How many inbuilt lights are there in Arduino UNO

There are four inbuilt lights available in Arduino UNO. The four lights on the Arduino Uno board are marked as ON, L, TX, and RX. ON Led is a  green colored LED light and the other 3 inbuilt Lights are of yellow color. All of the four lights are used for different purposes on the Arduino Uno board. You can see the  LED labeled ON parallel with the digital pin 2, and the other 3 pins, L, TX, and RX  parallel with the digital pin 13. 

Functions of the inbuilt LED light available in Arduino UNO

Inbuilt LED ON

ON named LED gives the signal that the board is receiving power. in a simple way if you connect the Arduino UNO board with the power source or with the battery the ON named green LED will let up. This led will glow continuously without a blink until the power is disconnected.  the main CPU of the Arduino UNO board is ATmega 328p.  to program it we need a bootloader.   if the bootloader failures then we cannot program the Arduino microcontroller. But still, if the Arduino board gets power this light will glow. 

Inbuilt LED L

 you can found this led in a row with it the LED TX and RX,  which is parallel with the digital pin 13.  this LED  is connected with the digital pin 13.  if you give high as output in digital pin 13 then this led will glow up.  you can test this led with the blink or fade program available in the example SketchBook in Arduino IDE.  you have to set the  LED output pin as digital pin 13 or You have to give 

pinMode(LED_BUILTIN, OUTPUT)

this command to set the output LED as digital pin 13.  and as I have mentioned that LED L  is connected with the digital pin 13.  then you will able to see the blink effect in digital pin 13.  after uploading the code to the Arduino UNO board the LED will start blinking according to the output of digital print 13,  given by the program. 

Inbuilt LED TX 

You can find this led just up of the LED RX or on the left side of the Arduino logo available on the Arduino UNO board. TX  simply represents the term transmission.  so if your board is transmitting any type of data then this led will glow up according to your speed of transmission.  the color of this led is yellow.  at the time of uploading the data, you can also see that the light TX is blinking.

Inbuilt LED RX

 you can find this led just bottom of the LED TX. RX  simply represents the term reception.  So, if the Arduino UNO board is receiving any type of data then the LED RX will glow up and start blinking according to your speed of receiving data.  the color of this is also yellow.  at the time of uploading your program to the Arduino board, you can see that the LED  is blinking.

So far you have come to know that what are the four inbuilt lights and how they work.  if you want to check all of the 4 inbuilt Lights Are glowing then you can upload the blink program available in the example SketchBook. 

 I have also given you a screenshot of that program you can run it and upload it to your Arduino Uno board then we will see that the ON led will glow without blinking as you have plugged in the USB cable to your Arduino board to upload the program. At the time of uploading both TX and RX LEDs will start blinking until the uploading finishes. Then the led L will start blinking according to the given program.

Blink Program to see all inbuilt lights are glowing

Hope this post was quite enough to make you understand, about the inbuilt lights available in the Arduino UNO board.  I have also been explained the functions and uses of these four inbuilt LED lights of Arduino UNO.  if you have any kind of thoughts or if you want to know something more feel free to comment down below.