Backlight for Sharp Memory LCD

[stoic]

Hello,

I’m using the 2.7 inch transreflective Sharp Memory LCD in a project and would like to provide a backlight to the panel.



I see there’s a display film, sort of a combined diffuser and reflector, that is used in the Pebble watch along with a few surface mount LEDs. This can be seen here:

Does anyone have any ideas on where to source this sort of film and diffuser? And what it is called?

Thanks very much,
-S

[Dongulus]

I found some information about display diffuser film stacks here [link]. In addition to diffusion film, the backlight layer should also have a light guide. Without that light guide layer, I suspect you wouldn't get much luck producing an even glow behind the screen.

I think an easy solution would be to buy a cheap display off ebay, or take one from some piece of junk you don't care about, then repurpose the backlight.

[stoic]

Thank you very much!

The material was very informative!

Based on your suggestion, I've decided to source some replacement iPhone backlight modules, which can be found on eBay quite cheaply. I plan to cut them to size and figure out a way to interface with the ribbon connector.

Thanks again!

-S

[sacherjj]

Thank you very much!

The material was very informative!

Based on your suggestion, I've decided to source some replacement iPhone backlight modules, which can be found on eBay quite cheaply. I plan to cut them to size and figure out a way to interface with the ribbon connector.

Backlight LEDs are generally in series parallel sets.  It is common to have series string of 3 LEDs and parallel those strings until you have enough LEDs to light the panel.  Ideally you would only have 1 LED, because the voltage is around 3V that most devices have available.  But the voltage is boosted to 7-10V to drive 3 in series, so that the backlight is even.  Forcing the same current through 3 LEDs makes the lighting even.  It is also much cheaper than making matched sets with characterization.

I mention this, because if you are cutting down a larger backlight, you need to cut between a set of series LEDs or fix the issues you create by not doing so.

[poorchava]

You can use El foil. It's cheap and easily available on fleabay,  aliexpress ,  dx and similar sites.

[Corporate666]

Is the sharp memory LCD transparent?  I thought it was opaque and just shifted from gray to black or from gray to metallic silver.

If so, a backlight won't work - you will need a front light.  Light injected into a light guide will reduce in intensity the further from the source it is (I think it will drop as the square of the distance?).  Anyway, it's not a linear drop-off.  To solve this, light guide manufacturers put little "dots" in increasing frequency as you move away from the light source.

In other words, light guides are manufactured to be a specific size... cutting one down won't make it work as designed.  It may be acceptable, but it won't be as good as it was designed to be.  May be fine for a hobbyist use, but for something commercial, just keep that in mind.

[Sebastian]

I am also looking for a backlight for that display. I found this: http://flexlighting.com
I don't know if they sell to end customers or if they have any distributors, but I already asked them. I will let you know what they say.

[mikeselectricstuff]

Is the sharp memory LCD transparent?  I thought it was opaque and just shifted from gray to black or from gray to metallic silver.

The OP did say transreflective, which usually means a translucent rear surface to allow backlighting.
For a 1-off, the best bet is to find an equivalent size LCD and cannibalise it for the backlight

[Corporate666]

Is the sharp memory LCD transparent?  I thought it was opaque and just shifted from gray to black or from gray to metallic silver.

The OP did say transreflective, which usually means a translucent rear surface to allow backlighting.
For a 1-off, the best bet is to find an equivalent size LCD and cannibalise it for the backlight

I didn't realize any of the memory LCD's were transflective.  I just went to their website and I saw they have quite a few transflective displays!  Guess I am way out of the loop... I have some of the same units you showed in your video (that's what prompted me to buy them to play with) and while they are mounted to a PCB, I don't think they are transparent at all (pixel off or on) - are they?

[LightYear]

Great to see someone else experimenting with this display. We have a commercial design that I want to use this display in, so I'm also searching for a suitable backlight.

I have a couple of prototypes on my desk of varying success. One was made by an unknown manufacturer who is very coy about its design. It has five white SMD LEDs along the bottom edge, drawing 62mA at 2.9V and while it not very noticeable in daylight, is very effective in the dark. There's something a bit clever about it though, because a prototype I've put together using a Chinese LCD backlight manufacturer with five LEDs (standard stack of reflector, acrylic light guide with lattice print, and diffuser) doesn't get nearly the same amount of light through.

The FLEx option looks terrific and I've started consultation with them to see if it's viable. It would be nice to have a backup though - anyone have any further insight on an effective backlight for this style of LCD? For reference, the transmissivity is specified as 0.25%, so it's not quite completely reflective, but is certainly not a completely transmissive display either.

[andyturk]

They are nice displays, but the ones I've bought from Mouser (128x128) are pretty opaque. Even with a flashlight shining behind the bare display, you can't really read anything on the front if there's no other illumination. Maybe the ones Pebble uses are different somehow?

[Sebastian]

I am also looking for a backlight for that display. I found this: http://flexlighting.com
I don't know if they sell to end customers or if they have any distributors, but I already asked them. I will let you know what they say.

Apparently they are not in production yet.

