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How does the backlight circuit on the 1.3" TFT display work?
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How does the backlight circuit on the 1.3" TFT display work?

by bootsie123 on Sun Sep 12, 2021 10:06 pm

Hi! I recently bought the bare 1.3" 240x240 TFT display (4520) for a project I'm working on and I've been looking at the schematics of the breakout PCB for this display to help guide me with how I should wire everything up. In particular, I'm stuck on how to go about wiring the backlight. I was wondering if someone could explain to me how the backlight circuit worked on the breakout PCB and how I could go about implementing it myself.

Just as a heads up, I am still a beginner so definitely correct me if I say something that's incorrect. Anyway, here's the rundown of what I think is happening in the diagram below.

Screenshot_1.png
Portion of schematic
Screenshot_1.png (24.57 KiB) Viewed 47 times


We have the backlight pin connected to a 2.2k resistor which is then connected to +3.3V from the onboard voltage regulator. From there, that goes into the anode pin of the LED backlight. The cathode pin of the LED backlight goes into a 10-ohm resistor which I think is bringing the voltage from 3.3V down to 3V with a current of 30 mA (basically now within the operating specs of the LED backlight). This is then connected to a MOSFET as the drain with ground being the source and the backlight pin being ground (not sure if I have this correctly).

Basically, what I'm stuck on is what's happening with the 2.2k resistor and the +3.3V line along with what's happening with the MOSFET. Any explanations would be greatly appreciated. Thanks!

Screenshot_2.png
Full schematic
Screenshot_2.png (43.91 KiB) Viewed 47 times

bootsie123
 
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Re: How does the backlight circuit on the 1.3" TFT display w

by adafruit_support_bill on Mon Sep 13, 2021 5:33 am

R6 is a current limiting resistor for the backlight LED. R8 pulls BL and the gate of the MOSFET HIGH to turn the backlight on by default. If you pull BL low, it will turn the backlight off.

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Re: How does the backlight circuit on the 1.3" TFT display w

by bootsie123 on Sat Sep 18, 2021 8:33 pm

Thanks for getting back to me! Sorry for the late reply. This makes a lot more sense now. Just curious. Do you know a good way I could use the same BL pin to then control the backlight for multiple screens at once? I assume I could probably use the same BL pin to the gate of multiple MOSFETs (one for each screen) right? Thanks!

bootsie123
 
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Re: How does the backlight circuit on the 1.3" TFT display w

by adafruit_support_bill on Sun Sep 19, 2021 5:51 am

You should be able to drive the gates of multiple MOSFETs from a single GPIO pin. You would only need the one pullup resistor (you could also just use the internal pullup on your GPIO pin).

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Re: How does the backlight circuit on the 1.3" TFT display w

by bootsie123 on Sun Sep 19, 2021 1:21 pm

adafruit_support_bill wrote:you could also just use the internal pullup on your GPIO pin

Good point. Thanks for the advice!

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