Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!oliveb!felix!fritz!dennisg From: dennisg@fritz.UUCP Newsgroups: sci.electronics Subject: Re: Liquid Crystals Message-ID: <4192@fritz.felix.UUCP> Date: Tue, 14-Apr-87 21:25:57 EST Article-I.D.: fritz.4192 Posted: Tue Apr 14 21:25:57 1987 Date-Received: Fri, 17-Apr-87 00:18:56 EST References: <337@sdics.ucsd.EDU> Reply-To: dennisg@fritz.UUCP (Dennis Griesser) Organization: FileNet Corp., Costa Mesa, CA Lines: 122 In article <337@sdics.ucsd.EDU> wargo@sdics.ucsd.EDU (Dave Wargo) asks some questions agout liquid crystals. (sound of guy rummaging in dusty corners of his mind...) There are two flavors of liquid crystals. One type responds to changes in temperature, the other is affected by an electric field. Actually, any liquid crystal responds to both, but the material can be "tuned" for the desired application and performance. This is why the LCD display on the clock in my car gets really strange in hot weather. Temperature-sensitive LCDs change color, usually in a rather narrow range that is set by the particular formulation in use. They are widely available as a single spot sealed in stickers to be placed on components that heat up. If the color changes, the part is getting too hot. Thermometers are commonly made with several spots of LCD material, each tuned to a different temp range. I have such a thing on my fish tank, and they are available as fever thermometers, to be placed on the forehead. This material is sometimes available bound to sheets that can be cut to size with scissors. Look in the Edmund catalog. I have seen it as paint, too. Electrosensitive LCDs are normally scattered every which way. Under an electric field, the crystals line up in the same orientation. This does you no good at all unless you have an external light source so you can see what the crystals are up to. Commercially available LCD displays are either transmissive or reflective. In transmissive displays, the deactivated (disorganized) crystals appear cloudy as light passes through them. Adding an electric field lines up the crystals and allows light to pass with less scattering. You can make this work the other way by adding a polarized screen on each side of the display (polarized 90-degrees apart). Without the display between the screens, no light passes, as the polarizing screens block each other out. When you insert an activated display, the crystals line up, and do nothing to the polarized light, so it stays black. A deactivated display scatters the light, letting it pass back through the polarizing screen on the other side. A reflective display works the same way, but has a mirror on the back, and relies on light from the front of the display to make a round trip through the crystals. This means that you need brighter light to use it, but you can get better contrast because the light is getting two trips through the crystals for the price of one. It is becoming common to find transmissive displays illuminated in back by a flat (electroluminescent) panel. Makes for a nice display on a laptop or portable computer. >Is the technology to a point that I can buy a sheet of the stuff or is >it all custom work for watches, displays, etc... The electrosensitive stuff isn't a sheet. It is a liquid, imprisoned between two sheets of glass or plastic. The front inside surfaces of the display are coated with a transparent conductor that allows the application of the control field. LCD displays have traditionally been difficult to multiplex, so you usually end up with a solid (transparent) electrode on the back, and a complex tracery of electrodes on the front, making up the segments of the display. Each segment receives direct drive from some logic output. Much of the LCD displays running around are custom. There are generic LCDs around that you could design into an application. A quick look in the Newark Electronics catalog reveals products by Hamlin and IEE. Making your own LCD displays is possible. It can be instructive, but is not cost effective, and seldom pretty. There was an article in Radio Electronics (?) about 15 years ago with details. The basic procedure involves buying glass plates pre-coated with transparent conductor, removing what you don't want (with a heavy duty eraser), and pressing the plates together with a teflon gasket and the LCD liquid in between. They gave sources for all of the strange parts... >Can liquid crystals be made to work on a thin film. I need a better question before I can answer it. >Are they binary or can they be make to be opaque? I'm not sure what you mean by this question, but it looks like you want to know whether a LCD can have settings other than "on" and "off". Yes. The more field that you apply, the better (and quicker) the crystals will line up. There are points of diminishing return. But if you apply a weak field, you will get weak alignment. There's a better way to do it than this, however. You can feed the LCD with pulses. By turning it on and off fast enough, you can get pretty much any density you want. If the pulses go to the backplane electrode, the entire display can be adjusted for contrast. Up to now, I have led you to believe that LCD displays feed on DC. In fact, if you reverse the polarity of the field, the crystals do a quick about-face and re-organize pointing the other way. So they should be non-polarized... The nasty fact of the matter is that LCD displays will run on DC for only a short time before the display is burned in. You have to keep twitching the crystals around to keep them from falling into a permanently aligned state. The easiest way to do this is by applying a square wave signal to the backplane, and driving each segment from the same source through an XOR gate. +------+ +-------------------------- common | | | to backplane | osc. |--------+ ____ | | | \ \ \ +------+ +-----------| | \ direct drive ' | | XOR |------ to segment 'a' segment 'a' ----------------| | / control line . / /____/ | | ____ | \ \ \ +-----------| | \ direct drive | | XOR |------ to segment 'b' segment 'b' ----------------| | / control line / /____/ With a control line at logical 0, the segment drive follows the backplane drive, and the segment in question stays inactive. Changing the control line to logical 1 puts the segment out of phase with the backplane and it does its thing. If you really want the LCD display to last, you will take special to avoid all sources of DC bias, so the gates will be CMOS or some other family that drives output close to the power supply margins.