Noise problem with MAX7219 displays

Hello all

I have a front control panel, that has it's own 7805 reg running it. 12v in, with 100uf elect and 0.uf poly caps either side. My usual setup.

This 5v supply runs some illuminated led buttons and 4x 8 digit LED displays with a Max7219 controller on board.

Similar to these:

They work fine, but are introducing noise on my ground line, which is creating noise through the amplifier back in the main control box.
If I unplug just the displays, the noise immediately stops.

I have put ferrite coils on the supply lines, and caps directly onto each MAX7219 module, but the noise persists.
I have no options to add a separate 12v power supply (no room and the unit has only 1x 12v input), so I need to eliminate it.

Anyone else had issues with these modules? Any ideas for other isolation methods?

Help would be gratefully appreciated.

Oh... and this forum doesn't seem to work in Firefox? Login features are missing

But you haven't tried a common-mode choke yet? That would be next on my list.

However, to be honest, I think it's all going to be trying to patch up a bigger problem somewhere else. I'm surprised this particular kind of module would generate noise that would be problematic, and if it does, then there is likely to be not optimal in terms of circuitry and PCB layout elsewhere.

So perhaps let's try to get down to the bottom - can you tell us a bit more about:

  • why/how the noise is problematic; i.e. what problems does it cause elsewhere?
  • What does the noise actually look like; can you post some scope traces?
  • What does the entire circuit look like; i.e. can you post a schematic?


No schematic (yet anyway)... it's kinda make it up as I go
No scope.... ain't that rich yet!

There is an MP3 module (JQ8400) that works fine off the Mega2560p. The only time I get a whining noise through the amplifier is when it is talking to the Max7219'S.

Once I have sent a display command to the Max7219, the whine kicks in and then becomes permanent.

Unplug the 7219 modules and that noise leaves. Maybe it's the data lines?

Hmm... don't have any chokes

I would expect the problem to be caused by poor layout of the ground (0V) conductors, with current from the 7219 flowing in the same conductors that make up the ground for the audio circuits. Please post photos clearly showing how everything is connected up, the problem might be obvious.

As a general rule you need star grounding, which just means all the different grounds come together at a single point. In some manufactured equipment this is obvious because there is a nut and bolt in the chassis with lots of ground wires all terminated at that point.

Look at this:

There are 2 separate circuits linked by a common ground, the power for the device 1 does not flow in the same conductor as the power for the device 2, so they cannot interfere with each other. The only current shared between them is the signal between device 1 and device 2, and even that does not share either of the 2 ground conductors. So, you have 3 different ground conductors, all linked, all carrying different signals and not interfering with each other.

The only thing different to what I said in my reply #4 is that in this example the star connection is not really a star, but that is for clarity in the schematic, you could (and probably should) bring the 2 separate ground points together, not have them as I have shown with a wire between them.

Try 5v-5v or even 12v-5v isolated dc-dc converter to separate your display supply from audio supply. Have a look at something like B0505S or B1205S. they come in 1W and 2W versions.

Noise problems rarely have off the shelf hardware solutions. It's a potpourri of issues. Hearing that it was, "make it up as I go" certainly adds another level of uncertainty. This makes images and diagrams mandatory.

Your "amplifier back in the main control box" may have grounding or shielding issues.

By the way, Firefox works fine on this site, for me.

Thanks all.

I have separate grounds run back to a bus at the PSU.

The amps (2x Velleman 4001's) are directly off the 12v input jack.
The MAX7219's do share a ground with the push button leds, but I didn't think that would be an issue.

I will continue poking around and maybe try and put together a schematic (although it's pretty complicated)


I would begin right at the display module. Often those don't have adequate supply filtering. If you can kill the noise at the source, you won't have to worry about how it propagates through your wiring network.

Honestly, the fast track for this one is probably not a schematic, but images... especially around the grounds, display module and amp.

Certainly. And a schematic to go with it, cause otherwise we'd still be clutching at straws. But the photos would certainly help. If only because they will elucidate why/if there's no room for a separate power supply for either the amps or the display. Things really have to be crammed in there if there's no room for a small linear regulator and a handful of passives.

OP reports a 7805 regulator in the display unit. But I don't like trying to imagine how a system is laid out. Especially when analyzing a noise problem.

Adding to all the other questions, how fast are you running the display interface?

If it is noise being picked up by an amplifier then this is probably made worse by the high input impedance has. In the past I have used a 4K7 pull down resistor and AC coupling to greatly reduce the noise pickup, like this.
Audio amp
The 100K log law pot acts as a volume control. And don't forget the decoupling on the amplifier's power input.

Saves me pointing it out.

But I'm waiting for actual information - schematic and pictures - before commenting further. :face_with_raised_eyebrow:

Noise problems with MAX7219 ? Look also at the almost identical MAX7221 which appears to have been introduced, partly to solve these. From the joint data sheet;


Mind you there is a substantial difference between EMI and audio noise. :face_with_raised_eyebrow:

Thanks all. Photos of this messy car crash will probably get me banned! Lol

I will draw up a schematic, and I started last night.... but I am using all the pins of the Mega2560p, so it's quite involved.

A for PSU... 12v comes in. Goes to a Stripboard PCB that houses 6x 7805 regulators. 100uf and 0.1uf on the incoming and outgoing of each.

These individually feed 6 sets of RGB Led strips.
12v goes directly to the Amp on a separate line directly from the 2.1mm input power jack, via a small relay that is switched using a TIP122. This allows me to turn off the amps when not needed.
12v Also goes to the front panel as mentioned earlier, that has it's own 5v reg to supply the MAX7219's and the illuminated buttons.

MP3 module is local to the Mega2560p and uses its onboard 5v supply. This seems OK.

Working during the day, so update later

How fast am I running the display interface? Not sure! Didn't know you could vary it.
I am using the "LedControl.h" library that works well (and I have used it before).
I send the text once and it stays like that until I need to need to update the text.

Thanks, Steve

Translation: I'm afraid that posting images will result in my having to clean up my wiring. I would rather that the problem be something else, and I will continue to dribble out verbal details until I get an answer that doesn't point to my wiring.

I spent the whole of last night drawing up the schematic. I was joking regarding the wiring.
But, if you want to be unhelpful, aggressive and sarcastic.... fine.

I have solved the issue, but I am sure you being so damn clever, you don't need to know what it was

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Looks pretty fancy! Nice you solved the issue.