Tutorial, Display on MAX7219 Tube Module from Arduino

November 17, 2014

A few folks have asked me how to connect a MAX7219, 8 Digit, 7 Segment, Tube Display Module to an arduino and code it to display information from Flight Simulator X (FSX) via Link2FS.
Below are pictures and explanation of one way to set this up.
At the bottom of this article is a working but limited Arduino Sample Sketch that will only display the Autopilot Altitude Set from FSX via Link2FS.  The Arduino Code has been mostly stripped down to this one function for simplicity and is remarked heavily for educational purposes.


In the picture above I am using an Arduino Leonardo Clone.  An Uno or Mega will also work.
To connect the MAX7219 to the Arduino, we use 5 wires connected to the 5 pins on the left side of the module.  Three of them are for communication and two are for 5 Volt and Ground connections.
In this particular example I use pins 10, 11, and 12.  If you compare the wire colors in the picture below with the picture above, you will see Orange, Data (DIN) is connected to pin 12, Yellow, Load (CS) is connected to pin 11, and Green, Clock(CLK) is connected to pin 10.
Red and Brown are connected to 5Volt (VCC) and Ground (GND), respectively.

The Modules have another set of pins on the right side of the Printed Circuit Board (PCB), that are labeled nearly identical to the left side. The right side pin labels are on the backside of the PCB.  They are VCC, GND, DOUT, LOAD, CLK.  If you have more than one module that you want to “chain” together then you do it through these pins.  The second module’s left side, DIN, CS, and CLK would go to the first modules right side DOUT, LOAD, and CLK.

NOTE:  Do NOT chain the VCC 5Volt and GND from one module to another.  As I understand it there is a diode in each module that will drop the voltage on the out going side.  It may work ok with 2 or 3, but if you chain more you may be looking at dimming digits or failure to function.  So, it’s easy enough to run the 5Volt and Ground to a common bus from the Arduino.


In the picture below you can see we are connected to Link2FS and receiving the Autopilot Altitude Set as it is present on the display.



Link2FS Output to Arduino


In the above picture you can see what Link2FS sends to the Arduino when the Autopilot Altitude Set is at 18000.
First the Arduino Code looks for the “=”.
After finding the “=” it checks the next character and find a “b”.
The “b” tells the “Switch Case” in the code to start storing and display the Autopilot Altitude set “18000”.



Hardware Notes:


The picture above is just to show how the displays can be tethered away from the module.  On my first go, I used male/female jumpers.  I found that the female connection to the displays would sometimes become loose and subject to vibration making the connection intermittent.  This would result in some or parts of the display digits to flicker or go off completely.  Jostling the wires would cause it to work again.  This picture has soldered connections on the display side, with heat shrink covering.

The Jumpers and Heat Shrink


Pictured above is the type of jumper wires needed to tether the displays away from the module.
The leads have round male connectors.
NOTE: Dupont brand wire jumpers with square male connectors that work fine with Arduinos, will not fit the female connection on the Max7219 display module.

The picture below:
If you find the need to do a lot of soldering and cover the connections with heat shrink, it is far cheaper to buy heat shrink by the 100ft spool.
I used 3/32 for the above wires.  I found the 100 foot spool on ebay for about $13 US.



Here is the Code with a lot of Remarks
I tried to format it for understanding.

This is a simple demonstration of code to Display information on a MAX7219 8 Digit 7 Segment Tube Display Module.
The Information is extracted from Flight Sim X via a program called link2fs_multi and then sent to the arduino.
There are snippets in this code that came from Jim, the author of Link2FS. 
Find him at jimspage.co.nz. Also, in the mycockpit.com forums under link2fs.

This example will only work with FSX's default Auto Pilot.  
Some planes like PMDG's 737 does not use FSX's default variables.
I use an Array variable to store the Auto Pilot Altitude Set Display.
Its probably not necessary, but its what I used in my Radio Box for quicker updates when using 
shared displays with a Mode/Function Selector knob.
              ***** Info on how the Displays work *****
              *****   This is an excerpt from my  *****
              *****        Radio Box Code         *****
      led_Display_1.setDigit(0,7,Com1Active[0],false);    // setDigit(0,7,Com1Active[0],false) The 0 is the first display, 7 is the first character in the 
      led_Display_1.setDigit(0,6,Com1Active[1],false);    // display, Com1Active[0] is the stored Integer to be displayed, false is no decimal point.
      led_Display_1.setDigit(0,5,Com1Active[2],true);     // "true" puts in the decimal point
      led_Display_1.setDigit(0,4,Com1Active[3],false);    // setDigit is for putting in numbers
      led_Display_1.setDigit(0,3,Com1Active[4],false);    // You can also use setChar for putting in some letters
                                                          // character, symbols, and ' ' space.
                                                          // NOTE: setChar has to use single quote 'A'
                                                          // Double quote "A", is for Strings and wont work.
      ****************** The 300 Level Remarks ****************************************
       In the "300 Level Class", you can use setRow to light one of the 7 segment
       displays using binary or hex equivalent. It would look like this:
       led_Display_1.setRow(1,4,B00110000,false) that would display the number 1.
                __A_      The segments on my displays are lettered A thru H
              F|    |B    However, using the setRow with Binary the H (decimal point) comes first.
               |__G_|             If you want an 8 with no decimal...
              E|    |C             H A B C D E F G
               |__D_|.H          B 0 1 1 1 1 1 1 1   = 8
                                 B 0 1 1 1 0 0 0 0   = 7

