EPU (Emergency Power Unit) panel Delicate panel for backlighting

Introduction

The EPU panel has just one 3-position switch, of which both positions have a switch guard to prevent accidental activation. The Jay-El indicator in the upper part of the panel has two separately illuminated halves. The upper half says HYDRAZN and the lower half says AIR. In the middle is a green indicator lamp which is lit if the EPU is active.
As you can see, the lower part of the panel is rather small (the part between the OFF position and the NORM position if the switch), but connections must run via that small passage for the backlighting.

Backlight circuit board
EPU backlight circuit boards

The EPU panel is small, and its thickness is just 6 mm. As the lower part has a rather smallish panel surface to the upper part of the panel, I hesitated to cut more than 3 mm of the material at the rear side with the Dremel tool. But I do need some space inside the rear of the panel for the circuit board and the SMD LEDs for the panel backlighting.

Normally, I use standard circuit board with copper foil at one side, but in this case that board would add significantly to the total thickness. So, I used circuit board which is just 0.6 mm thick!
The advantage of this thin circuit board is that you can cut it to the desired shape with normal kitchen scissors. However, as usual, with every advantage comes a disadvantage. First, you have to be careful when you cut the copper foil for the isolated copper "islands" for the LED connections.
Further, there may be surface pieces that are difficult to reach for cutting with the scissors. In this case it is the opening in the panel where the switch guard is mounted. I solved that by simply cutting the circuit board in two pieces.

The tiny black squares are the 0805 SMD series resistors for each LED. The SMD LEDs are the yellowish small rectangles. The green wire connects the ground GND connection to one LED at the right side. Two thin wires will connect the two PCB halves when they are installed in the EPU panel.

Important
When you cut away material at the rear side of the panel, do not cut away everything. You must keep a small ridge surrounding the square opening for the Jay-El, otherwise the bright light of the LEDs will be very visible at all sides of the push-button!

Connection panel

The connections for the EPU panel are (of course) at the rear side. For all panels I make a small connector plate which is, if possible, made from a piece of L-shaped aluminium profile 20 x 40 mm. The "L" shape gives the plate strength and will not bend if you plug in a connector.
The green 9-pin sub-D connects to a connector on the ENGINE & JET START panel and is for the switch signals. The ENGINE & JET START panel has several connectors and acts as a "signal distribution" panel, thus the connection from the EPU panel to the ENGINE & JET START panel. Some wires pass through from the green connector to the red 9-pin sub-D to connect the switches of the "neighbor" panel, ELEC. The white 9-pin sub-D connects the two Jay-El indicator lamp sections, and also the EPU panel backlight LEDs.

EPU module front view	EPU module side view (backlight wires not yet soldered)

Due to the length of this Jay-El push-button, the M4x80 mounting bracket screws were not long enough. The connection pin / wire end would be close to the metal bracket, and I would have to remove the bracket to insert the pins. That would certainly not be "maintenance-friendly". Luckily, I also have longer M4 hexagonal spacer nuts, and that made it OK. As this Jay-El has a holding coil, I connected those 2 pins also to the (white) 9-pin sub-D connector. That is no F-16 functionality, but I might have use for it in the future ... Now it is just 2 more wires, but later, if I actually would like to have the holding function, I do not need to solder "inside" the panel but just connect them on the sub-D plug.
One pin on the (white) 9-pin sub-D was not used, so I decided to have a separate + connection for the LED and a separate + connection for the lamps in the Jay-El. But the LED series resistor is dimensioned for +24 Volt, so it is still possible to share the + of the LED with the + of the lamps. I made the connection to the EPU LED completely separate, because I do not know whether the bright/dim functionality of the MAL & IND LTS switch on the LIGHTING panel operates on the LED indicator only, or also on the lamps in the Jay-El.

Short explanation of the holding coil
If the coil is not energized and you push the button, you get what you expect, a momentary action. However, if the coil is energized and you push the button, the button will physically return to the "rest" position, but the contacts remain closed. If you push the button *again* you feel the holding force, and with a little pressure, the contacts open and the switch is back to its initial state.
PS. I know that the Jay-El on the EPU panel is just an indicator, no push-button