EEVblog #1044 - LCD Technology Tutorial

hi this is the first video in a series of videos on LCD displays and more specifically how can you can go about get in or what steps are involved in designing and manufacturing your own custom LCD display hope you enjoy it let's take a look at it now LCDs come in all shapes and sizes as you can see here this is just some of them you've got your traditional low-cost a reflective static LCD display which will have a play around with in a minute so stick around and you've got your traditional sixteen to line by 16 character LCD display module and you've got character based ones and graphical based ones and you've got ones that are on like you know mounted on PCBs as part of modules you've got ones that are just a forever flat flex coming out of them you've got ones that have got drivers on them that you've got ones at a low pin count are serial interface ones you've got seven segments 14 segment displays like this one we'll have a look at you've got graphical display ones and all sorts of things let's take a closer look now of course you can buy fully custom modules like these which have all the drive-in circuitry and everything else on and you just talk through a pin based interface be at a flat flex like this or just your regular through-hole type stuff and these are fairly inexpensive these days but I can actually do better cost than these which is what this video series is ultimately about is designing and manufacturing your own low-cost custom LCD display first of all will briefly cover how an LCD works and these images most of the images I'm going to show you in here are taken from the very excellent microchip application note AM six five eight so I'll link that in down below to check it out to get a bit more detail and you can actually a deep dive down the rabbit hole on how LCDs actually work at the you know the chemistry and physical level but we'll just keep it very simple here now let's take a look at the basic LCD components there's a front polarizer on top then there's the backplane elect Road which is basically the for all intents and purposes like the negative common type ternal and then you've got the LCD fluid itself it's actually just a like a pocket of liquid crystals and we'll take a look at electron microscope photo of that in a second and then you've got the actual segments itched onto a glass layer and they go off via little conductive paths to the actual pins and they're the shape of those how you design those determines what shapes segments you get on your LCD and then you've got a repolarize ER at the back now it's these polarizers can be combined in various ways to get you a positive LCD which is what we're going to look at now or a negative one which is basically white segments with a back black round or positive will be black segments that you're more familiar with on the white background now light polarization is key to how LCDs work and I'll give you a crude demonstration with my watch and my polarized sunglasses here if I put my sunglasses like that you can see the LCD no problems at all but if I twist it like that you'll see it eventually vanish there you go and that's basically how the key concept are behind how L cities work now the LCD we're going to talk about here is what's called a twisted nematic or TN type display there are different technologies of how the actual liquid crystals themselves work you don't have to concern yourself too much but suffice to say that there are different states of these liquid crystals so most LCDs you're probably going to come across of the twisted nematic type these days and here's a really cool electron microscope photo of the liquid crystals and how they can line up under an electric field and this is the key to how LCDs work you got the top and bottom electrodes there with the liquid crystal fluid in there which is affected by an electric field not a knot essentially not a current it's just an electric field between the positive and negative place and that can change the orientation of the liquid crystals themselves and let's have a look here the LCD on the left there the LCD orientation with no electric field and you've got the twisted pneumatic liquid crystals in there in this particular case when there's no electric field applied the light will pass through reflect off the reflector at the bottom and come straight back out so you effectively the light is not blocked at all so you essentially see that white or you know silver type background segments not on but on the right-hand side here the when you apply an electric field all of the liquid crystals line up like this it's a bit counterintuitive they line up so you would think that the light would pass back through but it's actually the opposite case because of the polarizers in this particular case when they all line up like that the twisted nematic crystals in there aren't will not reorient the light so the light is actually passes straight through and then is blocked by the polarizers and the segment appears to be on or black and of course you can as I said you can change the polarizer surround the configuration to get either white characters on a black background or black segments on a white background and also temperature can play a role in the LCDs as well the poor little liquid crystals actually get pretty lethargic if you drop them if you drop the temperature and you can see this one here I've actually had in the freezer for a little bit and it's you can see it a little bit lethargic compared to the other one just takes a little bit of time for those segments to decay and that can take like you know hundreds of milliseconds if it gets cold enough once it gets towards zero degrees or below it can be very significantly affected so LCDs are not current driven they're actually electric field driven which essentially makes them zero power apart from the capacitive nature of them and you have to switch at a certain frequency to make a visible and then you have to switch that capacitance so you do get some Kappa current through that capacitance your basic reactant capacitive reactance formula there but I can actually demonstrate this by actually having any power at all I can actually turn