MobiFlight Community Support

Hello!

I am a noob in those things and I want to make a Boeing 737 MCP using 7-Segment Displays.

Connected 2 displays in daisy chain of 2 and looked like all was good, but sometimes it works fine for 5+ min, other if I start to change values the second display goes blank or shows random digits.(screen below how it's connected)


Also, there's another problem when I press run in MF runs only 1 display. If I press Upload config a few times, then the second display works too. (Screen below)


Also happened that started to get absolutely random digits(Screen below)


Tried to disconnect the 2 display, and when only 1 is connected, all looks fine. Tried to connect the second one alone, and he works fine too. 2 displays connected not in chain works good too.


Then I resoldered the inputs to the displays and the problem doesn't fix.

After all decided to connect 2 displays in daisy chain of 2 but connected the 5V of the second display directly to Arduino and the problem almost disappear, but some small parts of the second display becomes blank when I started to change values after 5 min of stand by.(Screen below)


After this, connected 2 displays in daisy chain of 2 but 5V and GND from the second display directly to Arduino 5V and GND and the problem looks solved, can't get any of the upper problem.

So the question is - in total I want to connect 4 MAX7219. 7-Segment Display, how do I need to connect all those displays to not receive these problems?
Thank you!

You have already discovered a developer error. The voltage decreases by 0.7 V from module to module. This is due to the diode D1, which is installed on each module to prevent reverse polarity of the operating voltage and to protect the 7219.
In this respect, it was absolutely correct to supply each module separately and directly with 5V. To protect the Mega from over-powering, I strongly recommend powering the display modules with a separate power supply.
This external power supply can be dimensioned a little more generously, then you can also operate stepper motors and other display modules.
When connecting external voltage sources (also with mega modules), please always ensure that the respective GND is connected to the first module. This is extremely important electrically!
So at least you shouldn't have any more problems with your displays.
Something else about the cables: Plug connections, especially with the MAX7219, can always lead to problems. The MAX7219 is extremely sensitive and therefore prone to failure when it comes to cables. I strongly advise against inexpensive, not to say cheap, DuPont cables! That's enough for breadboard applications, but totally unsuitable for good and solid and permanent connections. Good plug connections on one side and soldered connections on the other side are recommended here.
Please tell us about your experiences and whether it worked.

Thank you for the answer, Stephan.

After your answer I got another few questions)

• How do you recommend connecting each module separately, to what, and can I solder 5 wires from 5 displays into one?

• What power supply can you recommend, a default 220 adapter to 5/9/12v?

• How is it possible to over-power the Mega, because I thought it was impossible to overpower through USB?

• What connections can I use to the Arduino, i can solder the wire do the display but with Arduino I have no idea what to do. Unsolder the pins from the Arduino or use a protoshield?

• If I connect a 12v adapter to Mega, it will not help with a lack of power or over-powering?

I will definitely tell about the experience, I'm making a mini "blog" that I will post on Instagram.
I already connected each display with DuPont cables and after a few hours of testing looks like it's working without issues.

1. If the modules are connected as a daisy chain, the connections are simply looped through from board to board. This does not apply to the +5V connection.
Please always connect the 5V from the external 5V directly to the board.
2. Any power supply can be recommended as the power supply, the b output voltage of which is 5V and which is stabilized. Depending on the load, 1 - 3 A should be sufficient as a current. If a limit is also reached here, use another external power supply unit. Make sure to connect the GND to the other external power supply and both to the GND of the Mega or Megas.
3. A USB port can be overloaded (USB 2.0 up to 500mA, USB 3.x up to 900 mA). The voltage at this USB port then simply collapses and the connected components simply no longer work.
4. The most important connection or connections on the Mega are those from GND. Here all GND from other components should be brought together in one point so that there are no complications. Nothing has to be unsoldered or otherwise removed from the Mega for this purpose. You simply take a GND connection, for example, and lead it to an external point, where all the other GNDs are then routed. This ensures that all components run via a GND and therefore all have a common reference potential.
5. Connecting a 12V adapter to a Mega is unlikely to be successful, because:
- the voltage regulator on the Mega has to handle the full power, so 12V - 5V = 7V multiplied by the current gives the power of the on-board voltage regulator. At 1 A current, that would be 7 W of power, which the small voltage regulator would certainly not be able to cope with.
- An external voltage or current source would make more sense here, because the voltage would be brought in externally and would not burden the Mega. In addition, if necessary, several external voltage sources can be used.
My experience with DuPont cables is not the best. Too many have simply failed for that in the past. They can be used for normal prototyping, but they are less suitable for final assembly. Soldered connections are always preferable here. Soldering one side of the cable is also possible if high-quality connectors are used on the other side. Ultimately, I can only make recommendations based on my experience. If someone else has had good experiences that don't match mine, then everything can work wonderfully with that too.

Thank you so much for the answers.

In the second question, I meant how do I solder the wires to the Arduino?

You can use any plug connections, that fit into the mega sockets.

Just a little point to the main Problem.....

Whatever the problem was the power.... There is a fact that should be talked about, cause this happens also lots of times for other users maybe.

You say....

Tried to disconnect the 2 display, and when only 1 is connected, all looks fine. Tried to connect the second one alone, and he works fine too. 2 displays connected not in chain works good too.
...
Then I resoldered the inputs to the displays and the problem doesn't fix.

If both Displays work fine in a single Setup.... But as the "second" one in a chain it stop working....
THEN
We know 100% there must be a issue in the solder connection of the Right Side (Output) of Display 1 in the Chain.
Cause we know both work fine single (so left input Connectors must be ok)

Best way to test if you like to build chains....

1. Test all Displays you like to use as a single Setup (Not a chain).
If they work you know 100% the Left Input Side is Fine !
2. Build a Chain of TWO Display. (Only with Displays you still confirmed working in Step1)
If the second display work fine then you know 100% that the Right Output Side of the FIRST Display is also OK.
3. Remain the Second Display on its position but now use every Display one Time on Position 1 in the chain.
Always if the Second work fine you have confirmed the first is working good.

In easy words.... Do this procedure to confirm all Displays have a working LEft and Right Connector side.