LUMI - Secret Light Message Coin & Keychain

It looks like a plain white coin. Hold it over a light and a hidden message glows through.

LUMI hides your name, a word or a little icon INSIDE the print. In normal room light both faces are smooth and blank. Put it on a phone flashlight, against a window or a lamp, and the secret appears. Made for proposals, gender reveals, love notes, stocking stuffers and name keychains nobody can read until you let them.

HOW IT WORKS

• The message is a sealed hollow in the middle of the coin: 1.2 mm of white on top, a 1.2 mm hollow shaped like your message, 1.2 mm of white below.

• Light passes more easily where there is less plastic, so the letters glow brighter than the rest.

• Nothing is printed on the surface: nothing to see, scratch or peel off. Both faces stay blank.

• No supports, no pause, no glue, no magnets. Print it and gift it.

TESTED ON A REAL PRINT

Before publishing I printed a calibration plate with the top layer at 0.8, 1.0 and 1.2 mm. Only 1.2 mm stayed completely invisible in room light, so every file and the customizer use 1.2 mm on both faces. The real-life photos show the actual print: blank on the table, glowing on a phone flashlight (Bambu PLA Basic white + black, Bambu Lab X2D).

MAKE YOUR OWN (OPENSCAD FILE INCLUDED)

• Open LUMI_customizer.scad in OpenSCAD 2021.01 or newer (Window > Customizer). Fonts: Poppins, Fredoka, Luckiest Guy, Pacifico (free on Google Fonts).

• Up to 3 lines of text, auto-sized to fit the shape.

• 6 hidden icons: heart, star, Christmas tree, paw print, snowflake, or none.

• 4 fonts: Bold (tested), Rounded, Extra heavy, Script.

• 3 shapes: Coin (36-70 mm), Keychain / ornament with a loop, Wallet card (86 x 54 mm).

• Export with Export = White body only, then Export = Rim only, to get the two STLs.

Tip: short words and chunky fonts glow the sharpest. Letters of 7 mm or more look best; the customizer warns you below 6 mm.

PRINT FILES INCLUDED (4 plates in every profile)

• Coin BELLA with a heart.

• Keychain ISA with a star.

• Wallet card "Merry Christmas Bella" with a tree.

• Gift set of 4 coins: LOVE YOU, MARRY ME?, IT'S A GIRL!, MERRY XMAS.

PRINTING TIPS (already set in the profiles)

• Body in WHITE or natural PLA: it has to let light through. Bambu PLA Basic white was used for the photos.

• Infill 100% is essential. With sparse infill the pattern shows when backlit and the message gets lost.

• 0.20 mm layers, 2 walls, no supports, no brim, flat on the plate.

• The rim starts at 3.6 mm: one single colour change, any colour you like (or print it all in white).

• Not a toy for small children: keep away from children under 3.

HOW TO SHOW THE SECRET

Turn on your phone flashlight in a dim room and lay the coin flat on it, or hold it against a sunny window or a lamp. The brighter the light behind it, the stronger the glow. Film that moment for a reveal video!

GIFT IDEAS

• A "MARRY ME?" coin hidden in the ring box.

• "IT'S A GIRL!" / "IT'S A BOY!" for a gender reveal.

• A secret name keychain for a kid's backpack.

• A love note for a wallet, a pet's name with a paw print.

• A Christmas ornament that lights up on the tree.

• A code word for an escape room or a treasure hunt.

FILES

• 3MF (Bambu Studio projects, 4 plates each): LUMI_2colours_AMS_all_printers.3mf and LUMI_1colour_all_printers.3mf work on every Bambu printer (A1 mini, A1, P1S/P1P, X1, P2S, H2 series, X2D); LUMI_2colours_dual_nozzle_X2D_H2D.3mf is the no-swap version for dual-nozzle printers (LUMI_2colours_AMS.3mf and LUMI_1colour.3mf are the X2D versions).

• STL for every example: LUMI_<example>_body_white.stl + LUMI_<example>_rim_colour.stl (same origin, load both as one multi-part object).

• LUMI_customizer.scad: make your own message.

PRUSASLICER / ORCASLICER / CURA

• Import the body_white STL, then add the rim_colour STL as a part of the same object and give it your second filament (or add a colour change at 3.8 mm).

• Infill 100% (rectilinear), 0.20 mm layers, 2 perimeters, no supports.

SUPPORT THE DESIGN

Original design by A Arte De. If you print one, please post a Make with your reveal and give it a Like - it helps me keep publishing free models.

PRINT PROFILES (made on the Bambu Lab X2D, 0.4 nozzle, 0.20 mm)

• 2 colours - dual nozzle (X2D, H2D): Coin 15 min (9.1 g + 0.4 g); Keychain 15 min; Card 29 min (20.6 g + 0.6 g); Gift set of 4: 53 min

• 2 colours - AMS: Coin 13 min (9.0 g with purge); Keychain 13 min; Card 26 min; Gift set of 4: 48 min (one colour change per plate)

• 1 colour: Coin 12 min (8.8 g); Keychain 12 min; Card 25 min (20.5 g); Gift set of 4: 47 min (34.6 g)

NEW - Bambu Lab H3C: LUMI_2colours_H2D_H3C.3mf puts each colour on its own PrintTool (almost no purge), re-centred on the 330 x 320 mm H3C plate, with the official H3C machine G-code. Sliced in Bambu Studio 2.09.