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Frontplate design w LED light channels, help needed
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Frontplate design w LED light channels, help needed
weasel79
hey guys, i'm shamelessly crossposting my post from the midibox forum cause i am sure there are a ton of frontplate/material experts around here... i am building a desktop keyboard synth with these nice rgb led ring encoders, in a tiny form factor:

most recent demo video:
https://www.youtube.com/watch?v=FaMVOi2yt4E


So i am currently investigating frontplate options and i realized some small issues with these 2020 LEDs: they are in a completely transparent housing, as opposed to the white housing with a top lense of the 5050 2812s. So naturally i get quite some cross bleed between the neighbouring LEDs, at highest brightness setting even up to 2-3 LEDs far. In real life it is not nearly as noticeable as on those videos i posted but i am still trying to optimize it.

Following are some sketches of what i have so far and some ideas. The "light channel" drill holes got pretty long, 3.5-5mm over all, but that actually makes frontplate assembly super convenient and also should reduce the cross bleeding by creating a more narrow viewing angle. any input is welcome!

1. This is a description of what you see on the videos, what i started out with. just a 3mm mdf with 1.25mm holes for each LED, and another 1.6mm acrylic layer on top, same holes. they align nicely and sit right on top of the middle of the LEDs. The thin arrows show where i think most of the crossbleed comes from. the whole housing of the LED is transparent resin so the 3 tiny R G B LEDs all emit light in any direction of almost half a sphere.




2. So the first thing i tried to improve it was to cut a negative mask out of MDF to enclose each LED, leaving room for the condensators and solder points.







its black coated MDF and obviously the details are a good bit too much for the laser. but i am sure i could still improve on these first tries. it did improve the bleed a little but not to the extend i hoped for.




3. i tried the same technique but with black cardboard instead of the MDF, and the little bridges betwees the LEDs came out even better.







4. the next thing i want to try is different materials. i did a first cut of the same negative mask as above in solid acrylic but didn't get to test it yet. i have to play with the lasercutting margins and measures to make the fit as good as possible, eg. i left space for the extra solder points but not for the blobs of solder around the LED connectors. but right now i am more interested in alternative methods. what about using milliput or some other thick epoxy stuff, press it on the LED pcb so the top stays somewhat clear, and carve off remainders after hardening? something like this: (note: most likely my final front-frontplate will be aluminum..)



anybody has experience with using milliput/expoy like this?


another thing to try is the resin-based 3d printer in my local fablab which might be detailed enough to get a better mask going.

and yet one more idea is to get some thick lightproof acrylic paint and a tiny brush and try to paint the sides of the LEDs, or even spray paint it with circular masking tape on the top?



also i am not sure about how to fill the light channels in the frontplate, what @Antichambre did so beautifully with those inlays. i was also gonna try just fill them with epoxy and razer off any residue on the outside? or maybe get some 1.25/1.5 mm optical fibre, and stuff each LED hole with a 3-5mm piece that ends directly on the LED enclosure? might also help reduce bleeding.



Here are some more pics of @FantomXR's beautiful prototype SMD assembly job, you can see the shape of the LEDs really good. in before questions: shot on iphone with a 10$ hama macro lens i just bought on impulse at the local electronics megastore. keep in mind these LEDs you see are 2x2.2mm small...










oh and see my other thread if you wanna jump in on the group order for these.
cackland
Have you looked into light pipes?

jochem
I think i would use some heat resistant sealant to seal off the sides of the leds. Than you can use the mdf layer on top of that (after the sealant has dried).

You should be able to find a sutable sealant with a thin dispenser at a local car supply shop. It is commonly used as a gasket solution.
weasel79
cackland wrote:
Have you looked into light pipes?


yeah those look like what i meant with the optical fibre, these look super convenient though, thanks. they should help with the directly emmited "right" light, not with the bleed i guess? any idea if they exist specifically for 2020 leds?

jochem wrote:
You should be able to find a sutable sealant with a thin dispenser

good call, i will look into those. would you use a brush or spray paint? how thick/liquid is that stuff? keep in mind these are 2mmx2mm and i can't paint them before assembly.
whoop_john
How about a small piece of black heatshrink over each LED? A lot of work I guess.

I'd probably use a 00 size artist's brush and black acrylic paint.
jochem
The heatresistant gasket sealant i mentioned is like toothpaste.
It comes in a toothpaste like tube with different tips or in a thin can. It is mostly black en you dispense it on the object directly.
ersatzplanet
weasel79 wrote:
cackland wrote:
Have you looked into light pipes?


yeah those look like what i meant with the optical fibre, these look super convenient though, thanks. they should help with the directly emmited "right" light, not with the bleed i guess? any idea if they exist specifically for 2020 leds?

jochem wrote:
You should be able to find a sutable sealant with a thin dispenser

good call, i will look into those. would you use a brush or spray paint? how thick/liquid is that stuff? keep in mind these are 2mmx2mm and i can't paint them before assembly.


Light pipes work very well with isolating external light sources. At least some designs do. The nature of some of them, having a high incidence of reflection, means external lights not at the correct angle, reflect off the sides instead of going "through" the cylinder. Typically the light has to enter the ends to travel down the "pipe".
weasel79
ersatzplanet wrote:

Light pipes work very well with isolating external light sources.

Do you have any good source for tiny ones? Anything 1-1.6mm diameter. mouser etc start at 50ct/pc, i need hundreds-thousands...

also ordered that gasket sealant and some other epoxy variants. second day of lasercutting today was better but i don't think it'll be good enough. might try and mill a light pipe mask out of transparent acrylic with cnc.
extralifedisco
Wow, nice work so far! Here's some very basic stuff - this product training module on digikey explains the geometry of fitting lightpipes on SMD leds:
https://www.digikey.com/en/ptm/l/lumex-optocomponents-inc/light-pipe-a nd-led-matching/tutorial

The gist of it is that you want the light pipe to have a reflective surface on at least 3 sides of the LED which bounces the light up towards the panel, which gives you less bleed.


It may be worth picking up and older Akai/ableton APC40 which has nice LED rings and a custom "light pipe" part. You can see in this picture there's a black plastic ring with lots of tiny channels for SMD leds on the board. There's clear plastic pieces coming through the panel but I don't know if that's a separate part.


(If that hotlink doesn't work try the 5th pic here:
http://archive.monome.org/community/discussion/4966/apc40-taken-apart/ p1.html)
ersatzplanet
weasel79 wrote:
ersatzplanet wrote:

Light pipes work very well with isolating external light sources.

Do you have any good source for tiny ones? Anything 1-1.6mm diameter. mouser etc start at 50ct/pc, i need hundreds-thousands...

also ordered that gasket sealant and some other epoxy variants. second day of lasercutting today was better but i don't think it'll be good enough. might try and mill a light pipe mask out of transparent acrylic with cnc.


I'm afraid to say that in most cases, for the density and use you have in mind, it is a custom piece. This used to be an extreme expensive process with the molds costing lots of cash up front, but the cost are much more palatable nowadays. You may, if you plan on doing quantities, get the cost pretty reasonable.

I have to say though, I think the easiest and most cost efficient thing to do is live with the bleed. It is not like the bleed makes the unit unusable, the user will know the settings still. Alleviate it as much as you can with the methods you have shown above, and live with the results. It will always be cheaper.
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