LCD I2C connection problems

Question

I am hoping there is a simple fix to this problem.

I am playing around with a 16x2 lcd with a I2C connector attached. I wired it up to an elegoo uno R3.

I started with some code I've used before and ended up with one row of the LCD fully lit and the other dark.

After a ton of trouble shooting, I can not get the LCD to show anything but one lit and one dark row.

arduino with lcd attached

In this image:

Power: 5V, 1.5A

Pin	LCD	Uno
SCL	SCL	SCL
SCA	SDA	SDA
+	VCC	5V
gnd	gnd	gnd

I've also tried:

• putting the SDA to A4 and the SCL to A5
• Using different power supplies
• Powering from the USB connector

And I've tried several of the libraries available

They are all variations of the following from the Adafruit_LiquidCrystal library:

#include <Wire.h> 
#include <LiquidCrystal_I2C.h>
// Set the LCD address to 0x27 for a 16 chars and 2 line display
LiquidCrystal_I2C lcd(0x27, 16, 2);
void setup()
{
 // initialize the LCD
 lcd.begin();
 // Turn on the blacklight and print a message.
 lcd.backlight();
 lcd.print("Hello, world!");
}
void loop()
{
 // Do nothing here...
}

No matter what I do the display does not change.

• SBF: There should be a potentiometer for the contrast. Turning it ccw will make the text visible
• Andrew Staroscik: I've played with that. There is no position where text is visible. The top row is fully lit and the bottom row off. When I turn the pot, the lit crystals dim evenly to nothing.
• 6v6gt: Maybe try an I2C scanner to verify the I2C address which your code assumes is 0x27
• Andrew Staroscik: That fixed it. The scanner found the i2c at 0x3f. Everything is working. Thank you!
• st2000: @6v6gt, if you could create an Answer and Andrew if you could accept it, that would make this Q & A perfect for anyone else in the same predicament.

Answer 1

Some common problems with LCD displays which incorporate an I2C backpack include:

1. Not using the correct I2C address for the I2C backpack. Use an I2C scanner to discover the correct address.
2. Mixing voltages without proper level shifting, say using a 5v display with a 3.3v MCU
3. Failing to adjust the contrast potentiometer
4. Some I2C backpacks may use a non-standard pin allocation which may be corrected by specifying a pin mapping in the constructor to the display library. Fortunately, this is rare.
5. Using code samples intended for a different variant of the library supporting the display. Always use a basic test sketch which is packaged with the chosen library to verify the performance of the display.

Answer 2

Apparently I have the same problem. I did what was said over and over again but I couldn't get a display. I found that my problem was due to a manufacturer's defect. The 5th pin (rw) from the left of the I2C package was not soldered at all. However, I had seen this pin unsoldered before and thought it should be like that. I hope it helps someone.