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Thread: Hardware Dimming 2.8" color TFT from PJRC

  1. #1
    Junior Member
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    Hardware Dimming 2.8" color TFT from PJRC

    Hello,

    I've bought about ten 2.8" touchscreen TFTs from the PJRC store. But the five I bought this month are different from the ones I bought in the past. I am feeding the backlight LED pin power through a 200 ohm pot which allows me to hardware dim the screen, or at least it did!
    Unfortunately, it doesn't work on the "new" screens.

    Has anybody run into this problem and found a fix?

    I'm not too great at this stuff, so hopefully it's something simple that I'm just not seeing/getting...

    Here is the relevant part of the circuit:

    Click image for larger version. 

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    Many thanks for any help!

  2. #2
    Senior Member+ Theremingenieur's Avatar
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    Basically, a series resistor as a dimmer is nood goot practice. First, it’s not economic, transforming unneeded energy into heat. Second, its effect depends strongly from the target impedance which might vary as you experienced it with the new displays. The way to go for efficient dimming the LED backlight of whatever TFT is using a variable PWM signal.

  3. #3
    Senior Member
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    All the screens I've met recently have the backlight fed from a transistor - so throttling the current to the LED pin will not do anything until you bring the transistor base current down to the point it is in the non linear region (and getting hot). You could trace the board and try to bypass it or as Theremingenieur suggests drive it via a PWM pin (which is why they have the inbuilt transistor, so the pin current is low).

  4. #4
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    Yep, that is what changed. There is a transistor there now where there was none before.
    PWM control is working :-)
    But the problem now is that I have 30 PCBs that will need to be hacked a little bit as this device is designed to use a thumbwheel pot for dimming - not a software slider.
    I'm working through that now...

    Thank you Gentlemen!

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