[IDEA] [Tut] How to turn off Backlight of a Waveshare 7" HDMI LCD using GPIO and screensaver

[Lucatze]

As far as I can tell, the display control circuit is not affected by the backlight switch. The touchscreen keeps definitely working, even with switched off backlight and there is no kind of "resync " process when turned on. However, its true that the backlight leads react sensitive to very low voltage as it might happen, that it doesn't turn on at all if you dim it till the point it turns off by itself. Meanwhile I replaced the relay with a transistor for smoother dimming. It works really good as long I don't dim too much. Though, it is possible to drive it at very low brightness. As I said , the downside is, that it won't tuRn on again until you give it more power. I suspect there is another transistor in the backlight circuit that needs a specific voltage to kick in.

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The more i think about it, you might be right. Probably, even though its claiming "Backlight" on the PCB, the switch controls the whole display electronics excluding the touchscreen, which would be a much more reasonable explanation to the "doesnt turn on anymore" than the transistor theory i had before. I just checked the LCD with a bright flashlight and there is absolutely no image displayed, if the backlight is turned of. Interesting, that the electronics are able to be undervolted that far and still continue working. I think i will try to find the actual leads that are responsible for the backlight leds. Thanks for the hint.