As you can see in the photos, there are 6 LEDs (in parallel) for backlighting. Any idea on which component is driving these LEDs? The chips with partnumbers are for touch panel and audio DAC. If I can find component that provides power to the LEDs, then I can isolate that component and I can inject my own PWM signal (and not via on-screen display).
Unfortunately, for these particular Waveshare displays, PWM control of the background is very difficult, as they take power from the 3.3 V and 5 V Rails which are fixed on the Pi. The simplest way to get around this one is to rewire the pinout of the Pi on a breadboard in order to provide a modulating signal to the appropriate pins on the Waveshare display. Please let us know how you go and if there’s anything else that I can do for you.
PS. Be sure to check the current and voltage outputs of the GPIO and use a MOSFET or similar device to devise an appropriate way to power the display and provide control signals from the pins.
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