Audio interference problem with WS2812B strip

[scottmarley]

Hi all. I designed and built a circuit based on the Arduino Nano to make light-to-sound VU meter. The same 12V supply runs the amplifier and the VU meter (which is stepped down to 5V by a buck converter). Stupidly, I forgot to test the circuit with the same power supply powering both the amplifier and the VU meter before I got some PCBs made. Images of the schematic, block diagram and board are here.

When the lights are operating, an annoying whine is heard from the speakers via the amplifier. See video here. At first I thought is might be the buck converter, so I made up a version using a linear regulator and it behaves the same. Then I thought it might be the nano (it's a knockoff!), but if the lights are lit whilst the nano is being reset, I still get the interference. It also occurs using a genuine Uno.

I can cut out the whine entirely by using a ground loop isolator between the output of the VU meter and the input of the amp, but I was wondering if I have made some sort of obvious mistake in my design that someone might be able to point out? That way I can correct it at the component level rather than adding an isolator after the fact.

Thanks everyone, and please ask me to clarify if anything is unclear.

[stj]

your audo is capacitivly coupled to the cpu power rails.
either hush the cpu with lots of ceramic decoupling and maybe a choke,
or consider a 1:1 transformer or optocoupler

[scottmarley]

Thanks for the response. I added a lot of decoupling and it has definitely gotten quieter. There wasn't a huge change until I added 2200uF to the power lines for each strip, now it's more or less acceptable. I also bulked up some of the ground connections which also made a small difference. Cheers.

[Audioguru]

You show two capacitors marked "1mF". What is that since capacitors are marked in uF, not mF.

Light to sound?? What do you want the sound to do? Increase the frequency as the LED display goes up? Or increase the loudness of a single tone as the LED display goes up? Or both?

[scottmarley]

Hi, I meant of course Sound to Light, I'm sure you're aware of what a vu meter does. I'm also sure you're aware that 0.1mF is 100uF and yes, I know mF isnt normally used, it was changed by accident. I can't help but feel that you're being somewhat disingenuous with your response. Please remember that this is a beginners forum and we don't always get everything correct. As I mentioned in my reply to stj's much more helpful response, I've added a lot more decoupling and its now a lot quieter. Thanks.

[tooki]

Please upload the images as attachments here, so they don’t break if/when imgur goes offline, leaving the thread useless. (I’m in the process of doing that to my old threads, which suffer the same problem.)