Blink sketch of IDE example doesn't work

dvanburik · January 11, 2026, 5:28pm

Hi I’m uploading the standard blink code to my Arduino UNO R3.
Although the upload is complete without any error the LED next to L is not ‘blinking’ but is slowly starting to shine and then remains on.

Can someone help me out how to fix this?

LarryD · January 11, 2026, 5:30pm

Always show us a good schematic of your proposed circuit.
Show us good images of your ‘actual’ wiring.
In the Arduino IDE, use Ctrl T or CMD T to format your code then copy the complete sketch.
Use the < CODE / > icon from the ‘posting menu’ to attach the copied sketch.

GolamMostafa · January 11, 2026, 5:35pm

Figure-1:

I don't see any LED near L? Are you saying that L does not blink?

dvanburik · January 11, 2026, 5:36pm

/*
  Blink

  Turns an LED on for one second, then off for one second, repeatedly.

  Most Arduinos have an on-board LED you can control. On the UNO, MEGA and ZERO
  it is attached to digital pin 13, on MKR1000 on pin 6. LED_BUILTIN is set to
  the correct LED pin independent of which board is used.
  If you want to know what pin the on-board LED is connected to on your Arduino
  model, check the Technical Specs of your board at:
  https://docs.arduino.cc/hardware/

  modified 8 May 2014
  by Scott Fitzgerald
  modified 2 Sep 2016
  by Arturo Guadalupi
  modified 8 Sep 2016
  by Colby Newman

  This example code is in the public domain.

  https://docs.arduino.cc/built-in-examples/basics/Blink/
*/

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(LED_BUILTIN, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(LED_BUILTIN, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(1000);                      // wait for a second
  digitalWrite(LED_BUILTIN, LOW);   // turn the LED off by making the voltage LOW
  delay(1000);                      // wait for a second
}

dvanburik · January 11, 2026, 5:37pm

Hope this is the information you were looking for?

dvanburik · January 11, 2026, 5:38pm

correct

LarryD · January 11, 2026, 6:22pm

Wire an external LED as seen below, does this LED flash ?
D13—[220R]—A[Led]K—GND

If you see no flashing, try:
D12—[220R]—A[Led]K—GND

Change code to pinMode(12, OUTPUT);

dvanburik · January 11, 2026, 6:32pm

The external LED and L blinks very fast 3 times and then stops.

The second suggestion (using D12) doesn’t do anything.

sonofcy · January 11, 2026, 6:41pm

Show us the complete verbose upload log. Paste it into code tags < CODE/ >

dvanburik · January 11, 2026, 6:47pm

FQBN: arduino:avr:uno
Using board 'uno' from platform in folder: /Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6
Using core 'arduino' from platform in folder: /Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6

Detecting libraries used...
/Users/dennis/Library/Arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/variants/standard /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/sketch/Blink.ino.cpp -o /dev/null
Generating function prototypes...
/Users/dennis/Library/Arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/variants/standard /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/sketch/Blink.ino.cpp -o /private/var/folders/68/8r98s0vd6kq4vf7wyl0blhm80000gn/T/728795528/sketch_merged.cpp
/Users/dennis/Library/Arduino15/packages/builtin/tools/ctags/5.8-arduino11/ctags -u --language-force=c++ -f - --c++-kinds=svpf --fields=KSTtzns --line-directives /private/var/folders/68/8r98s0vd6kq4vf7wyl0blhm80000gn/T/728795528/sketch_merged.cpp

Compiling sketch...
/Users/dennis/Library/Arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/Users/dennis/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/variants/standard /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/sketch/Blink.ino.cpp -o /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/sketch/Blink.ino.cpp.o
Compiling libraries...
Compiling core...
Using precompiled core: /Users/dennis/Library/Caches/arduino/cores/arduino_avr_uno_6b219e4f567e0999dfd9d35704cb19c2/core.a
Linking everything together...
/Users/dennis/Library/Arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-gcc -w -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections -mmcu=atmega328p -o /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.elf /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/sketch/Blink.ino.cpp.o /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/../../cores/arduino_avr_uno_6b219e4f567e0999dfd9d35704cb19c2/core.a -L/Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937 -lm
/Users/dennis/Library/Arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy -O ihex -j .eeprom --set-section-flags=.eeprom=alloc,load --no-change-warnings --change-section-lma .eeprom=0 /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.elf /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.eep
/Users/dennis/Library/Arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy -O ihex -R .eeprom /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.elf /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.hex
/Users/dennis/Library/Arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-size -A /Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.elf
Sketch uses 924 bytes (2%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.
"/Users/dennis/Library/Arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17/bin/avrdude" "-C/Users/dennis/Library/Arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17/etc/avrdude.conf" -v -V -patmega328p -carduino "-P/dev/cu.usbmodem1301" -b115200 -D "-Uflash:w:/Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.hex:i"

