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CFAH2004AP backlight LED resistor and LCD reset


New member
This question may have been answered in a previous thread, but that thread is no longer accessible.

I'm running a CFAH2004AP-TFH-EW 20x4 character LCD and the backlight LED seems dim. I've tried using resistors of 330 Ohms and 220 Ohms between Vcc (5v) and pin 15 on the LCD, the Anode. Are these values too high? Would I risk damage to the LED if I tried lower value resistors?

Second question. The sample Arduino code available for this LCD contains a reference to LCD_RESET and SD_CS. Neither of these lines appear in the datasheet for the LCD. What purpose do they serve in the code?

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The datasheet shows that the backlight has a Vf of 3.5 volts at typically 48 mA current. So you'll need to drop 1.5V from the 5V supply to get 3.5V. At 48 mA the resistor required would be 1.5/.048 = 31.25 ohms. So try using a 33 ohm resistor (100 mW rating or more).
I can't answer the question about the Arduino sample code, but I guess that the code is applicable to different versions of this display which have those inputs.

CF Kelsey

Staff member

For the backlight, it looks like you've already got good advice. Did that fix the dimness?

SD_CS is chip select for if you add an SD card to your set up, otherwise it can be ignored. Some of the demos may only work if an SD card is attached, but setting any SD card required demos to off and ignoring the SD card connections is completely fine.

As for, LCD_RESET, it looks like this connection is a legacy from changing this code from parallel to SPI. It should be no connection.

I'll go ahead and fix/clarify these questions in the code comments. Let us know if you run into anything else!