With a headless computer system, no display, some kind of a run/sleep light is needed. If things go south with the Raspberry Pi there needs to be some way to shut things down and reboot without corrupting files. See Design Consideration for a longer discussion.

When shutdown, the Raspberry Pi really just goes to a deep sleep really low power mode. However, all the GPIO pins are at 3v3dc, All the 5vdc pins are at 5vdc to keep any attached HAT happy. So there is no way to know if the Raspberry Pi is running or shutdown. By enabling the UART transmit pin, GPIO 14, Tx will go low when the processor is shutdown, and high, 3v3dc, when the system is running. GPIO pin 14 is connected, with a current limiting resistor, to the green LED on the traffic monitoring bracket. Also see the PWM HAT for accessing GPIO pin 14.

  • In /boot/config.txt
    • Add 

# Enable GPIO 14, TxD for blinky RUN light