Extra consideration is required with power supply design if you intend to use a Nextion HMI in a project with audio

Low level noise was being generated by a Nextion (and therefore a TJC) display when the display brightness was reduced down from 100%.

This noise was traveling back down the PSU line into the audio circuits powered off the same mains to 12V PSU.

The backlighting level is controlled by PWM, and that’s the source of the problem.
You’ll hear a low level ~300Hz (and harmonics ie; 300Hz, 600Hz, 900Hz… ), and it’s quite evident on an audio spectrum analyzer.

For my project, I tried powering the Nextion off a separate 5V regulator to the MCU, but that didn’t fix the noise problem. The noise was still getting back into the audio equipment.

When I temporarily powered the Nextion-based project and the audio equipment off separate 12v PSUs, the problem went away.

The solution was to have a relatively large inductor (10mH wire wound with a suitable current rating, 270mA in my case) and a large electrolytic on the input of the 5V regulator feeding the Nextion. This prevented the noise traveling back down to the common 12V PSU.

Typical wire wound 10mH inductor:

A 12V rail feeds this 5V regulator which then feeds the MCU board and Nextion: