How to redirect pi heartbeat to GPIO pin

I built myself a custom rackmount case for some raspberry pi’s using a Bitscope blade quattro, a USB powered ethernet switch and some other bits and pieces I had lying around.

As they were headless, I thought I should add some activity LEDs to the case so I could see at a glance if the Pi’s were working as expected.

I hooked up the LEDs to pin 21 on but to get the heartbeat to show on the external LED rather than the one on the board I had to edit the config.txt file that resides in the boot directory. By adding the below, the LED is powered by activity on the Pi.

dtoverlay=pi3-act-led,gpio=21
dtparam=act_led_trigger=heartbeat

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