on these segments it's a bit he called he piggledy but you can see that I'm just just the electric field picked up by my body and then superimposed across in this case I've got a ground hooked up to the common pin it's going back to mains earth and you'll see that where I already get the segment's to come on and stay on and you can see that they do actually decay away like that so there can be a bit of charge buildup in there on on the actual display and then it can take time for them to fade out like that cool huh now you can actually drive LCDs with a DC voltage but don't because that is the incorrect way to do it you'll ruin your LCD it will destroy the liquid crystals in there and it magic smokeless Cape and it won't work anymore the only correct way to drive LCDs is with an AC voltage and I'm actually driving it at the moment as you can see with a basically a DC voltage just six volts peak-to-peak with the three volt offset so that's basically a six volt TTL type you know digital signal at a nought point one Hertz so it actually flashes off and on so you can technically do that but don't what you actually want to do is have a proper AC signal like that so there's no offset so I've just got a six volt peak-to-peak AC signal so you can see in the middle there it's going positive negative positive negative so the DC the average DC value is zero and that's what you want when you drive in an LCD you want is an average DC value of zero otherwise you'll eventually kill it now I'm actually driving this segment at 6 volts peak-to-peak AC with a hundred Hertz this datasheet actually says to do 5 volts but doesn't specify RMS or p2p now we can actually adjust the amplitude here I'm going down if I do it a 5 volts you can see that it starts to affect the contrast and that's four and a half it's basically almost vanished and at 4 volts it's Gonski 4 volts peak-to-peak for this particular one is not enough a common drive voltage might be you know at least three volts or something like this this one needs at least six volts peak-to-peak to be dark and of course you can go up and that might increase the contrast but don't go too high otherwise you can get ghosts in between segments now I'm actually driving this at a hundred Hertz at the moment but you can't just say switch down to one Hertz because if I do that you'll see it's still basically on trust me I've got that at 1 Hertz now if I drive this at no point 1 Hertz you can see it's sort of fading out like that that's not the correct way to do it the proper way to actually drive it is to apply your frequency or not so that that's a hundred Hertz and you can drive it you know you don't have to drive it any lower than that that's just you know so it doesn't flicker or anything like that so we're driving that and 100 Hertz and on off on off on off that's it but you definitely don't want any DC bias if you want to go off you actually just try state your pin you drive a pin like that now this is your most basic type of LCD you've basically just got your or LCD glass and the pins like that there's no additional circuitry now these actually come in two types one is with the through-hole pins like that was just clamp top and bottom basically like that or you can just specify the exact same LCD but without the pins it's exactly the same hourglass but if you order them without the pins then you need to use them with what's called an elastomeric connector otherwise known as a zebra strip which is basically if we can get that off there it's a conductive rubber strip like that which just connects through to the glass you might be able to see the actual contacts on the bottom of the glass down there so you can either tell the manufacturer to just give you the raw glass like that and no worries are using a less turmeric or Zipporah strip or you can ask them to provide a strip to your specification or you can just order it with the through-hole pins attached now the next step up from your just your basic are glossy the pin or a less turmeric connectors every strip connector is a flat flex connector once again it's exactly the same glass but you can get them to I use a conductive VAR glue that basically sticks the flat flex cable onto there and it goes straight out two pins like this you're not actually reducing the number of our pins in any way you just merely they're they're just three different types of interfaces that you can get to a regular LCD glass like that and the thing with any of those three interconnect solutions you've basically just got your LCD glass and you need a separate driver so you need all the driver circuitry on the bottom there either a microcontroller which has the built-in LCD a controller which we'll go into in a future video or whether or not you use a dedicated LCD a driver chip or you can bodge your own driver circuitry we might go into that but you need some sort of AC drive circuitry for the display because these things don't drive themselves you can't just hook these up to a microcontroller and expect them to work they're just a raw glass they need that AC signal and they're going to be a summer often multiplex start displays so very complicated to actually drive these things you need a dedicated driver chip basically so basically our the next step up from your glass is of course these dedicated modules which have all the driver circuitry and everything built in but that's quite expensive you've got a PCB you've got you know components and all sorts of stuff on there you know and that's a rig but you can't get these reasonably cheap but if you're talking really high volume you know it's not that terrific and the height form factor with the PCB and everything else can impact your design you can't do like really ultra tiny designs and things like that so your next step up from that is to get you drive a chip build onto here can you sit you sit that's it down in there now this is actually called a chip on glass or C og solution and that's actually the driver chip built into there so you can see this is actually hasn't got many pins at all this is actually a hundred and 32 by 32 dot matrix graphic LCD displays so the LCD display is all multiplex and everything it's got a lot of lines on there a lot of