[ovnr]

I didn't realize any of the memory LCD's were transflective.  I just went to their website and I saw they have quite a few transflective displays!  Guess I am way out of the loop... I have some of the same units you showed in your video (that's what prompted me to buy them to play with) and while they are mounted to a PCB, I don't think they are transparent at all (pixel off or on) - are they?

Some of them are pretty much completely transparent when on (or off? I forget). They're pretty neat displays - shame they're kind of hard to find and also a bit expensive.

[elektronchika]

Hello,

any updates on the topic? I also consider using this display and I need a backlight too. It seems like Flexlighting product is not in the manufacturing phase yet.

I looked for solutions and I found that 3M has a new product that might help - 3M™ Envision™ Diffuser Film. It's a vinyl film, that is designed to control the light distribution. I'm still not sure how exactly to use it. The simplest solution would be to place LEDs on the PCB, place this folio on top of them and then place the display. Interesting to try.

Cheers!

[Buriedcode]

The film in question isn't what really makes a backlight - that is the light pipe that has a pattern etched into the backside, to changed the angle of internal reflection so light can 'escape' through the face of the panel.  Often these are a series of dots (moulded in) or bumps (moulded protrusions) but can also be V-grooves where the lines get progressively closer the further from the lightsource it is, to give an even brightness.

This creates a panel with lots of small light 'dots' or lines, the diffuser film then..well.. diffuses this to create an even light source across the panel.  There is also a mirror/reflective film on the back because the lightpipe will kick out light from both flat sides, so at the back it needs to be reflected back into it.  As LCD's (including TFT's of course) use a polariser, 50% of the light is wasted when a pixel is transparent (usually 'on' on TFT's, as they are negative displays).  Some companies like 3M have created a polarising diffuser - this allows one polarisation to pass through, diffused, whilst reflecting the other polarising back, recycling the light.  It's clever and can increase the brightness, or reduce the power consumption.  I believe these diffusers are standard these days.

You could search for a PSP (play station portable) backlight replacement, as this is the whole assembly.  Although I suspect this is slightly too large for that display. Rolling your own can be very tricky.  Its easy to epoxy some LED's into the edge of acrylic, but to get an 'even' brightness across the panel requires very specific machining/moulding of the lightpipe.  Should you wish to make your own, I highly recommend using a bright laptop screen for parts - it'll come with all the films you need, plus a big, thin, lightpipe to cut to size.  I've made illuminated posters this way for less than £10.

[Lomax]

You don't mention the dimensions of the panel, or what thickness you have available - or what your budget is - but I've recently added backlight to (monochrome alphanumerical) LCDs using these backlight panels from Adafruit. They can easily be cut to size with a fine saw, and you can probably replace the LEDs with other colours/better quality fairly easily. Light output is not completely uniform, but not bad for the price. Photos in this thread.

[Lomax]

Some companies like 3M have created a polarising diffuser

You can find information about the different kinds of display films from 3M/Vikuti on this website. They do sell them as well, but minimum quantity is 10 11x11" sheets, at £600 HOWEVER, after much searching and head-scratching, I realised that the rear polarisers on some smartphones/phablets are transflective (notably from Samsung and Apple), and that replacement films for these displays are available for as little as £3. I've bought a couple of these for experimentation, but didn't need to use them for my latest hack - a plain white diffusing film proved more than good enough for my needs.

[elektronchika]

Yes, but the display is transflective- when you put a light source on the back, just a tiny bit of the light passes through. It makes it almost impossible to have a good backlight on this display. Front light can solve the problem, but then a question arises - how front lights are done? Some light guide is used and an LED. So far so good, but doing this for small amount of devices is almost senseless. Any ideas guys?

[Lomax]

Yes, but the display is transflective- when you put a light source on the back, just a tiny bit of the light passes through. It makes it almost impossible to have a good backlight on this display.

I have some of these films and the transmissivity looks to be about 33-50%. I haven't tested one behind a display yet, but it looked plenty bright when held in front of an LED backlight. If you look through the film you can still see quite well, a bit like through mirror finish sunglasses. Depending on what kind of backlight you have you might need a layer of diffusion as well, which would cut down the brightness a bit more, but I don't think you'd have trouble getting enough light through (LEDs after all, can be very bright indeed). Having some plan for how to black out the inevitable light-spill is probably a good idea though.

Front light can solve the problem, but then a question arises - how front lights are done? Some light guide is used and an LED. So far so good, but doing this for small amount of devices is almost senseless. Any ideas guys?

I have never seen a good front-lit LCD display. The main problem is that you need to use some form of light guide to distribute the light evenly from the edges (since you can't put the light source directly in front). But there's a catch; in order to get the light to exit the light guide (and shine onto the display), you have to "frustrate" the internal reflection somehow, and this inevitably causes some amount of diffusion. The same is of course true for light guide materials for backlighting, but here the diffusion is not a problem.

Edit: I found this (old) PDF brochure for Vikuiti's transreflective films.

[timb]

Open up a Pebble watch and see how they do it.

It looks like there's a light guide on the top or both sides of the display, possibly with SMD LEDs under it? I assume that since these displays are reflective it causes the light to scatter evenly or something. I say that because the light appears very uniform over the display.

