- Disconnect anything from Uno pins 0 and 1.
- Confirm the board and port.
- Try a known data-capable USB cable.
- Close other serial-monitor applications.
Check LED polarity, resistor placement, and the lesson pin table. Do not remove a resistor to make an LED brighter.
Select No line ending in Serial Monitor. The maintained sketches ignore CR/LF, but a custom sketch may not.
Disconnect power first. Check the external supply, shared ground, driver enable pin, driver truth table, and motor stall-current rating. Measure driver output without holding the motor shaft.
This usually indicates supply noise, excessive current, missing protection, or poor grounding—not a reason to draw motor power from GPIO. Recheck the complete driver and power design.
Compilation only verifies source compatibility. Compare the actual circuit to the lesson pin map, then reduce the system to one LED or one unloaded driver channel.