Building a physical KMA20 panel 🔗

Posted by Médéric Ribreux 🗓 In projects/flightgear/

#electronic #flightgear


KMA20 is an audio panel from Bendix King with which you can transfer audio signals to speaker or headphones. You can also configure which microphone will be used. There is also an ILS light controls for MM and OM signals.

There is no commercial equivalent product (nothing on Saitek side as far as I have searched).

What does it look like ?

The face looks like this:

KMA20 Panel



On the left, you can see a 3 position rotary switch. It is the microphone selection switch. First position is to select COM1 frequency to answer, second position, you will be talking to COM2. Ext means to use the microphone to speak to you copilot. To emulate this switch, we just need 1 three positions rotary switch.

Then you have the main group of 8 switches which is used to send the audio signal of the different systems to the speaker, to nothing (silence) or to headphones. Those 8 switches have 3 positions: up for speaker, middle for silence, bottom for headphones. To emulate this group of controls, we need 8 three positions toggle switches. Those positions will be ON-NONE-ON.

On the right, there is a three positions switch which is used to set the sensitivity of the beacon marker (HIgh or LOw). If the switch is toggled to the bottom, all of the bottom left LEDS are switched on (for test purpose). To emulate this control, we need 1 three positions toggle switches.


There a few outputs for KMA20. There is just 3 LEDs on the bottom left. A LED means that you have crossed an Airway marker, O means you have reached an Outer beacon marker and M means you have reached a Middle beacon marker. Those markers are lighted on ILS procedures (as far as I know).

Supplemental inputs

We need more than the inputs shown previously. First, we need 1 ON/OFF switch to power the KMA20 console. Then, if you look closer at the KMA20 panel at the bottom right, just before the King logo, you will see a photo-diode. It is used to adjust the 3 lights dimming.

As far as I am concerned, I'd prefer to have a potentiometer to adjust LED brightness manually. So we need 1 rotary potentiometer. But we could combine those two controls by using only 1 switch rotary potentiometer.

Microcontroller dimensioning


We will use PDIP µcontrollers as it is easier to solder with hand.

We need at least 25 I/O pins (two analogs and 3 PWM). After a bit of search, there is no PDIP28 AVR that can be used on this project. We need to focus on 40 PDIP µcontrollers.

We could reduce the 18 pins of the toggle switches to one analog pin using an R-2R ladder. But we would have to use about 36 resistors which is not cost effective (the price difference between PDIP28 and PDIP40 AVRs are about 2,5€) ! Even with some molded R-2R ladder it is not cost effective.

Definitively, we should use a PDIP40 µcontroller. In Atmel range, we should use ATMega !

RAM consumption

We need to track the following inputs

Minimum of 13 Bytes of RAM for KMA20 variables.

Communication with flightsim computer

We will use USB to power the KMA20 and to send/receive information to/from the flightsim computer. We can use V-USB library and a few discrete components or use an FTDI uart-USB converter.

We will use USB-B connectors as they are cheaper than mini-B ones and are much more easier to solder by hand. Furthermore, USB A-to-B cables are nearly as cheap as USB A-to-mini-B ones


Here is what I've found inside the installation manual (dimensions are in centimeters, as a european, I love metric system !):

schema of KMA20 Panel dimensions

You can download the SVG source to make your own adjustments. Beware, all of the sizes are deduced from the original size: I've taken the first panel bitmap, imported it in inkscape, adjust the size to what I'v found in installation manual and make manual measures inside inkscape: those figures are not really precise.

I've gone further and made an SVG replica of the panel of the KMA20:

KMA20 Panel replica

The goal is to print this panel on a adhesive paper (with a black'n white laser printer), to cut the grey holes (for switches) and to stick it to a metal panel (with holes also) to have something that looks like a KMA20 panel.

You can download the SVG source to make your own modifications or to print with better quality.

For the moment it is just pure theoric work: I've not attempted yet to build the thing. It will surely need adjustments...


We need lots of cables to connect switches and LEDs to the Printed Circuit Board (PCB). But how much ?

Here is an estimation:

Here are the color codes for each connection:

Lengths by color codes:

Bill of materials

Component Number Manufacturer Datasheet Price
3-positions rotary switch 1 Lorlin CK1032 Datasheet 1,94 €
3-positions toggle switches 9 E-Switches 100SP1T4B1M1QEH Datasheet 2,12 €/by 10: 20,3 €
switch rotary potentiometer for powering up and LED dimming 1 Bi Technologies P170S-FC20BR10K Datasheet 0,94 €/by 2: 1,88 €
current limiting resistors for LEDS (140 Ohms) 3 Yaego MFR-25FBF52-140R Datasheet 0.101 €/by 6: 0,606 €
Microcontroller 1 Atmel ATMega 164A Datasheet 4,84 €
DIP Socket 40 1 FCI DILB40P-223TLF Datasheet 0,54 €
20 Mhz Quartz oscillator 1 CTS Electronic Components ATS20B-E Datasheet 0,362 €
20 pF capacitor 2 Xicon 140-100N2-200J-RC Datasheet 0,086 €/by 2: 0,172 €
Cap for rotational switch 1 Eagle Plastic Devices 450-AE186 Datasheet 0.648 €
Cap for switch rotary potentiometer 1 Eagle Plastic Devices 450-4760 Datasheet 0,81 €
5mm Blue LED Bright 1 Cree C503B-BCS-CV0Z0461 Datasheet 0,211 €/by 3: 0,633 €
5mm Amber LED Bright 1 Cree C503B-ABS-CY0Z0341 Datasheet 0,201 €/by 3:0,603 €
5mm White LED Bright 1 Cree C513A-MSN-CW0Y0511 Datasheet 0,252 €/by 3:0,756 €
USB B connector 1 Molex 67068-9001 Datasheet 0,98 € or samples
USB A-to-B cable 1 Molex 88732-9002 Datasheet 2,25 €
White cable 4 ft Te Connectivity 55A0111-24-9 Datasheet 0,29 €/ by 4: 1,16 €
PCB Headers 5 pins 1 Molex 22-28-4050 Datasheet 0,254 €
PCB Headers 12 pins 2 Molex 22-28-4120 Datasheet 0,399 €/by 2: 0,8 €
PCB Headers 16 pins 1 Molex 22-28-4160 Datasheet 0,531 €
Crimp Housing 2 pins 1 Molex 22-01-2021 Datasheet 0.102 €
Crimp Housing 3 pins 10 Molex 22-01-2031 Datasheet 0,174 €/by 10: 1,74 €
Crimp Housing 4 pins 3 Molex 22-01-2041 Datasheet 0,255 €/by 3: 0,68 €
Crimp Terminal 100 Molex 05-50-0113 Datasheet 0.036 €/by 100: 3,6 €
Wood for enclosure 1 Medium 3mm 2,45 €
A4 Sheet of paper with KMA 20 console printing 1