Using Side View LEDs in place of Reverse Mount LEDs

TwinkleTwinkieTwinkleTwinkie wrote 04/11/2019 at 03:15 • 7 min read • Like

So if you've seen damn near anything I've done for the last 18 months then you've seen examples of using Reverse Mount LEDs and also manually mounting LEDs upside down. 

A sample of my projects from 2018, all using reverse mounted LEDs
Reverse Gullwing LED & 1206 LED Properly Abused

I like shining LEDs through PCBs, it's a great lighting effect and it isn't as harsh a light as you get from having the LEDs on the top side of the board.  However there are a handful of draw backs to using them.

Reverse Gullwing LEDs:

Reverse Mounting Normal LEDs:

So between these two solutions there is the classic "Cheap, Fast, Good, pick two" dilemma.  Reverse Gullwings are Expensive, but fast and good.  Manually soldering LEDs upside down is Cheap, Slow, but also Good.  Now let's look at another solution: Side View LEDs.

Side View LEDs

Side View LEDs are exactly what the name says.  They're LEDs that instead of shining up or down they shine at a right angle.  Side View LEDs are cheap, pick n' place friendly, and you can get them in every standard LED color including UV and RGB.  My initial interest in Side View LEDs came from wanting to use the UV variant on some projects to get that eerie glow you get when UV light is shined onto FR4.

The glowing eyes are from the UV Light, they are not back lit.

This is an effect I still intend to use on future projects but I also saw the possibility that I could use Side View LEDs in place of Reverse Mounted LEDs.  Unfortunately it isn't as simple as just replacing one foot print for another.   Side View LEDs were not designed to be used for this purpose, they're designed to, well, shine light at an angle and are typically used as indicator lights in most consumer applications, think something along the lines of status indicator lights on a Laptop.  As such there is a tremendous amount of light bleed that you have to control.

The other major issue with using Side View LEDs is that the light that does shine below them isn't as bright as using a Reverse Gullwing or a typical LED mounted upside down. Due to their vary nature the light they output isn't in a circle it's more like a 1/2 Circle or a very wide cone so you have to rethink your designs to take advantage of that as well.

Pink Side View LEDs

I designed a simple prototyping board so that I could get measurements and figure out how to best work with these LEDs before I put them in a project.  I am very glad I did this because despite having seen other examples of Side View LEDs being fantastic drop in solution for back lighting PCBs I instead found it had its own draw backs.

Green LEDs from the "Front"
Green LEDs, other side of the board.

I used a Sony A5100 Mirrorless Camera to take these pictures, my iPhone X that I use for most of my photos blows out the light too much and makes it look brighter and more distributed than it actually is.  My A5100 still makes it look better than it really does but it's the best I could do.

So, how can we salvage this?  Simple: Hot Glue.

To control the light, make it spread out evenly, and to dramatically reduce the light bleed we need the light to have a medium to travel through so I chose Clear Hot Glue.  Hypothetically you could use Epoxy too but Hot Glue is cheap, readily available in large quantities and can be easily removed if need be.

Same Green LEDs but with Hot Glue

This solution worked even better than I had expected.  In some cases I was able to reduce the number of LEDs from 5 to just 2 while still covering the same area and getting an acceptable brightness level.   This doesn't even take into full account of positioning the LEDs in different directions.   As long as the LED shines into hot glue there won't be remotely as much light bleed and it will be distributed fairly evenly.

Below you'll see a lot of examples of this technique in action.  Unfortunately my Glue Gun has been used mostly with Black Glue Sticks so my results were hampered a little by black and gray artifacts that wouldn't normally be there.  Even with these artifacts using Side View LEDs with some sort of additional medium can yield very impressive results.

Yellow, showing before and after applying hot glue.
Pink, Before and after Hot Glue with only 2 or 3 LEDs

White Before Hot Glue with 5 LEDs vs After with 3

Red After Hot Glue with 5, 3, and 2 LEDs, demonstrating how good the coverage can be even indirectly.

Of course this technique isn't perfect and it's still a fair amount of work after assembling the board but I think the results justifies the extra steps.  I'm looking forward to seeing future results and additional iterations on this technique, especially after a little more practice applying the hot glue itself.



Ivan wrote 03/21/2021 at 14:58 point

have you tried using small pieces/blocks of acrylic? I’ve found they bounce the light around from normal LEDs quite nicely. I’ve drilled a hole the size of the led to embed it and get max use of the light output. If you sand the outside of the acrylic with coarse sandpaper, that diffuses the light very well. 

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Elliot Williams wrote 04/17/2019 at 20:48 point

1) Hot glue 

2) Different color LEDs

3) ??? 

4) Dan Flavin.  (e.g.

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UnnecessaryComplification wrote 04/17/2019 at 20:36 point

I've wondered about 3D printing directly on a PCB. Would it stick? If it works you could do some cool things with the diffuser.

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alireza safdari wrote 04/11/2019 at 06:54 point

Awesome, the look so cool

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Jarrett wrote 04/11/2019 at 05:44 point

Hah, awesome. I solved the same problem with my side-mount LEDs with hot glue the same way:

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