Dimming backlight on 2.8'' TFT ILI9341

Hi, I bought a 2.8’’ TFT display from banggood with ILI9341 controller and I am trying to change the backlight through PWM. I have seen in some forums that they have done it by disconnecting the cathode of the led, soldering a transistor and controlling it with a pin but I don’t know what to disconnect and where to solder everything, and every attempt at finding a pinout or schematic has failed even with the buyer.

I will atach a picture of the back of the screen and a link to the product

Thanks!

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I can’t help much but I did find this in the document pack following your link. It has a 37 pin connector between the screen and the main board.

Module size.PNG

You can google for “TFT Module Schematic” and look at the images. You may find something similar. Clearly the pins broken out to data lines will not be candidates for the back light(s).

To do anything, you’ll almost certainly have to separate the screen from the main board.

I did look at some of the questions and answers on the product link you supplied. Judging by the answers, it appears that posting a question there would be hopeless.

I did more or less the same thing for a different screen and was fortunate to find a pinout diagram. https://forum.arduino.cc/index.php?topic=490421.0

I see little point in dimming the backlight.
Change the series resistor to set a lower backlight current.

I suspect that the 5V pin only supplies the AMS1117-3.3 regulator for the backlight. So it should be fairly easy to intervene by lifting the output pin.

Insert a PNP transistor between 5V and AMS1117 to switch on or off.

It is your responsibility to investigate the pcb traces. Photos on the internet are not always accurate.

David.

Hi, I have just found in this page

a schematic (Image below) and I see that the LEDs are controlled by the 3.3 regulator but other thing are connect to it too. Won’t be there a problem if I use PWM in input of the regulator with other things in the diplay?

Thank you!

I also found this user manual with the connections of CON1
https://class.ece.uw.edu/474/peckol/doc/TFT-Display/2.8inch%20touch%20screen%20user%20manual(Arduino-EN)V1.00.2017.03.22.pdf

Thanks for the schematic. I had assumed the Blue Mcufriend pcb were similar to the original Red Mcufriend pcb (which had an LDO regulator).
The original Red pcb used the 3.3V pin for the electronics and the 5V pin fed the backlight via the LDO.

You would either have to either intercept pin #16 of the ribbon connector or pins 17-20

If the backlight is too bright, I would put a 10R or 22R resistor in the LEDA line.
PWM would need a drive transistor (and series resistor)

If you do modify the Blue pcb you could glue the external transistor and components like groundfungus 6v6gt.
And use pin A5 to switch the transistor as on/off.
Or use 10 or 11 as a PWM pin. (and lose the microSD)

David.

Edit. corrected poster's name.

That was good work finding the documentation.
I generally use at least two brightness levels. One for when the room is dark (at night with the lights out), one for normal use (full brightness or the maximum brightness the display can tolerate without getting too hot) and maybe other levels in between.
With multiple levels, it is good to apply some hysteresis to brightness algorithm to avoid flicker. See Hysteresis - Introductory Tutorials - Arduino Forum