for $1. :wink:

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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Will,

Actually it is as simple as the post states.

NOTE the backplane is driven with a square wave as are the front electrodes.
However the function of the EXOR gates is to act as a selectable inverter.
So when selected the backplane and the electrode will be driven antiphase and segment will light up. When deselectedthe segment and the backplane will be driven in phase hence no driving potential.

As far as the 5V level is concerned 3.5Vrms is a "nice" driving voltage.
As simple as that.

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For a single backplane system your solution would work.
I'm more used to dealing with multibackplane devices. Over the last 20yrs
I've written test programs for dozens of LCD driver chips and so far
I haven't come across a single one with only 1 backplane. That's not
to say they don't exist, I just haven't had any expirience with them. :-)

Will