avrdude: Version 6.3-20190619
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2014 Joerg Wunsch

         System wide configuration file is "/Users/dennis/Library/Arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17/etc/avrdude.conf"
         User configuration file is "/Users/dennis/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping

         Using Port                    : /dev/cu.usbmodem1301
         Using Programmer              : arduino
         Overriding Baud Rate          : 115200
         AVR Part                      : ATmega328P
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :

                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom        65    20     4    0 no       1024    4      0  3600  3600 0xff 0xff
           flash         65     6   128    0 yes     32768  128    256  4500  4500 0xff 0xff
           lfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           lock           0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00

         Programmer Type : Arduino
         Description     : Arduino
         Hardware Version: 3
         Firmware Version: 4.4
         Vtarget         : 0.3 V
         Varef           : 0.3 V
         Oscillator      : 28.800 kHz
         SCK period      : 3.3 us

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e950f (probably m328p)
avrdude: reading input file "/Users/dennis/Library/Caches/arduino/sketches/991D17817CFAD6364CB78CE1DC66E937/Blink.ino.hex"
avrdude: writing flash (924 bytes):

Writing | ################################################## | 100% 0.13s

avrdude: 924 bytes of flash written

avrdude done.  Thank you.

cattledog · January 11, 2026, 7:05pm

In the IDE under File> Preferences check the box to Verify code after upload and see if there is successful read back.

dvanburik · January 11, 2026, 7:11pm

I checked the box and tried again. Now I see an error code coming up

avrdude: verifying ...
avrdude: verification error, first mismatch at byte 0x0002
         0x5d != 0x5c
avrdude: verification error; content mismatch

Any ideas how to fix this?

cattledog · January 11, 2026, 7:30pm

Has this board been used before?
Is there a program currently loaded on it?
If so, is the TX led on the board lit up or constantly flashing?

sonofcy · January 11, 2026, 7:34pm

Remove power from the board, restart the IDE then verify and upload the following empty sketch. Does the same error happen?
If it does, maybe you have low humidity and zapped the board. I use an esd mat connected to the ground pin of the nearest 110vac outlet. The fix is unfortunatly $$$

void setup() {}

void loop() {}

cattledog · January 11, 2026, 7:37pm

Is this what you see when you press the reset button? It's an indicator that the bootloader is present.

GolamMostafa · January 12, 2026, 5:48am

Upload the following sketch to blink LED at DPin-12:

void setup()
{
    pinMode(12, OUTPUT);
}

void loop()
{
     digitalWrite(12, HIGH);
     delay(1000);
     digitalWrite(12, LOW);
     delay(1000);
}

dvanburik · January 13, 2026, 10:47am

The verification code is slightly different but still an error

avrdude: verifying ...
avrdude: verification error, first mismatch at byte 0x0002
         0x5d != 0x34
avrdude: verification error; content mismatch

dvanburik · January 13, 2026, 10:49am

Yes I used the board before. There was a program loaded on it but I upload an empty sketch. The TX led is constantly lit up.

dvanburik · January 13, 2026, 10:50am

Correct, It happens when I press the restart button.

jim-p · January 13, 2026, 11:13am

@dvanburik
Is this a genuine Arduino R3 or a clone?
Have you tried a different USB cable?

Topic		Replies	Views
Arduino Uno code uploads successfully but nothing happens Programming	33	2705	May 2, 2023
I cann't uploade Blink Uploading	21	392	October 2, 2025
Programmer is not responding Uploading	41	909	June 7, 2026
Upload fails: "programmer is not responding" Uploading	17	282	August 6, 2026
The arduino doesn’t work: simple program upload but not running Programming	31	3817	April 11, 2023

Blink sketch of IDE example doesn't work

Related topics