13- making the Korry switches

Translation by Christian Girardet


 

This method was inspired by the following site: http://home.hccnet.nl/jwopdenakker/building%20tip.html. Check it out!

 

There are many “Korry” or “Eaton” push buttons in the cockpit, especially on the overhead panels. Most of them are OFF-ON switches comprising several electronic circuits, but there are also some switches with lock-in inverter functionality with the same form factor.

 

Once in a while, some “Korries” show up on eBay, but they remain expensive. Don’t even think of being able to afford new ones…!

 

The only economical option for a home cockpit is therefore to build your own Korries.

It is likely that the ergonomics, the reliability and the look will never match that of the real Korries, but at least we can aim at reproducing these switches functionality very close.

 

Korry is a trademark name of the manufacturer. In order to simplify, we will use the term “Korry” or “Korries” instead of “push-button switch derived from those made under the “Korry” brand”.

 

Our version will be slightly different :

 

Real world Korries used 5 volts incandescent lamps (now they have LEDs) which are turned on by one of the circuits inside the switch itself. Circuit A activates a function while circuit B (triggered at the same time) sends 5 Volts to the lamp. In our version, the LEDs do not check if the switch has been closed or opened. The signal to turn them “on” or “off” is not by the switch closing or opening sent, but by Flight Simulator or P3D, through FSUIPC and SIOC. The Korries’ LEDs are connected to one of the 45 outputs of a Master card and programmed accordingly with SIOC.

 

There is a little problem though: the Master card puts out 25 mA max at each of the connections. This is only enough to supply one LED per output connection provided that LED is not too powerful… If several LEDs have to be activated by the same output connection, use a USB Output card. Remember that in this case, the LEDs’ wiring will be different as the common ground is positive and NOT negative.

 


Overview of our Korries

The Push button itself is made out of a square 20 x 20 mm Plexiglas rod and is 20 mm long.

 

This button comprises two compartments separated by an opaque insert. We will use only one LED per compartment. That LED will need to have a wide scattering beam (Wide angle LED) and to produce an intense light output. The button (Plexi block) will be glued or soldered onto a small PCB. This PCB will also house the LEDs and their respective resistors: 100 Ohms (optional) when connected to a Master card J2 output connection or 150 Ohms (mandatory) if connected to a USB Output card supplying 5 Volts.

The Plexi block will travel through the front panel : the latter has to be thick enough to provide good side support to the block and the hole in the panel has to be barely larger than the block size so it slides precisely.

 

The display label is made of 2 layers in order to completely prevent the light to show through the black areas of the label. The first layer is printed in black and white (the white areas will behave as if transparent with the light beam). The second layer is printed in color and glued on top of the first layer after careful alignment. As the Plexi profile is cut into individual actuator blocks, the cut side does not show any polish and after a slight sanding, it will be perfect for diffusing light.


Assembling the Korries

Plexi actuator (Plexi block)

 

We started off with some 20 x 20mm cross section Plexiglas profiles, 1 meter long (about 3 feet). We got ours at http://www.plexiglas-shop.com/fr/

Two 1,000 mm lengths are sufficient for 80 Korries.

 

1- To block the light coming from the LEDs, these Plexi blocks will have their vertical sides painted. Prior to chopping the individual blocks from the profile, remove the polish from the sides as well as on the edges. The latter are too sharp for the paint to hold on properly.

 

2- Paint the whole Plexi profile with white acrylic paint. We use acrylic paint because it is the only type of paint presenting enough mechanical resistance to abrasion: it is hard to scratch it with a nail even without using a primer for platics.

 

It is white as it is very important for the light to reflect and bounce around inside the Korry’s compartments. This would not be a problem when looking at the Korry from the front. But one notices a huge difference between white and black paint when you look at the Korry from an angle. This is perfectly illustrated on the following picture. In that case, one does not see the LED itself but only the reflected light.

Some will ask if the light would be better without sanding the sides of the Plexi blocks… Our comparisons tests do not show any difference and, as the paint will hold better with sanding, let’s sand!

 

3- Chop the Plexi profile into 20 mm long blocks.

