What materials are used in LED 7-segment displays?

[Corporate666]

I need to fabricate a custom display.  The idea is to use a PCB with SMT 0603 discrete LED's (3 in series) for each 'segment', then put some sort of opaque material on the front with the segment cut-outs.  Then fill the cut-outs with a light diffusing material to get them working as desired.

I don't have a lot of time to experiment to get things just right, so I was hoping someone had some experience in this area.  I was thinking to get the cutout laser cut from opaque acrylic.  I am really not sure what to use for the diffusing material.  Perhaps some sort of epoxy filled with glass beads or something like that?

[Alex Eisenhut]

http://www.ebay.com/itm/SKY-5-5-0-7-Segment-Red-LED-Display-Common-Cathode-LD05-/181456215070?pt=LH_DefaultDomain_0&hash=item2a3fa21c1e

[chickenHeadKnob]

You can buy cast-able acrylic (PMMA) but you may be over thinking this. I have seen people just mount a dark (blue,green or red) transparent bezel in front of the PC board which has  a custom pattern of leds  as close to the surface of the leds as mechanically possible. The cosmetics are reasonably good.

[pickle9000]

Using a routed pcb slot with back mounted leds works well. No diffuser required, tinted / lightly smoked window contacting the pcb to deal with bleeding.

[chickenHeadKnob]

Thought of another possible way, similar to pickle's idea. Get, or make yourself, a light aperture stencil using the same techniques* you would employ to make a solder stencil. Glue that opaque light aperture layer to the back of your bezel.

*by same techniques I mean laser-cut, cri-cri , or chemical etch methods, don't even have to make it yourself and bob will be your uncle.

[calexanian]

For the diffuser you can go cheap and just sand any clear plastic to matte it and it works well enough.  polycarbonate or abs would work with a little 220 of finer sand paper does the trick right.  It also works well on clear LED's when you want frosted.

[salbayeng]

GE make a range of embossed lexan films, I have used these in LED annunciators.
It has part numbers like 8B35 for the coarse and 8B35F for the fine , any of your local screen printers , graphics art suppliers or plastics suppliers will usually have this in stock. Here's a random data sheet
http://www.usnameplate.com/files/documents/resources/specs/GE-Lexan-8B35F.pdf
Combine this with some dark red acrylic sheet, and it will look like a bought one.

[Corporate666]

GE make a range of embossed lexan films, I have used these in LED annunciators.
It has part numbers like 8B35 for the coarse and 8B35F for the fine , any of your local screen printers , graphics art suppliers or plastics suppliers will usually have this in stock. Here's a random data sheet
http://www.usnameplate.com/files/documents/resources/specs/GE-Lexan-8B35F.pdf
Combine this with some dark red acrylic sheet, and it will look like a bought one.

Ooooh, that might work perfectly!  Thanks!  How did you use the material - like, what sort of area were you trying to illuminate and what sort of distance did you need from the LED die to get reasonable diffusion?

[Corporate666]

In regards to my original question, I may not have been clear.

I already have an array of LED's "drawing" a shape, but it's not sufficient.  Imagine you had a character - let's say the letter 'A' in a specific font - that you wanted to light up.  You could use a bunch of LED's to "draw" it the way you want, but going a step further and actually cutting that letter in that font out from a piece of opaque plastic, and then filling in the cut-out area with some sort of light diffusing material (think - something like hot glue) would make the character appear might higher quality in the actual font and format you want, not just an approximation.

This is for a commercial design and the quality of the illuminated characters is the main focus of the project so it has to look good.  I've dissected a few 7-segment displays and they use this same technique, a 'riser' with cut-outs and a diffusion material (some sort of epoxy) filling the cut-outs.

I'll give the film mentioned above a shot - but if there's any more ideas on materials that would offer good diffusion, I am all ears.   I found these guys:

http://www.micro-beads.com/Products.aspx

they are size-controlled acrylic spheres that are specifically designed for light diffusion and matting.  I had thought of trying those, mixed in with a clear epoxy which would then be put into shapes cut from an opaque piece of plastic using a laser cutter.  But bead-blasting media is also little acrylic spheres and available locally for a few bucks per pound.  You can also buy retroreflective microspheres (the stuff that is added to paint for street signs to make them so reflective).  I thought maybe adding those to clear epoxy would do the trick also.  Or maybe some other kind of pigment/diffusion media to clear epoxy.  Just figured rather than embarking on weeks of expensive testing, someone might have some experience and know what works.

The GE film is a good start, I'll give it a shot.  Any other ideas welcome.

[Kjelt]

I would really suggest visiting a plastick specialist shop. You should search in your vicinity for such a place, here I just take my led project and ask them what I should use, they have hundreds of different materials and sheets in different thickness in stock and show me on the spot how it looks.
They even can cut it for me in exact shapes.
You can experiment a lot but since you do not have the time there is only one solution: go to a business who already knows the optical and mechanical properties of all the plasticks and have it in stock for you.

[salbayeng]

I think it was about 10mm standoff from LED to the film,  here's a picture of the finished unit , it has 1 sheet of 8B35F on front  (frosty side out) that has been screen printed in black on the inside. , behind this is another plain sheet of 8B35F, then a 2mm sheet of acrylic, then the ice-cube tray assembly.


