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Backlit Panels

For a long time I had shied away from adding backlighting to the cockpit panels. My reasoning was that good backlighting is difficult to make and that it will be used seldomly, since most simulated flights happen with daylight. On the other hand, nobody can deny the beauty that lies in a nighttime cockpit with all panels glowing mysteriously . . .

Over the years I experimented with different techniques for backlighting, but never got satisfactory results. One problem was the light source: The LED’s I tried were too dim and had only a very narrow opening angle (so I would need large numbers of them), mini-bulbs were too hot and too yellow. Another problem was my printed, self-adhesive decals on the acrylic glass: The black parts of the print were never black enough, so the backlight would not be blocked completely. Instead, it was shining through the black, rendering it kind of grey – very unpleasant to look at.

But the dedicated cockpit builder never despairs! After a lot of tinkering I finally got it right. The result are backlit panels with sharp, well lit letters and symbols on an otherwise black background. When lit, they really make a difference in my cockpit. Here is the how-to, using the EFIS panel as an example:

The image on the left is printed with a laser printer on a grey, translucent plastic foil with a self-adhesive back . Everything that should later be visible on the unlit panel is printed on this foil.

The outer border is a bit thicker than needed. This allows for less than perfect alignment of the foil on the acrylic glass plate. Note the crosshair marks in the corners; they are important for the next step.

This image is laser printed on a transparent, self adhesive foil. All surfaces that appeared grey on the previous image were replaced with 100% black. The black is supposed to block the passage of light – except for the letters and lines.

At least 2 layers (i.e. 2 prints!) of this foil are needed to completely prevent the backlighting from shining through the black areas.

When joining the 3 printed foils, utmost precision in positioning is recommended. Here is where I use the crosshair markers. I found that it is best to print full A4 pages (putting as many panels on a page as possible to reduce cost), because good alignment of the three foils is easier to achieve when using the long edges of the sheets for reference.

Let me again emphasize the need for precise alignment. The result of bad alignment is ugly, blurry letters (when backlit). I’ve been there, so believe me: That sight can ruin your otherwise perfect day!

The panel after installation. Make sure the cutout in the wooden frame or support structure is big enough. Letters, lines or symbols that will be backlit must not be covered.

The unpainted plywood parts on the left and right are spacers that will hold the LED board.

As a light source, I use white Super-Flux Spider LEDs. They have a luminous intensity of 2000 mcd and a viewing angle of 90 deg. This means they are both bright and illuminate a wide area – perfect for backlighting. They are sold by Lumitronics GmbH, Germany.

Specs: forward voltage 3.1 V, red. forward current 18 mA. Due to their construction, Spider LEDs are rather heat resistant.

My backlighting is powered by the 12 rail of an old PC power supply.

For simplicity and reduced energy consumption I usually use groups of 3 Spider LEDs connected in series, with a voltage limiting resistor. A good web-based resistor calculator can be found here.

The LEDs are mounted on a circuit board that has roughly the same shape as the panel it is supposed to backlight. The placement of the LEDs depends on the panel layout, but spaces between LEDs should be more or less equal. If the distance between two LEDs is too small, you get bright spots. The idea is of course to have uniform lighting.

I found that a distance of 4 cm between the LED board and the back of the panel gives good results with the Spider LEDs. If the backlighting is too bright, either reduce the voltage or use bigger resistors.

I use a board with parallel lines, so cabling can be kept to a minimum. Just make sure these lines are interrupted in the right places to avoid short circuits! The resistors are soldered right onto this board.

12 DC current is supplied via the black connector.

The LED board after installation. It is held in place by the plywood spacers. The board slides into a gap on the right spacer and is fixed with a small screw on the left spacer. This makes disassembly quick and easy, for example in case an LED needs to be changed (I expect that to happen every 30.000 hours or so…).

Above the LED board there are now some more plywood stripes. They hold a piece of cardboard that prevents any light from coming out this side of the frame.

The cardboard lid is in place, and labeled – in case I forget one day what was underneath…

The reward for all that effort: A panel with crisp, uniformly backlit letters and symbols!

As a bonus, I even got lit buttons, because the edges of the holes drilled into the panel are illuminated. Quite a nice effect, both on the push buttons and flip switches.

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