Going back to electronic after a 20 years break

The first part is a sort of generic main board including : Arduino nano, ST7735 screen, level translators for the screen, usb input, and easy to connect Axxx and Dxxx lines Nothing special here The couple of mandatory pics : At first i had problem with the screen, that looked dim It appears that powering the screen through the arduino is not a good idea. That's why there is a separate micro USB connector that powers everything (not sure it can handle "high" current as it is a micro usb to DIP adapter board with rather thin tracks). That main board then connects to a daughter board, sending it power + Axxx lines + Dxxx lines

I have a lot of 18650 batteries, salvaged from laptops or power banks. Charging them is always a chore, so let's make something smart and arduino based. The requirements are as follow : Safe Cheap Can manage 3 or 4 batteries Display charge level and current consumption Be usable with a regular 2.1A usb charger  For the last point, we can either dumb down the max current per unit i.e. for example max 0.5 A / battery x4 ~= 2 A, or delay charging until a slot becomes available. We'll go for the latter. The system will be made of 2 boards, stacked on top of one another : A display board : With a nice  and big color screen and the arduino (3.75$ screen ST7735 based) + outputting Axxx and Dxxx lines from the arduino over dupon connectors + 5v/GND A control board, managing 3x batteries  The idea is that the display board can be easily reused for other projects, such as 2S battery charging. Initially;  i intended to use Nx INA...

Since i bought them, when i connect my QC25 to my Yamaha amplifier there is a medium pitched humming. It is barely noticeable when noise cancellation is disabled, but very present when active. The low end DSO scope confirms that : There is a small 180 hz signal on the wire, some noise generated by the amplifier. That signal  is amplified a lot by the noise cancellation circuit of the QC25 and is  really  annoying. Since it is 180 Hz, it cannot be simply filtered out, else we'll damage the input. Solution : Use the airplane adapter, the buzzing becomes not noticeable. But what's inside ? It's actually very basic :