user57851: I am working with an I2C lcd display with an arduino uno board I have connected the gnd pin to gnd of arduino uno and the vcc pin to 5V of arduino uno and the display is not even lighting up there is only a red led lighting up on the back so I tryed connecting the rest of the pins and run a code to detect the i2c display and it worked successfully I got the adress but the lcd did not light up. The lcd display is a new one I bought yesterday here are the pictures:
https://photos.app.goo.gl/x9QPAWig98yLg4DW9
The script I tryed to display text was this one with the NewLiquidCristal library:
//Libraries 
#include <Wire.h>  
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the 
LCD I2C address, if it's not working try 0x27.

void setup(){
lcd.begin(16,2);   // iInit the LCD for 16 chars 2 lines
lcd.backlight();   // Turn on the backligt (try lcd.noBaklight() to turn it 
off)
lcd.setCursor(0,0); //First line
lcd.print("I2C LCD Tutorial");
lcd.setCursor(0,1); //Second line
lcd.print("*Ardumotive.com*");
}

Here is the model nmber of the I2C: PCF8574A And the lcd is a QAPASS Ui also I don't have a spec sheet and the LCD seems to show empty chars when rotating the pot.

user57281: Try this:
#include <Wire.h>  
#include <LiquidCrystal_I2C.h>

// LiquidCrystal_I2C lcd(0x27, 16, 2);
// OR
LiquidCrystal_I2C lcd(0x3F, 16, 2);

void setup(){
lcd.begin();   
lcd.backlight();
lcd.setCursor(5,0);
lcd.print("TA DAA...!");
}
void loop{
// do nothing here
}

If that didn't work probably your LCD have a different address than 0x27 which in that case you should run a I2C scanner to show you the addresses of I2Cs that connected to your Arduino:
#include <Wire.h>
// I2C scanner

void setup()
{
  Wire.begin();

  Serial.begin(9600);
  Serial.println("\nI2C Scanner");
}


void loop()
{
  byte error, address;
  int nDevices;

  Serial.println("Scanning...");

  nDevices = 0;
  for(address = 1; address < 127; address++ ) 
  {
    // The i2c_scanner uses the return value of
    // the Write.endTransmisstion to see if
    // a device did acknowledge to the address.
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0)
    {
      Serial.print("I2C device found at address 0x");
      if (address<16) 
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");

      nDevices++;
    }
    else if (error==4) 
    {
      Serial.print("Unknow error at address 0x");
      if (address<16) 
        Serial.print("0");
      Serial.println(address,HEX);
    }    
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");

  delay(5000);           // wait 5 seconds for next scan
}

Check the jumper behind the I2C module (plug it out and in again).
Check the voltage of LED using a multimeter if there's no voltage there, maybe it's just a "bad LCD" try replacing it.

57851: <p>I am working with an I2C lcd display with an arduino uno board I have connected the gnd pin to gnd of arduino uno and the vcc pin to 5V of arduino uno and the display is not even lighting up there is only a red led lighting up on the back so I tryed connecting the rest of the pins and run a code to detect the i2c display and it worked successfully I got the adress but the lcd did not light up. The lcd display is a new one I bought yesterday here are the pictures:</p>

<p><a href="https://photos.app.goo.gl/x9QPAWig98yLg4DW9" rel="nofollow noreferrer">https://photos.app.goo.gl/x9QPAWig98yLg4DW9</a></p>

<p>The script I tryed to display text was this one with the <a href="http://www.ardumotive.com/uploads/1/2/7/2/12726513/lcd_i2c_16x2_tutorial.zip" rel="nofollow noreferrer">NewLiquidCristal</a> library:</p>

<pre><code>//Libraries 
#include &lt;Wire.h&gt;  
#include &lt;LiquidCrystal_I2C.h&gt;

LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the 
LCD I2C address, if it's not working try 0x27.

void setup(){
lcd.begin(16,2);   // iInit the LCD for 16 chars 2 lines
lcd.backlight();   // Turn on the backligt (try lcd.noBaklight() to turn it 
off)
lcd.setCursor(0,0); //First line
lcd.print("I2C LCD Tutorial");
lcd.setCursor(0,1); //Second line
lcd.print("*Ardumotive.com*");
}
</code></pre>

<p>Here is the model nmber of the I2C: PCF8574A And the lcd is a QAPASS Ui also I don't have a spec sheet and the LCD seems to show empty chars when rotating the pot.</p>
 57281: <p>Try this:</p>

<pre><code>#include &lt;Wire.h&gt;  
#include &lt;LiquidCrystal_I2C.h&gt;

// LiquidCrystal_I2C lcd(0x27, 16, 2);
// OR
LiquidCrystal_I2C lcd(0x3F, 16, 2);

void setup(){
lcd.begin();   
lcd.backlight();
lcd.setCursor(5,0);
lcd.print("TA DAA...!");
}
void loop{
// do nothing here
}
</code></pre>

<p>If that didn't work probably your LCD have a different address than <code>0x27</code> which in that case you should run a I2C scanner to show you the addresses of I2Cs that connected to your Arduino:</p>

<pre><code>#include &lt;Wire.h&gt;
// I2C scanner

void setup()
{
  Wire.begin();

  Serial.begin(9600);
  Serial.println("\nI2C Scanner");
}


void loop()
{
  byte error, address;
  int nDevices;

  Serial.println("Scanning...");

  nDevices = 0;
  for(address = 1; address &lt; 127; address++ ) 
  {
    // The i2c_scanner uses the return value of
    // the Write.endTransmisstion to see if
    // a device did acknowledge to the address.
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0)
    {
      Serial.print("I2C device found at address 0x");
      if (address&lt;16) 
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");

      nDevices++;
    }
    else if (error==4) 
    {
      Serial.print("Unknow error at address 0x");
      if (address&lt;16) 
        Serial.print("0");
      Serial.println(address,HEX);
    }    
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");

  delay(5000);           // wait 5 seconds for next scan
}
</code></pre>

<p>Check the jumper behind the I2C module (plug it out and in again).</p>

<p>Check the voltage of LED using a multimeter if there's no voltage there, <em>maybe</em> it's just a "bad LCD" try replacing it.</p>
