Building a Backlit Annunciator Box

A backlit annunciator is one of the most satisfying parts of any panel build. Done right, only the engraved text glows — the rest of the face plate is solid black. This guide walks through the full process from a blank Fusion 360 file to a wired, working annunciator ready to drop into a panel.

The box dimensions vary per project and aircraft. This guide covers the technique — see the specific component page for your aircraft for the exact measurements.

CAD & Design

The annunciator box consists of three parts designed in Fusion 360:

1. Box housing — the 3D-printed shell that holds everything and mounts to the panel
2. Face plate — laser-cut from translucent white acrylic, spray-painted and engraved
3. Light diffuser — a thin frosted sheet that sits between the LEDs and the face plate

Box Housing

Design the housing as a simple rectangular shell. Key dimensions to parametrise:

• Face opening — the visible area of the face plate (e.g. 30 × 12 mm for a single annunciator)
• Depth — enough to fit the LEDs and wiring (typically 15–20 mm)
• Wall thickness — 2.0–2.5 mm minimum so the box is rigid and opaque
• Mounting flange — a 3–4 mm lip around the front face for panel attachment; include M3 insert boss holes at the corners
• LED aperture — a hole or slot at the back or side for the LED leads

Make the inner walls dark and opaque. If printing in a light-coloured filament, add a 0.8 mm inner wall at 100 % infill to block light bleed between adjacent annunciators.

Face Plate

The face plate is the visual heart of the annunciator. The technique is:

1. Start with 3 mm white translucent acrylic — this transmits LED light evenly
2. Spray-paint the front face with primer
3. Spray-paint the front face matte black (2–3 thin coats, fully cured)
4. Laser-engrave the legend text from the front — the laser burns through the black paint and reveals the white acrylic underneath
5. When lit, only the engraved text glows

Design the face plate 0.2 mm smaller per side than the housing opening for a clean press-fit with no visible gap.

Light Diffuser

A 1–2 mm frosted acrylic sheet or a layer of white PETG between the LEDs and the face plate eliminates hot spots — without it, the individual LED point sources are visible through the engraved text rather than a uniform glow.

Cut the diffuser to the exact inner dimensions of the housing so it sits flush against the inner lip, directly behind the face plate.

Manufacturing

Printing the Box Housing

Print with the open face down on the build plate. This puts the critical mounting flange surface on the first layer (smoothest side) and avoids supports inside the box.

After printing, install M3 brass heat-set inserts in the corner boss holes.

Preparing the Face Plate

Cut the face plate from 3 mm white translucent acrylic on the laser cutter. Cut slightly oversized, then fine-trim to final dimensions if needed.

Spray painting:

1. Clean the acrylic with IPA — no fingerprints
2. Apply 2–3 thin coats of matte black spray paint to the front face
3. Allow full cure: 24 hours minimum before laser engraving. Engraving into uncured paint smears and blurs the edges

Laser engraving:

1. Place the acrylic paint-side up in the laser cutter
2. Engrave the legend text — aim for 0.1–0.15 mm engraving depth through the paint only, not into the acrylic body
3. Test on a scrap piece first: too shallow leaves paint residue in the text; too deep creates a rough texture that scatters light unevenly

Cutting the Light Diffuser

Cut the diffuser from 2 mm frosted acrylic or thin white PETG sheet. Size it to slide inside the housing with 0.1 mm clearance per side — tight enough to stay in position without glue.

Assembly

1. Press the light diffuser into the housing. It should sit flush against the inner lip with no rocking
2. Press-fit the face plate into the opening from the front. A small drop of acrylic cement on two corners holds it permanently without a visible bead
3. Verify the face plate is flush with the mounting flange — any proud edge will leave a visible step in the panel

If the face plate is slightly loose, a strip of 0.5 mm foam tape on the inner housing lip takes up the slack and prevents rattling.

Electronics

LED Selection

For most cockpit annunciators, a single 3 mm LED per annunciator cell is sufficient with a good diffuser. Use warm white for general illumination panels, amber for caution indications, red for warnings, and green for normal/active states.

Resistor Calculation

Every LED needs a series resistor. See the Resistors & Components page for the full formula. Quick reference for 3.3 V GPIO:

For annunciators, run LEDs at 5–8 mA rather than the maximum 20 mA — panel brightness is more comfortable and the LEDs run cooler.

Wiring

Thread the LED leads through the aperture at the back of the housing. Solder the resistor inline on the anode lead before it reaches the ESP32 GPIO. Connect the cathode to GND.

For panels with many annunciators, group cathodes on a common GND bus and run individual signal wires to each GPIO pin.

Testing

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1. Wire a single annunciator to a bench supply before installing in the panel. Set current limit to 50 mA as a precaution
2. Check that the engraved text glows evenly — if hot spots are visible, the diffuser is either missing or positioned too close to the LED
3. Check for light bleed between adjacent annunciators — if visible, the housing walls are too thin or the infill too low
4. Mount in the panel and verify the face plate is flush and the legend is readable at normal cockpit lighting distance (around 60–80 cm)
5. Test firmware control: each LED should respond to its assigned GPIO independently with no cross-talk