It is highly recommended to use a good miter box and a fine tooth saw blade.

With a miter box, you can set a block (here a piece of wood) to cut all pieces to the same length. The Plexiglas piece can also be held tightly during the cutting phase. It is not a bad thing to make a first Korry prototype to check that everything goes well. Eventually, with the large number of Korries to make, you will save a lot of time by doing the work in batches. Therefore, once your miter box is set, cut all the 20 mm long pieces in one batch.

4- In a 0.6 mm thick aluminum sheet, trace several 19.5 x 15 mm rectangles with a blade cutter. By repetitive folding, break off the individual rectangles. Make as many rectangles as there are Korry switches.

5- Make a template based on the following picture; the goal is to hold each Plexi block perfectly vertical.

Adjust the location of the template in the miter box so that the saw blade lands exactly in the center of the Plexi block. Use a Vernier to check the location. Tighten everything and cut each of the blocks vertically, 16 mm deep from the top side. Make a mark with a pen on the template or the saw blade to not overshoot the 16 mm depth.

The Plexiglas is not a soft material: the cutting will take quite some time. Be patient! Juan Cordon came up with another method: he cuts the Plexi with a circular diamond blade (the type usually used for floor tiles). As the base of this saw sits in water, this is very efficient: the Plexi does not heat up or melt.

 

6- Apply Epoxy glue on both sides of one of the aluminum rectangles and slide it into the slot of one Plexi block. Make sure you apply enough epoxy for a good adhesion and that nothing sticks out of the white painted sides. Let it dry. The aluminum piece must be flush with the top of the Plexi block, but it not as important if it does not go all the way to the bottom of the slot.

 

7- Build a template more or less like this one:

The 2 wood pieces are set at a right angle. Once a Plexi block is inserted, its edge should stick out by a few millimeters. Take a sanding block and make a small bevel on each edge of the top side of the Plexi block just for the sake of aesthetics.

8- Using acrylic black paint, paint the 4 vertical sides of the Korry and with the flat side of the brush, apply the black paint on the 4 bevels we just made to the top surface. Let it dry hard.

 

9- After the sides are dry, paint the bottom of the Plexi block to avoid light leaks at the back of the switch.

 

10- Place a 500 sand paper sheet on a perfectly flat surface and gently sand the top side of the Plexi block to uniformly remove the Plexi polish as well as any traces of paint.

 

11- Make a drilling template so that the holes for the LEDs and the switch actuator are perfectly centered.

This template should be made out of metal such as aluminum and must slide snugly on the Plexi block. To put it together, glue the sides of the template around one of the 20 x 20 mm Plexi blocks.

Once dry, punch the three holes and drill. The size of the hole for the switch actuator will obviously depend on the switch type you selected. In our case things were even more complicated as we chose a square actuator! The diagonal of that square was 3.5 mm: drilling at 3.4 mm provided a very tight fit while still making it possible to take it apart as no glue was used. Make some experiments in order to select the perfect diameter: this will ensure a tight fit based on your type of switch. Holes up to 4 mm can be drilled directly. Beyond that diameter, first make a smaller hole and then enlarge to the final diameter.

The holes for the LEDs are a bit trickier to make. The 2 LEDs will be soldered onto a PCB and they are rigid because their legs cannot bend. The LEDs will therefore slide into the Plexi inside a hole slightly larger than their widest diameter. With our 3 mm LEDs the base of the LED bulb was 3.5 mm and we drilled at 4 mm.

Remember that regular drill bits are not designed to properly drill into Plexiglas. There is a good chance that the Plexi will end up breaking as you drill though it. To avoid this, start with a smaller hole and then enlarge it. Most importantly, make sure the Plexi is held tightly and certainly NOT by hand! Select a slow drilling speed (250 rpm) to avoid heat build-up: this creates tensions in the Plexi and weakens it.

 

After several trials (during which some Plexi blocks flew across the room) we finally perfected our method: the drilling template has three 3.4 mm holes. The three holes are drilled into the Plexi without removal from the template, 4 mm deep for the central hole and 6 mm deep for the LEDs holes. The Plexi block is then immobilized in a hand vice and the LEDs holes are re-drilled to 4 mm.