#include "LedControl.h"              
                               // ******   LedControl Library is required to drive the MAX7219 7 segment LED displays.  *******
                                                    // led_Display_1 is the variable name of my set of displays chained
LedControl led_Display_1 = LedControl(12,10,11,8);  // together running off of pins 12,11,10.  
                                                    // Pin 12 is the DataOut 
                                                    // Pin 11 is Load or CS
                                                    // Pin 10 is clock. 
                                                    // The 8 is for how many displays you have to chain.
                                                    // I have 4 but put 8 incase of expansion. It cost no memory.
                                                    // You can run any pins you want in the instruction, 
                                                    // just plug the module in correctly.
                                                    // LedControl(Data,Clock,Load,Displays)
                                                    // I used 12,10,11 so I could have a straight ribbon connection
                                                    // from the module without crossing leads.
int CodeIn;      // used on all serial reads from Jim's code
String Digit;    // Variable as a string to take from getChar()
int Count;       // This variable used in loops in the EQUALS() function
int AP_alt_set[5];   // AP_alt_set[5] is an array of the 5 digits that are the Auto Pilot Altitude Set

//*************( START of void setup() )********************

void setup() 
    //The MAX72XX is in power-saving mode on startup, we have to do a wakeup call
    delay (500);
    led_Display_1.shutdown(0,false);      // I have 4 displays, these start them up
    // delay (500);                          // I put the delay in so they all dont start drawing
    // led_Display_1.shutdown(1,false);      // current at the same time.
    // delay (500);                          // I had an issue with running on only USB power and getting a display glitch.
    // led_Display_1.shutdown(2,false);      // The delay seems to have fixed this issue.
    // delay (500);
    // led_Display_1.shutdown(3,false);
    // Set the brightness to a lower than medium values 
    // led_Display_1.setIntensity(1,8);
    // led_Display_1.setIntensity(2,8);
    // led_Display_1.setIntensity(3,8);
    // and clear the display 
    // led_Display_1.clearDisplay(1);
    // led_Display_1.clearDisplay(2);
    // led_Display_1.clearDisplay(3);
//**************( END of void setup() )********************

//*************( START of void loop() )********************

void loop() 
    if (Serial.available()) 
        CodeIn = getChar();
        if (CodeIn == '=') 
            EQUALS();              // The first identifier is "="
   // if (CodeIn == '<') {LESSTHAN();}// The first identifier is "<"
   // if (CodeIn == '?') {QUESTION();}// The first identifier is "?"
   // if (CodeIn == '/') {SLASH();}// The first identifier is "/" (Annunciators)
//*****************( END of void loop )********************

//************( START of getChar() Function )**************

char getChar()// Get a character from the serial buffer
  while(Serial.available() == 0);   // wait for data
  return((char)Serial.read());      // Thanks Doug
//***************( END of getChar Function )***************