Hmmm not sure how to include image, possibly attached below?

[Corporate666]

I would really suggest visiting a plastick specialist shop. You should search in your vicinity for such a place, here I just take my led project and ask them what I should use, they have hundreds of different materials and sheets in different thickness in stock and show me on the spot how it looks.
They even can cut it for me in exact shapes.
You can experiment a lot but since you do not have the time there is only one solution: go to a business who already knows the optical and mechanical properties of all the plasticks and have it in stock for you.

I buy quite a lot of plastic from a few places who also do lighted signs and the like.  The problem is that in this case, the size and aesthetic of the design is the driving force... the diffuser must be made to work with the existing size/shape.  I have a PCB that will have a somewhat intricate piece of plastic cut out (intricate because it will have a lot of cut-outs for LED's and must accomodate the full size of the 0603 LED's).  Then I need to either fill those cut-outs with a diffusing material or cover them with a diffusing material.  The plastics folk I've talked to are well versed in using their off-the-shelf materials and then having someone work the electronics into their design, but not the other way around.

But simply, what I'm doing is pretty much identical to the folks who manufacture 7-segment displays, so if I could find the specific materials they use for diffusion, I'm sure it would work well.

[Corporate666]

I think it was about 10mm standoff from LED to the film,  here's a picture of the finished unit , it has 1 sheet of 8B35F on front  (frosty side out) that has been screen printed in black on the inside. , behind this is another plain sheet of 8B35F, then a 2mm sheet of acrylic, then the ice-cube tray assembly.


Hmmm not sure how to include image, possibly attached below?

That looks pretty damn good!  I'll give it a shot:)

[janekm]

In regards to my original question, I may not have been clear.

I already have an array of LED's "drawing" a shape, but it's not sufficient.  Imagine you had a character - let's say the letter 'A' in a specific font - that you wanted to light up.  You could use a bunch of LED's to "draw" it the way you want, but going a step further and actually cutting that letter in that font out from a piece of opaque plastic, and then filling in the cut-out area with some sort of light diffusing material (think - something like hot glue) would make the character appear might higher quality in the actual font and format you want, not just an approximation.

This is for a commercial design and the quality of the illuminated characters is the main focus of the project so it has to look good.  I've dissected a few 7-segment displays and they use this same technique, a 'riser' with cut-outs and a diffusion material (some sort of epoxy) filling the cut-outs.

I'll give the film mentioned above a shot - but if there's any more ideas on materials that would offer good diffusion, I am all ears.   I found these guys:

http://www.micro-beads.com/Products.aspx

they are size-controlled acrylic spheres that are specifically designed for light diffusion and matting.  I had thought of trying those, mixed in with a clear epoxy which would then be put into shapes cut from an opaque piece of plastic using a laser cutter.  But bead-blasting media is also little acrylic spheres and available locally for a few bucks per pound.  You can also buy retroreflective microspheres (the stuff that is added to paint for street signs to make them so reflective).  I thought maybe adding those to clear epoxy would do the trick also.  Or maybe some other kind of pigment/diffusion media to clear epoxy.  Just figured rather than embarking on weeks of expensive testing, someone might have some experience and know what works.

The GE film is a good start, I'll give it a shot.  Any other ideas welcome.

Those micro-beads sound perfect for the job. Diffusion sheets are not as good for this application because you need to use multiple diffusion to eliminate the hotspot from the LED die (sure you could stack a bunch of sheets with spacers but that sounds tedious...)

[chickenHeadKnob]

The one time I tried clear epoxy in this kind of thing I wanted the opposite effect; optical clarity, and found I couldn't de-gas all the bubbles out. Eventually gave up, but I didn't try too hard. Those bubbles also made for some diffusion, but not in any way I could get uniformity.

[Alex Eisenhut]

I see now. How about an edge-lit display?

http://www.instructables.com/id/Edge-Lit-Displays/

Also found this

http://en.wikipedia.org/wiki/Uniformity_tape

Never heard of this before, but might be fun to look into?

[Things]

If the OP is still following this thread .. I've done a similar thing for a giant clock I built. Basically I used styrofoam as the riser, as it's cheap, light, and fairly reflective on the inside, being white. Then I used another MDF cutout of the segment, which gives the segments nice sharp edges and hides the light spill into the foam. I then simply used some frosted acrylic sheet on the front - almost all plastic suppliers should stock it, usually in various transparencies.



If the budget stretches, you could use the same plastic to make diffusers for each segment. In my case I just used a layer of baking paper - but it's not a professional product

[alank2]

Seedstudio sells segments to make an 8", 5", and 3" digit in green and red:

http://www.seeedstudio.com/depot/7-Segment-Display-8-Inches-Green-p-1226.html?cPath=34_78

You can position these segments any way you like...

[sacherjj]

If the budget stretches, you could use the same plastic to make diffusers for each segment. In my case I just used a layer of baking paper - but it's not a professional product

A professional product is any product that you get paid for.  I've disassembled LCD panels from Chinese makers that used wax paper for the diffuser for the back light.  So parchment paper that you used is professional product level, just the very low end.