contacts and they go into the embedded chip which they mount on the glass on there hence why it's called chip on glass because the glass actually extends right out to the edge here and then they've only got like a simple serial interface so it takes all the headache out of driving this LCD this one's you know nice and thick because it has like a huge backlight and things in there but if you actually take that module out without the backlight it's pretty thin the chip is just on the glass and another variation of that one here we go this one's also got a backlight but the backlight just pops off like that there you go and that like just the same really as our glass that we had before but it's got the driver chip built onto it the chip on glass this one's not a serial interface this is like a parallel in a face one just like like it actually might simulate the interface on your standard sixteen by two LCD I think this one is a sixteen by two LCD dot matrix actually and it's got your standard interface but all the drivers surgery it's built on there look how thin that is awesome or if you're not a fan of chip on glass co G you can actually this one before this wasn't just the flat flex coming out if you look at there's a little bulge in there that's actually what's called chip on flex so this one actually has the driver chip once again a standard you know Hitoshi LCD driver interface but the driver chip is built on to the flat flex like that and that you may prefer that for some system interconnect reason or something like that rather than a pin base one like this but just be aware the the result is exactly the same whether or not the driver chip is mounted on a flex like this or as mounted directly on the glass when you're getting a custom LCD made like this you can just specify I want see og please I want chip on glass and they will just extend the glass out for you they'll put the chip on there like a standard Hitoshi chipset or whatever it is whichever type of interface you specify and they will give you that for like I don't like 50 Cent's extra $1 extra per display or something like that in you know reasonable like thousand you know a thousand odd volume or something like that so it's not necessarily a hugely expensive solution but it can actually be ultimately cheaper as we're going in a future video - just get the raw glass and use an external driver chip that way this sensor these aren't too expensive but if you know you're counting every single cent then this may be more solution than this plus a driver chip now to confuse the issue even more LCDs all different types come in three different varieties in addition to those are three different solutions we saw for the interconnect this one has to do with the optical properties of the LCD or glass and the modular itself the first one is reflective the next one is trans afflictive and the third one is transmissive oh goodness now this particular one here for those playing along at home there's the did you keep part number this is a reflective LCD display and what that means is that it is not compatible with a backlight you might be able to edge light the thing or something but it's basically not compatible with a backlight it's got a mirror reflector embedded on the back in there and you basically have to shine the light from externally through the glass and then it reflects back out now these reflective types in my opinion have by far the best contrast and view angle and stuff like that although that varies with the process technology and other things so but basically if you want the ultimate contrast in LCD you'll want a reflective type but the disadvantage is they're no good in low light because you've got no backlight once you you know in like dark conditions you're screwed but the reflective type basically once you've got good external light it's the holy grail of display quality so your next type is what's called transflective and that's what this one here is and it's hard to sort of you know tell the difference one has like a white background and one has this kind of you know a silvery type or it could be you know some other type of backing on it and transflective tries to get the best of both worlds tries to get the optical properties of the reflective one while still allowing light to shine through from the back here and actually give you low lighting conditions and you can see this one has the backlight you can see the two PCB mount pins here this is just the LED on here that allows you to have your backlight like that and it shines through so you can use it with or without the backlight but the disadvantage is transflective isn't quite as good our optical contrast quality as you'll get on a pure reflective like this but it's a probably the best overall solution if you want to use your display in all sorts of lighting conditions you simply choose to turn the backlight offer on just like you know you're used to your multimeter but most of them good multimeters will have a backlight on it they're using transflective displays and the third type is what's called transmissive and i didn't actually have an example here in the lab of that but it basically has been graphic about it you basically must have the backlight on it are there to make it work at all there is no reflective or semi reflective back in on it it's purely relying on the light source coming from the back instead of coming from the front and that's great if you're always want a backlight and you want it to work in all conditions it can but then the backlight is always drawing power you don't have the option to turn that backlight off it'll basically come unreadable if you remove that light source from the rear now transmissive our displays are very common like your TV for example we use a transmissive display it's always on it requires that backlight but if you want to use him outside then you've got to have a really bright backlight on it continuously to overcome the external ambient sunlight and that can be really difficult for outdoor type displays or more to the point very power hungry but in small displays like this you won't often find like a small you know that you're using a product like this transmissive isn't all that common