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16x8 LED Matrix Flicker/noise
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

16x8 LED Matrix Flicker/noise

by pcmofo on Wed Jul 15, 2020 10:39 am

I'm using 4x 16x8 LED matrix displays in my project. https://www.adafruit.com/product/2043.

Currently powering them via 5v USB on a Feather M4 Express or a dedicated 5v external supply.

Displays are on brightness level 2 and is only being drawn to once and never updated again.

The displays emit a high pitched whining noise when lit.

They also have a slight flicker randomly that looks like glitching pixels that slightly fluctuate as if the display is refreshing every few seconds. It's rock solid stable then a wave of random subtle brightness shifts on random LEDs then back to solid again.

This happens on a dedicated 5v PSU and when powered directly from the microcontroller.

I'm not sure if the noise and flicker are related but I would like the display to be rock solid as it is a clock and on 24/7

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Re: 16x8 LED Matrix Flicker/noise

by adafruit_support_bill on Wed Jul 15, 2020 11:28 am

The displays are multiplexed. That is likely the cause of the noise and perceived flicker. With 4 of these going, your peak current demands are going to be in the 640mA range. (Which is stretching the limits of what most USB ports will supply.). Can you isolate the source of the whine? Is is coming from a specific board? Does the pitch change if you unplug one or more boards from the system?

The multiplexing can interact and create 'beat frequencies' with non-continuous ambient light sources such as fluorescent and most LED lighting. As incandescent lighting is becoming obsolete and replaced by LEDs, we are seeing more of these effects.

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Re: 16x8 LED Matrix Flicker/noise

by pcmofo on Thu Jul 16, 2020 3:34 pm

adafruit_support_bill wrote:The displays are multiplexed. That is likely the cause of the noise and perceived flicker. With 4 of these going, your peak current demands are going to be in the 640mA range. (Which is stretching the limits of what most USB ports will supply.). Can you isolate the source of the whine? Is is coming from a specific board? Does the pitch change if you unplug one or more boards from the system?

The multiplexing can interact and create 'beat frequencies' with non-continuous ambient light sources such as fluorescent and most LED lighting. As incandescent lighting is becoming obsolete and replaced by LEDs, we are seeing more of these effects.


Thanks for the reply Bill. I switched to a dedicated PSU and the display flickering went away. Here is a video of the whine noise. https://www.youtube.com/watch?v=Wrj_AGH3FmM
It seems it only happens when the LEDs are lit and the noise also makes it to the Sound board and comes out the speaker in addition to the displays themselves.

I'm still looking for a good way to power everything in such a way that I can leave everything hooked up but still plug in USB to re-program. I've measured at least 1A when the display is on being pulled by everything so I am guessing I may need 1.5-2A to be on the safe side. Currently using a LM317 which of course overheats when not enough current is being drawn. I could go right from a 12v2A wall wart into some simple adjustable switching power supplies that are set to 5V and maybe have independent switches and diodes going into each board. That way I could turn off power to the M4, plug in USB and program it while the external PSU powers everything else.
IMG_1272.JPG
IMG_1272.JPG (743.7 KiB) Viewed 30 times

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Re: 16x8 LED Matrix Flicker/noise

by adafruit_support_bill on Thu Jul 16, 2020 4:12 pm

The sounds you are hearing on the sound board are most likely due to voltage fluctuations on the 5v rail caused by the current surges from multiplexing the display. This can probably be addressed by some bypass caps on the power rails - and/or not having the sound board share a power rail with the display. In this case, the breadboard is not helping matters because of the combined contact resistance from the jumpers & pins.

The noise coming from the display sounds like vibrations from the chips themselves. Switching of significant current to the LEDs creates changing magnetic fields within the devices which creates the vibration. You can hear similar effects from open-frame switching supplies - or even some LED light-bulbs when you try to dim them. An enclosure for your clock will probably go a long way towards blocking that noise.

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Re: 16x8 LED Matrix Flicker/noise

by pcmofo on Tue Jul 21, 2020 5:08 pm

adafruit_support_bill wrote:The sounds you are hearing on the sound board are most likely due to voltage fluctuations on the 5v rail caused by the current surges from multiplexing the display. This can probably be addressed by some bypass caps on the power rails - and/or not having the sound board share a power rail with the display. In this case, the breadboard is not helping matters because of the combined contact resistance from the jumpers & pins.

The noise coming from the display sounds like vibrations from the chips themselves. Switching of significant current to the LEDs creates changing magnetic fields within the devices which creates the vibration. You can hear similar effects from open-frame switching supplies - or even some LED light-bulbs when you try to dim them. An enclosure for your clock will probably go a long way towards blocking that noise.


Thanks again Bill for helping me with my project.

I'm planning on having my own PCB made to an exact size to fit my project and essentially be a 4x version of the existing 8x16 LED matrix. I was hoping to resolve these kinds of issues and prove my prototype works well before having a PCB made as it is about $60 in components for just the LED/driver chips.

I happen to have some 100nf, 1nf, and 1uf caps from an audio project that I tried on the 5v line as decoupling caps. I also tried various combinations of powering different boards from different supplies with grounds tied together. Nothing seemed to have any effect on the noise in any way that I could hear. An oscilloscope might show me something but I don't have access to one.

I tried placing the caps in different locations on the breadboard. I'm now wondering if one or more of the matrix chips is producing the noise and if it would be worth tearing apart the display to get the caps closer to the chips.

Any other suggestions or things to test?

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Re: 16x8 LED Matrix Flicker/noise

by adafruit_support_bill on Wed Jul 22, 2020 7:08 am

If you press on the chips with your fingers, does it damp or alter the noise?
Also try with the on-board capacitor (the little tan-colored component).

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Re: 16x8 LED Matrix Flicker/noise

by pcmofo on Fri Jul 24, 2020 11:09 am

adafruit_support_bill wrote:If you press on the chips with your fingers, does it damp or alter the noise?
Also try with the on-board capacitor (the little tan-colored component).


I was able to partially disassemble my display assembly and run a single 16x8 segment at a time. If I press on the chip on the back the pitch and volume of the noise both drop.

Each of the 16x8 segments has this issue and appear to be summing the noise outputs increasing the noise when they are all on. The volume of one is barely noticable, but combined it is annoying within 2-3 feet. Combined with the audio fx board which also amplifies the noise in the system.

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Re: 16x8 LED Matrix Flicker/noise

by adafruit_support_bill on Fri Jul 24, 2020 11:46 am

The noise in the Fx board is likely due to noise on the power and ground rails and can probably be cleaned up with some bypass caps and separate power and ground wires direct from the power supply.

The acoustic noise from the chips is not something you can suppress with capacitors. It would need to be dealt with via damping and/or shielding with an enclosure.

Some thermal tape and a heat sink on the back of each would probably function similarly to your finger. https://www.adafruit.com/?q=heat%20sink
An enclosure of some sort would further block the noise.

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Re: 16x8 LED Matrix Flicker/noise

by darsk on Thu Sep 03, 2020 8:43 pm

I had a similar problem which was solved by using the M0 board instead of the M4.

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Re: 16x8 LED Matrix Flicker/noise

by pcmofo on Thu Sep 03, 2020 9:19 pm

darsk wrote:I had a similar problem which was solved by using the M0 board instead of the M4.

Interesting. How/why do you think the M0 solved the problem?

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Please be positive and constructive with your questions and comments.