//*************( START of EQUALS() Function )**************

void EQUALS()               // The first identifier was "="
    CodeIn = getChar();     // Get another character
    switch(CodeIn)          // Now lets find what to do with it
        case 'b':           //  AP altitude set
            Count = 0;               // clear the Count
            while (Count < 5)        // we will count to 5 from 0 to 4
                Digit = "";          // clears the string variable Digit
                Digit += getChar();  // Makes the string Digit what ever getChar() is holding
                AP_alt_set[Count] = Digit.toInt(); // Turns the string Digit into an Integer and
                                                   // then stores it in the Array AP_alt_set[] 0,1,2,3,4
            led_Display_1.setDigit(0,4,AP_alt_set[0],false);   // First digit of Alt Set is displayed  
            led_Display_1.setDigit(0,3,AP_alt_set[1],false);   // Second digit of Alt Set is displayed 
            led_Display_1.setDigit(0,2,AP_alt_set[2],false);   // Third digit of Alt Set is displayed 
            led_Display_1.setDigit(0,1,AP_alt_set[3],false);   // Fourth digit of Alt Set is displayed 
            led_Display_1.setDigit(0,0,AP_alt_set[4],false);   // Fifth digit of Alt Set is displayed 
                                                               // false denotes NO decimal point
    *********    IF YOU TRY TO USE THE BELOW CODE IT WILL NOT WORK    ************
    *********        IT HASNT BEEN FULLY SET UP IN THIS SAMPLE        ************
        case 'c':    //  AP vertical speed set
            Count = 0;
            while (Count < 4)
                Digit = "";
                Digit += getChar();
                if (Count == 0)
                    if (Digit == "-")
                        AP_vs_minus = '-';
                        AP_vs_minus = ' ';
                    Digit = "";
                    Digit += getChar();
                AP_vs_set[Count] = Digit.toInt();
        case 'd':    //  AP heading set
            Count = 0;
            while (Count < 3)
                Digit = "";
                Digit += getChar();
                AP_hdg_set[Count] = Digit.toInt();
            if (AP_hdg_set[0]==0 and AP_hdg_set[1]==0 and AP_hdg_set[2]==0)
                AP_hdg_set[0]=3; AP_hdg_set[1]=6;
        case 'e':    //  AP course set
            Count = 0;
            while (Count < 3)
                Digit = "";
                Digit += getChar();
                AP_crs_set[Count] = Digit.toInt();
            if (AP_crs_set[0]==0 and AP_crs_set[1]==0 and AP_crs_set[2]==0)
                AP_crs_set[0]=3; AP_crs_set[1]=6;
       //etc etc etc
       // You only need the "Case" testing for the identifiers you expect to use.
    ******************************************************************************  */
    } // ***  End of Switch Case ***
//***************( End of EQUALS() Function )**************


FSX Radio Stack, My First Design

August 5, 2014

This project was conceived and started with the ordering of parts around Sept-Oct, 2013.
Functional construction and coding was completed mid Nov, 2013.
A friend and I worked on this together.  He was also building a radio.  His was a bit different than mine with a smaller footprint for a fighter cockpit setup.

20140107_105555_640x480If you are an ate-up, Flight-Sim geek, then you may have a setup similar to that above.
In the picture we have MicroSoft Flight Simulator X (FSX),  CH Yoke, CH Throttle Quadrant, CH Rudder Pedals (on the floor), three 24 inch displays in span view, a single 21.5 inch overhead display, TrackIR (head tracking device), and on the right side of the desk, lit up with red 7-segment displays, is a custom built Radio Stack of which this article is about.


Here is a close up with a $1 bill for scale.


This project would not have been possible without a software interface program that allows Arduino cards to talk to FSX.  The program is called Link2fs and was written by a man named Jim who lives in New Zealand.  I have been following Jim at jimspage.co.nz for a number of years.  He has built at least two incarnations of a flight simulator motion platform.

A motion platform is the dream.

I had dabbled a bit, in the past, with Arduino.  Once I built an interface for a recumbent stationary bike-to-PC using an Arduino, a magnetic reed switch, and a game controller.  It allowed the user to run in first person shooter games by pedaling the bike.
When I read that Jim had discovered Arduino and he had written the Link2FS interface software to talk to FSX, I had to give it a shot.



Let me warn you…
If you start down this path of building interface devices for Flight Simulator, it is a dark path and it never ends.  Each new buzzer, bell, or whistle that you make will give you a “discovery and accomplishment” high.  It will make you feel as if there is nothing you can’t do.  But the high is fleeting.  It does not last. You will soon become discontent with the status quo.  Then you start thinking, “Well it would be easy to add this or to add that.”  It won’t stop until you have a full scale replica cockpit consuming substantial square footage of living space in your home.  If your family is lucky, you prefer small general aviation aircraft over commercial jets.


First We Crawl

The first thing I needed to do before ordering some of the more pricey parts was some proof of concept testing.  I had downloaded Jim’s software and had confirmed that it did indeed connect to FSX and could change things, like radio frequencies.
Next, I hooked up a simple push button switch on an Arduino clone and modified some of Jim’s example code, to make the switch change one of the radio frequencies in one direction.
That’s all it did, you press the button and the Mhz go up by one tick.
As simple as it was, it gave me the first rush.
One of the other things I did, not pictured, is wire up some LEDs as Landing Gear status lights and used some more of Jim’s sample code to make them work.  By experimenting with the Link2FS software and different Arduino code, it allowed me to learn more about the nuance and communication relationship of FSX to Link2FS and Link2FS to Arduino.

20131003_155237-640x480Arduino Leonardo Clone with simple push button switch.

Pictured below is a snapshot of an older version of Jim’s Link2fs_multi program.  It is the program that interfaces FSX to Arduino.  It is the middle man, that sends and receives from FSX and also sends and receives from Arduino.  The “multi” version allows you to control multiple Arduinos with one instance of the Link2fs program.  Need more Arduinos than one instance can handle?  Not a problem.  You can also run multiple instances of Link2fs_Multi.
FSX <-> Link2FS <-> Arduino.

