While nominal currents for typical LEDs are around 25mA, running this much current through a modern SMD LED will be too bright for simple PCB indication. Depending on color, modern LEDs can run on microamps of current, and in typical indoor lighting conditions will be blindly bright to be useful.

Here is a short note on resistors for LEDs for indication purposes on PCBs. The numbers are for 3.3V systems. I collected these values while assembling my DCC command station. The goal was to obtain brightness that does is noticable from several meters away, but doesn’t feel uncomfortable from up close.

  • Red: 500 ohm

  • Green: 10k for constant light or long pulses. For short pulses, use 4k-5k

  • Cool white: 5k

  • Yellow: 1k

  • Blue: 4k-5k