The PCB

Three outputs are needed: the ON-OFF switch connected on the Master card (2 wires) and 2 independent LEDs, requiring 3 wires going to the J2 of the Master card. One of these wires is common: that will be the “–“ in the case of a Master card or the “+” for the case of a USB Output card. As a result, pay attention to the LED direction when you install it. This is a very simple circuit as shown on the following drawing. This circuit cannot be bought at ERCIM, so you can make it in a small test bread-board.

If the + is the common, inverse the LED direction.

The lock-in push switch I bought at APEM is a double throw; we will use only one of the switch’s circuits and connect it to the Master. The LEDs are independent of that switch. JeeHell can run about all the overhead’s LEDs, so it is “just” a matter to put them all in place and to wire them.

The spacing between the LEDs on the PCB is about 13 mm corresponding to the spacing in the Plexi block.

 

Note that the prototype on the following picture included a HE14 connector. In the long run, this did not work out too well as when several Korries were next to each other, these HE14 connectors got in the way. The final choice was to have simple wires held in place with a bit of hot glue. This is much easier than installing connectors and in the end, more reliable.

It is important to make sure that all the LEDs legs are of the same length as this will give us homogeneous switch travel and lighting in the Plexi blocks of all the Korries. To achieve this, the Plexi block is installed on the switch actuator and the switch is pushed all the way to the end of the travel course. Slide the LEDs in all the way until they hit the bottom of their holes and solder them in place. When the switch is on the OFF position, there will be a gap of about 2 mm between the LEDs and the end of the holes.


LEDs color

The best results were obtained with color LEDs, not with white ones. Use yellow, green, blue and red LEDs, 3 mm diameter with 8,000 mcd (more than enough), wide beam and 20 mA. The LEDs can also be painted with stain glass windows paint, which works remarkably well for the yellow.


Printed labels

Only laser printed labels will work as the ink is strong enough not to require any additional exterior protection. They can be printed either in color or in black & white: the resulting color will be exactly the same and depends only on the LED itself. We chose to print the labels both in black & white and in color. The black & white label blocks the light with the black ink while the light goes through the white part. The label on top of the black and white is in color and will dim the inscriptions on the Korry when the switch is off. Normally one should not be able to read the switches labels in full day light when the switches are off. We did not exactly get that result, but it is nevertheless better than just having just the black and white label.

All the labels are grouped on 2 sheets. The following is a reduced picture.

Just as for the panels, the only glue which actually works for setting the labels is contact glue; it will hold very well over time. Follow the same process with a tape “hinge” and to save time, glue 2 or 3 rows at the same time before cutting them individually with a blade cutter, inside the white line.

 

Once dry, push down on the corners of the labels to avoid “catching” but DO NOT trim the sides as the end result is not satisfactory as seen on the following picture.

Once all the Korries are done, take a very thin water color brush. Turn on the LEDs in the Korry and with small touches of black acrylic paint, block the light leaks if there are any. If the 2 labels have been glued properly, the paint should not seep between the labels, but still be careful!

 

 

Comment #1:

The square Plexi block will not jam or have too much wiggling if you pay attention to the following:

Make sure that the square hole in the front panel is a tight as possible while still functional. Go easy with the file and check the fit often with one of the Plexi squares.

The Front panel should be at least 5 mm thick: this is mainly that thickness which will provide a smooth and precise travel to the Plexi block.

If you have a sticky Korry, do not screw the PCB too tight and leave some play in the screws: the Korry will end up finding it place by itself.

 

Comment #2:
remember that a Korry switch is quite large and may block the back-lighting of some of the inscriptions on the panel. Make sure you have enough LEDs or in the case of the overhead, use some long strips of LEDs covering the whole length of the overhead. As the light comes from the side, the risk of projected shadows is eliminated.

 

Comment #3:
this whole chapter described the process for only ONE Korry. You just have to make another 80 in order to get a complete cockpit…

Still it is faster than it seems as you can work in batches.