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I've bought myself one of these 3.5" TFT panels: https://www.aliexpress.com/item/32829704426.html and an accompanying 40 pin fpc to header adapter so that I can plug it into a breadboard. (Specifically, "ili9488 no touch")

I've got a project I've been working on that uses the fairly typical module that comprises a screen, touch sensor and sd card reader (http://www.lcdwiki.com/3.5inch_SPI_Module_ILI9488_SKU:MSP3520) and it works fine.

I've compared the pin listings on the store page to the schematic and whilst they're not a perfect match, I'm still pretty sure I figured out how to connect what to where.

Ali Express store page pin listing lcd wiki schematic

However, once I'd hooked everything up, nothing happened.

My approach now is to try and connect it up in stages. I'm not sure how feasible that is though, but I do know that it should be possible to simply just connect the backlight up and at least see the screen light up (if nothing else).

I know with the more beginner friendly module I simply need to hook up VCC, GND and LED and it works. Looking at the schematic for the module, the LED pin doesn't actually power the backlight, but is instead connected to a transistor that completes the circuit if power is supplied to the LED pin.

Looking at the schematics, I'm guessing the "LEDA" or "A" pin is power in for the backlight, and the various "K#" or "LEDK" pins are GND. I'm finding the datasheet a little impenetrable, but I do see the "A" pin described as an "anode" and the "K" pins as "cathodes" which backs up my theory.

LEDK to GND via transistor schematic

However hooking up only these pins still doesn't do anything (I also tried hooking up all the GND and VCC pins, but no difference there either).

For reference, I'm using the 3.3V and GND pin of a Teensy 4.0 to supply power to the LCD.

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  • \$\begingroup\$ if we assume the backlight led forward voltage is 3.2V and the drop across your transistor is ,say, 0.7V and we want to have 120mA flow, then the voltage across the resistor should be V= IR = 0.12 * 5.6= 0.672 then a supply of 3.3V isn't going to work. You need something like 4.4V minimum. \$\endgroup\$
    – Kartman
    Mar 27 at 5:48

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First of all, you're right about A for "Anode" and C for "Cathode" and what they do.

Under the LCD wiki link you provided, there is a "3.5 inch TFT Specifications" link, which takes to the panel datasheet. (this link)

The datasheet is written in "Engrish", but there is information on backlight. On page 5 we have:

enter image description here

First of all, on the bottom it says we have 12 LEDs. Second of all, in the backlight section under "forward voltage" there is 6x3.0V. Which makes me think that 12 LEDs are organized as 2 parallel chains of 6 LEDs each. Thus, you need to supply enough voltage to provide forward current through 2 parallel 6 LED chains, which would be 18V-20V, which is a reasonable voltage. Backlight circuits are always higher voltage than everything else, voltages of this range are to be expected.

Notice it states below that constant current has priority over constant voltage. LEDs can be different, there can be some mismatches, but LED brightness depends on current through it, so you need constant current source that can output at least up to 20V (better up to 24V, you don't want to push it to the max, there is plenty of choice in parts). You will need a boost circuit in any case, that's just how backlights work.

For comparison, LCD backlight in laptops can go up to something like 50V, in phones it can reach 20V.

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  • \$\begingroup\$ So does this mean there's a boost circuit in the LCD module? This schematic doesn't have anything like that: lcdwiki.com/res/MSP3520/… Most of it is just the various connections, the most complex bit is the transistor, but there is also a voltage regulator which I guess is to protect it incase the user plugs it into the 5V line instead. I don't see anything resembling a boost circuit in the schematic. \$\endgroup\$ Mar 26 at 23:16
  • \$\begingroup\$ Also the comparison to a phone screen (let alone a laptop screen) seems a little extreme - this is a very small, low resolution screen compared to the kind of fidelity you'd get on a phone. \$\endgroup\$ Mar 26 at 23:20
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    \$\begingroup\$ backlight and lcd matrix have nothing to do with each other, it doesn't matter if your screen is full hd or 480p, that's LCD matrix power, backlight is de-facto a completely independent device that has nothing to do with screen characteristics, only with LEDs inside it. Besides, it was just an example. \$\endgroup\$
    – Ilya
    Mar 26 at 23:56
  • \$\begingroup\$ As for backlight part, is that schematic a supply to a module with LCD or panel alone? If it's a module, it can have a boost on board. You can always attach some constant current source like LM334 or a bench power supply to anode and see what kind of voltage you need for current to start flowing. It's a pretty safe technique. In datasheet you need a lot of current, but you only need to estimate when diodes start to conduct, so let it be 10ma for test, or even 1ma maybe. Backlight working from fixed 3.3V is strange and very unlikely. Even datasheet says it expects constant current, not voltage \$\endgroup\$
    – Ilya
    Mar 26 at 23:59

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