updated for LEDE firmware:

# leds off
*/5 21-23,0-5 * * * find /sys/devices/platform/leds-gpio/leds \! -path '*usb*' -name brightness -exec sh -c 'echo 0 > {}' \;
# leds on
*/5 6-20 * * * find /sys/devices/platform/leds-gpio/leds \! -path '*usb*' -name brightness -exec sh -c 'echo 255 > {}' \; &&  /etc/init.d/led start
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