My Nemesis: 7 Segment Displays

Updated: Apr 6, 2020

If you're building a sim on a budget like myself, then you will have encountered the MAX7219 boards with there two sets of 4 digit 7 segment displays. If you haven't got that far in your build there are certain issues one will encounter when playing with these components.

The main issue is daisy-chaining the units. Mobiflight allows up to 8 boards to be chained together only using 3 data pins. The problem arises when you try to add more than 4 boards in the daisy chain, the data becomes corrupt and you end up with garbled displays. This is after using an external power source set to 5.21V and parallelling VCC & GND to each board with a dedicated GND to the arduino and power supply)

As I was digging through my old aircraft scrap parts I came across shielded twisted pair cable from an old bell 212 de-mod I carried out last year. I then wondered if I could use this for the data lines to the 7 segment displays. Normally you would connect the shield to ground and only use the two-wires in the cable. However, instead of using the shield as I ground, I connected that to the CS (Chip Select) pin and used the Blue wire for CLK (clock) and red wire for the DIN (Data In).

Connecting the 737 Sim Ardunio to MaX7219 Via shielded twisted pair wire
Shielded Twisted Pair Wire

I daisy-chained 8 MAX7219 boards in a row using this cable method. I still DID NOT get 8 boards working when daisy-chained. I did manage to get 6 boards working very reliably. Using this cable, I was also able to increase the distance from the Arduino to the first Max7219 board up to 3 meters. This was the maximum length of twisted cable I had available. With this success, I was also able to increase the distance between the boards to 1 meter. This is a massive step forward for my sim, but I haven't even googled the price of shielded twisted pair cable (LOL - I'm too afraid of the price) But I do have 3 aircrafts worth of the stuff sat behind me right now.

With this new-found knowledge, I noticed that out of the 50 MAX7219 boards, there were some that just refused to work. When I built the MCP's I encountered the same problem. Some were very easy to spot. Such as no solder on the pins at all and others just decided to heat up instead of operating. After testing all my boards, I was hitting a failure rate of 1 in 7. All my boards are from eBay and from the cheapest seller, usually located in China.

This led me to think, "wouldn't it be awesome, if I could just detach the 7 segment boards from the boards and then plug the boards into a socket. Easy swap ability". So I stole another idea from our aircraft at work, which have patch panels. This unit takes all the inputs & outputs and combine them into the relevant connectors. So I decided that this was also the way I wanted to go for the pedestal. If I could take the Inputs & Outputs from the Arduino, feed them to a patch panel which has the MAX7219 boards on, then kick the final result out to its dedicated Pedestal panel, that would make any maintenance a dream! It also allows each unit to be removed from the pedestal with ease.

Hot swap-able max 7219 boards

What is fantastic about doing it this way, is you ditch the requirement to have the Max 7219 board in the panel itself. Usually, the boards are too big for the position you require the displays to be in and the displays are the wrong amount of digits. This allows you to connect any amount of digit configuration up to to 8 digits. For theVHF COMM pedestal panel, I only require one unit of 6 digits. This allows me to go from 24 pins of the max board to only 14! Each radio requires two displays, that equals 28 pins, leaving plenty left over for I/O in a 40 pin connector.

The complete setup, Arduino to patch panel, ribbon cable to 737 VHF Radio

During this process of prototyping which has now seen many variations, I also discovered TinyCAD for wiring diagrams. This is a huge step forward for the sim and website. When it comes to connecting everything, it's now just a matter of connector the dots (lines)! You can see here below, the 6 digit display connect on the left and the two 3 digit displays connected on the right. This makes connecting any size display a piece of cake!


With all this knowledge, which has taken a few weeks! I was ready to create my first panels. I'm not going to lie, I didn't want to create custom PCB 's until I knew this would work. I set out with prototyping boards and solid core wire and armed with a soldering iron. This is how it went (video to follow):

737DIYSIM attaches the 7 segment displays to the radio panels
Radio Displays Mounted

Once the displays were mounted in the correct position, solid core wiring was used to create the tracks to the 20 pin ribbon connectors in accordance with the wiring diagram.

737DIYSIM radio panel Wiring
Displays connected to the 20 Pin Ribbon Connectors

Job Done! Below is the first 737 NAV radio panel.

737DIYSIM Nav panel Prototype
737 VHF NAV Panel Prototype

Below is the finished VHF COMM. VHF NAV & ADF Panels from the rear.

737DIYSIM Prototype Radio panels


The first ADF panel with LITEFLOW lighting under test

It's been a long slog for the last 4 weeks, It's been so much fun doing it this way so far. This means that now getting PCB's created is a proven design. If you have no electrical engineering knowledge, as long as you can solder, all that would be required is to add the components and connect via 40 pin connectors. This would make it a very easy and cheap way to progress anyone else's sim that is using Arduinos and Mobiflight.

If you have any questions or further ideas, drop me a line! Video coming soon, but I have taken over 4 weeks of video footage, and that's a lot of editing!

Best wishes, Stay safe,


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