20131003_155349-640x480Link2fs_Multi, interface program for FSX to Arduino.

The Acquisition

Now that I have proved that it is possible to change things in FSX from an Arduino, it is time to start finding components to build a Radio Stack.
Many different aircraft avionics utilize “Dual Concentric Rotary Encoders”.  These are basically 2 dials in one.  Elma makes a good one in the E37.  Unfortunately, they aren’t cheap and they aren’t easy to come by.  There is one guy on the planet, that I know of, that sells them in single quantities and that’s Leo at leobodnar.com in the UK.  At the time of this writing one E37 with knobs, from Leo, shipped to the US via Royal Mail Airmail would be $40.26 USD.  In Dec of 2013, I had enquired at Elma to see what I could get if I wanted to buy bulk.  At 100 units they would cost $17.50 per each not including knobs.

I bought three E37s from Leo, one for me, one for a friend and one for a spare or future project.

20131026_214818-640x480Elma E37 Dual Concentric Rotary Encoders.

Since I didn’t want to drop big money for multiple Encoders, I decided to use a Rotary Switch as a function selector on the Radio.  You dial the function you want with the switch and then use the encoder to change the frequency.
Functions that I coded in were: COMM/NAV 1, COMM/NAV 2, TRANSPONDER, ADF, and a limited AUTO PILOT function for experimentation.

20131029_125459-640x480Rotary Encoder on the left.  Rotary Switch on the right.

The Elma E37 Rotary Encoders are not plug-and-play.  They come with a small Printed Circuit Board (PCB) that needs to be soldered to the encoder.  The PCB has marked pinout holes in it that allows you to also solder wire leads to it.
When I first coded the Arduino to work with the Encoder, again taking Jim’s sample code and modifying it for my purpose, I got the accomplishment rush again,  “I am turning a knob and the frequencies in the sim are changing!  So cool, so cool!”   When I showed it to my friend, his response was a satisfied,  “Ok, that’s perfect.  I’m done.”  As if that is all he needs to make him happy, he needs no more.  But no, that was just his first “high“.  We didn’t even have the 7-Segment Displays yet.

20131029_125714-640x480MAX7219 8-Digit 7-Segment Tube Display Module

While looking for a 7-segment LED display solution, I came across a number of possibilities.
Some were serial driven modules that had 16 digits.  I had also seen other folks actually drive multiple single digit 7-segment displays by multiplexing outputs on Arduino.  An incredible accomplishment with an insane amount of wires but way too complex for me.  Plus it uses too many Arduino input/output pins.
I finally stumbled onto the MAX7219 8-Digit 7-Segment Tube Display Module.  The chip on the Module is the MAX7219 which is the driver for the displays.  In short, it receives a serial instruction, displays what the instruction told it to display, and holds that display until told to do otherwise.
These displays work with the LedControl library for Arduino.
The beauty of these display modules is that they are comprised of two 4-digit displays that are not soldered to the board.  They are in sockets.  This got me to think that I can tether these displays with wires and place them any where I want to on my radio without the PCB getting in the way.
The other great thing about these displays is that you can chain up to eight of them together and only have to use three pins on the Arduino.

20131029_021651-640x4803 MAX7219 Display Modules chained together for testing, pre tethering.


20131102_023537-640x480Here are 3 MAX7219 Modules with the 7-Segment Displays tethered away from the boards.
Also pictured is the Rotary Encoder, Rotary Switch and Arduino and experiment board for wire junctions behind the Modules.

Box It Up

The hardware and software are now working together.  We need to get all of those wires in a box.

20131107_121005-640x480This is the face plate that my friend helped me come up with.


20131107_232320-640x480The rough fit.  I added a 4th MAX7219 Display Module for Transponder and DME.

20131108_234100-640x480The back side fitting after paint had been applied.

20131109_024326-640x480Lexan cover for the face plate.

20131109_021000-640x480Backside with a dollar bill for scale.

Acceleration on the Encoder and updated functions.


After months of use and testing, some things I would do differently…

The female end of the tether wires on the 7-segment displays doesn’t cut the mustard.  Sometimes, from movement, the segments on the displays will flicker because of the female ends slipping.  A little jostling and they work.  To do it better, the tether wires need to be soldered to the 7-segment displays.  The other end of the tether wire can still be plugged into the MAX7219 Module without problem.

I am planning on designing and building a new Radio Stack with multiple displays and multiple encoders instead of sharing the main display and encoder with multiple radios.

The Sickness Continues…

20140124_212133-640x480Scale print out of the next project, a Mode Control Panel (MCP)/Auto Pilot for a Boeing 737.