you usually get the yaar transflective type or the fully reflective type so your standard reflective display like this one is your cheapest and your simplest because it doesn't need a backlight it's going to be the thinnest whereas your transmissive one actually you know you're going to have to need the backlight on the back of it because you wouldn't want to use a transmissive one without the backlight if you know you're not going to use the backlight you're not even going to integrate it into your product it'll still work but man not as good as a reflective one so you've got to decide upfront whether your product is going to have backlighting or not but there we go I can get I can generate numbers beautiful anyway we'll go into drivin them in a future episode but there are basically two different types of LCD this one that we've got here is a static type if you take a look at the datasheet it's what's called a static or a non multiplex start LCD display so what that means is that it's got one common pin down here and then a pin for each particular segment on the display here and basically are there's no need to multiplex anything you're still AC driving the pin but you don't actually need to multiplex different segments and this is common for like a display like this which doesn't have many segments on it in this case it's just a seven segment display like that and you know we have X number of pins this one actually has two pins per segment so you don't need that many pins and you know maybe a like a three and a half digit multimeter you know one of those really old cheapo once they might be static as well because you can get away with like 40 pins or whatever and you can drive those statically there's no need to multiplex but once you get into a more complex LCD which is what we're going to actually design in this series of videos and actually get manufactured you'll see that in the next video is that you need to multiplex the pins you need to have more than one common pin on here and then you need to multiplex the segments otherwise you'd end up with hundreds and hundreds of pins and you don't want that because that's logistically very difficult to drive you need a multi-hundred pn LCD driver chip and everything like that and of course it goes without saying that your are graphical ones like these or you you know even you text-based graphic or dot matrix once you this is an eight character by two line and this one here is a a 16 character by two line LCD that you're familiar with they're actually dot matrix and because there's just so many dots on there these ones are by default they have to be multiplex there's just no other way to do that with the sheer number of dots but if you've got one like this one with is an 8-digit 14 segment look at this puppy oh it's one of these our starburst displays I love these classic old still school starburst right this has if you count up the number of segments on there it's 8 by 14 segments clearly we don't have that number of pins so what this one is got is it's actually got four Commons what's called four common pins instead of this one here which just had the one common pin and then you have to actually multiplex if you want to turn like in theory your product could turn all these segments on at once and if you want to do that of course then you have no choice but to multiplex the display using those four different commons just to like reduce the number of our driver pins that you got on there oh sorry I love these I just love him starburst brilliant old school stuff I'm if you don't know about starburst it's just a way to allow you to display alphanumerical characters in a standard like seven segment type format but it uses r14 and you can also get 16 segment ones as well but they're very cool and you'll find that most LCD driver chips build into micro controls or typically have a maximum of four comments but we'll go into this in the design video in the next one so there you go that's just an example of a multi common multiplex display but the problem with your multiple common displays like this is that they require the more comments you have they are more bias voltages that you actually require to bias the various Commons and and that's probably a video in its own right but suffice it to say they're a bit more complicated to drive than just your general aesthetic and display or non multiplex display like this one which has basically one common so you really do need a dedicated driver chip that handles the multiple our bias voltage levels for these multi common displays so there you go that's a look at basically the three different interconnect type solutions for custom LCDs and also the three different tie up tickle properties of LCDs like this and you can get a or in any weird and wonderful combination that you like so in the next video we're actually going to look at actually designing your own a custom LCD display and ultimately in future videos actually getting that manufactured and then driving that for our custom product so stay tuned for that one so I hope you enjoyed that and if you did please give it a big thumbs up because that always helps a lot and as always discuss down below in the comments or on the EEV blog forum and don't forget to subscribe and you know do that bill icon so you get email notified and all that stuff YouTube has to you know youtubers say oh and yes thank you to my patrons over on patreon Tom I've had a patreon page for ages where people support me directly because of the ad pocalypse these days I'm not losing a huge amount of money on videos but I'm getting more and more videos deem monetized so a lot of youtubers like myself are moving over to direct sponsorship from the viewers on that places like patreon really appreciate it catch you next time [Music]

Channel: EEVblog

Views: 120,241

Rating: 4.9602895 out of 5

Keywords: eevblog, video, lcd display, lcd, liquid crystal display, liquid crystal, how it works, tutorial, lcd tutorial, transflective, reflective, transmissive, light polarisation, lcd polarisation, polarisation filter, flat flex, tab, tab construction, chip on glass, cog, cog lcd

Id: mo4_5vG8bbU

Length: 26min 46sec (1606 seconds)

Published: Fri Dec 01 2017