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Pushbuttons Switches featuring built-in LED

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I found these push button switches have a built-in LED in the center. Looks like it is time to revisit the 737FMC project, this time with Version 3 featuring backlit buttons?

The yellow version is a bit closer to orange (a lot more orange than in the photo below), but I think it’s fine. The LED is on a separate circuit from the switch itself.

They are USD15.20 + 2.49 shipping for a pack of 100 from AliExpress (https://goo.gl/KtKj3C)

Stuff to solve:

  • Wiring. There are 69 keys altogether on the 737 FMC CDU. Ledcalc.com shows one current limiting resistor with 4 LEDs in series. Why 4?
    • ANSWER: It’s to do with the voltage drop from each LED. If each LED drops 2.4V, then with a 5V supply you can only have 2 LEDs in series under one current limiting resistor, and with a 12V supply you can only have 4 LEDs in series under one current limiting resistor. The other LEDs have to be run in parallel, and therefore need their own current limiting resistor.
  • Making the key caps. The keys themselves can be 3D-printed in two parts, with black surrounds and a transparent center. But the challenge is still how to make the “mask” at the top surface nicely.
    • My current plan is to print the numbers and letters as outlines (clear text, black background) on two pieces of transparency sheets glued together … similar to how I did the annunciators on the overhead panel, but this is messy and tricky to do neatly

Originally created with EverNote at 20171015T164045Z

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