Arduino Uno Three LEDs Lit Meaning | Generated by AI
The CH340 has been disconnected and no new Arduino board is visible on USB. So if you’re looking at a physical Uno board with 3 LEDs lit, the board is either powered via USB from another cable or external power, and it’s running whatever sketch was last uploaded to it.
Let me explain what the 3 lit LEDs tell you:
On the Arduino Uno R3 boards
| LED | Label | What controls it | Likely cause if ON |
|---|---|---|---|
| 1 | ON (green) | Power rail | Board has power — normal |
| 2 | L / pin 13 (yellow) | Your sketch (LED_BUILTIN) |
Sketch has digitalWrite(13, HIGH) stuck on |
| 3 | TX (yellow) | ATmega16U2 serial chip | Serial data being transmitted — or 16U2 in noisy state |
| or | RX (yellow) | ATmega16U2 serial chip | Serial data being received — or 16U2 in noisy state |
If you see ON + L + TX all steady
- ON = normal, power is OK
- L = your sketch set pin 13 HIGH and isn’t running the blink loop — the 100ms blink at 5Hz is fast but still visible as rapid flashing, not steady-on. If it’s steady, either:
- The board froze / crashed after setting pin 13 HIGH
- Or a different sketch was already on the board
- TX = the 16U2 is driving its TX line high (could be idle-line state with USB disconnected)
What to try
- Press the RESET button on the Uno — if it’s a running sketch, it restarts: L should start blinking visibly
-
Plug it back into USB and check:
ls /dev/ttyACM*Real Unos appear as
/dev/ttyACM0, not/dev/ttyUSB0. - If it shows up, upload the standard Blink example (1s delay) to confirm the board is alive