[Lomax]

The Pebble uses (AFIAK) an electrophoretic display, like most e-book readers (aka "e-paper" or "e-ink"). Due to the opaque nature of these screens I don't think it's possible to backlight them. There are e-book readers that have front lit displays of the same type. Never seen one in the flesh though!

Edit: Having looked at The Evolution of Front-Lit e-Reader Displays it seems like the technology has advanced quite a bit. It's still the same principle (light guide with diffusion pattern), but they've managed to make the individual diffusion sites so tiny that they're not really noticeable(?).

Edit: The first front-lit e-book reader came from Sony in 2008, and was atrociously bad:



But the Amazon Kindle Paperwhite that came in 2012 improved things dramatically:



It seems I have to re-evaluate my scepticism towards front-lighting; done correctly it appears to work surprisingly well!

[timb]

The Pebble uses (AFIAK) and electrophoretic display, like most e-book readers (aka "e-paper"). Due to the opaque nature of these screens I don't think it's possible to backlight them. There are e-book readers that have front lit displays of the same type. Never seen one in the flesh though!

Nope, it's not an electrophoretic display. I believe Pebble used (uses?) the term "eInk Display" in reference to the displays used in their watches, which may be what you're thinking of. They are in fact Sharp Memory displays, not e-Paper.

An e-Paper display would have far too slow of a refresh rate for the type of graphics you see on a Pebble. Think about the second hand of a watch smoothly gliding around the face, now think about what happens every time you turn the page on a Kindle. (e-Paper displays will refresh the display by making it completely black, then completely white, then applying the new image. They don't *have* to do this every frame, but it does need to be done every few frames, otherwise artifacts will build up and cause ghosting.)

[Lomax]

Nope, it's not an electrophoretic display. I believe Pebble used (uses?) the term "eInk Display" in reference to the displays used in their watches, which may be what you're thinking of. They are in fact Sharp Memory displays, not e-Paper.

You're right that the Pebble isn't electrophoretic, and having just watched some videos clearly much faster. But e-ink is based on elecrophoresis - it's just another name for it. If they used the term e-ink in the past, then that must be because they used to have electrophoretic displays.

Edit: I just realised this is what the thread is all about. I had forgotten. Total confusion

Edit: Ok, so the Sharp Memory Displays use a technology called Polymer Networked Liquid Crystal - this is an LCD relative. Nothing in common with electrophoresis, other than appearance (to some extent). Backlit versions do exist.

[timb]

Nope, it's not an electrophoretic display. I believe Pebble used (uses?) the term "eInk Display" in reference to the displays used in their watches, which may be what you're thinking of. They are in fact Sharp Memory displays, not e-Paper.

You're right that the Pebble isn't electrophoretic, and having just watched some videos clearly much faster. But e-ink is based on elecrophoresis - it's just another name for it. If they used the term e-ink in the past, then that must be because they used to have electrophoretic displays.

Edit: I just realised this is what the thread is all about. I had forgotten. Total confusion

Edit: Ok, so the Sharp Memory Displays use a technology called Polymer Networked Liquid Crystal - this is an LCD relative. Nothing in common with electrophoresis, other than appearance (to some extent). Backlit versions do exist.

Okay, maybe they called it an e-Paper display? Yeah, I just checked their original Kickstarter page and they advertised it as an "E-Paper Watch for iPhone and Android".  So, substitute all mention of eInk with e-Paper in my last post, and vice versa. I had it backwards.

I remember after the Kickstarter, there was some confusion over the screen technology. People assumed since they called it E-Paper it was an electrophoretic "eInk" display. They clarified it was a Sharp Memory LCD, which at the time was being called E-Paper.

Since that time, the Memory LCD technology itself has become much more well known and the E-Paper moniker seems to have been dropped, probably because it was too easily confused with eInk.

Personally, I love these little displays. I have a bunch of them I've used in various ultra low power projects. They can literally run on a couple of uA. The only drawback is that they take a lot of memory on your uC, since you can't read back the data from the displays, like you can with some LCDs; so you have to use frame buffer.

I actually came up with a solution for that awhile back: I put some SPI memory between the uC and display (MOSI, MISO and SCLK connected to the uC and memory; then the MISO/MOSI lines reversed from the memory to the display). Then I could read/write small blocks to/from the memory/uC. When the frame was ready, I'd put the memory in sequential read back mode (where the address increments with each clock) and toggle the display's CS pin. Then the contents of the SPI memory (which was the frame buffer) would dump straight into the Memory LCD. It was basically hardware DMA. Worked perfectly and super fast.

[RobertoLG]

Hello,

I’m using the 2.7 inch transreflective Sharp Memory LCD in a project and would like to provide a backlight to the panel.



I see there’s a display film, sort of a combined diffuser and reflector, that is used in the Pebble watch along with a few surface mount LEDs. This can be seen here:

Does anyone have any ideas on where to source this sort of film and diffuser? And what it is called?

Thanks very much,
-S

I think that's this you're looking for   http://main.mvs.by/node/221  diffuser, reflective, prismatic, lightblocking films and plastics