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STRML/studio-led

Pulse the Mac Studio's front power LED with load (SMC LSLN + board power)

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studio-led

Turns the Mac Studio's front power LED into a load meter. It pulses while the machine works, brighter the more power it draws, and holds its normal glow when idle.

board power (SMC PD0R)        LED
  idle, < 50 W        ─▶  steady, stock brightness
  50 W                ─▶  slow pulse, dim peak
  60 W                ─▶  pulse, peak ~75%
  180 W and up        ─▶  pulse, full peak

Tested on one machine: a Mac Studio with M5 Ultra, macOS 27. Other Apple Silicon Macs may name their keys differently. Run sudo ./led-probe.sh to find out.

Run it

make
sudo ./studio-led

It needs root because every SMC write does. Ctrl-C, SIGTERM, or a dropped ssh session puts the LED back the way it found it.

Flag Default What it does
-w WATTS 50 board power that counts as working
-F WATTS 180 board power for a full-brightness peak
-p SECONDS 2.4 one breath
-l HEX 0000 the dim end of each breath
-f PATH none also count as working while this path exists (a lock dir your long jobs hold)
-k KEY LSLN the brightness key to drive

The 50 W default is a guess, since the idle draw hasn't been measured yet. Check yours with ./smcdump PD0R before you rely on it.

Run it at boot

make
sudo make install     # /usr/local/libexec/studio-led + /Library/LaunchDaemons/local.studio-led.plist
tail -1 /var/log/studio-led.log

sudo make uninstall stops it, which restores the LED, and removes both files. It has to be a LaunchDaemon rather than a LaunchAgent, because a LaunchAgent runs as your user and SMC writes need root. The binary is copied to a root-owned path, so the daemon never runs a file your account can edit. To pass flags, add them to ProgramArguments in local.studio-led.plist before you install.

The keys

Apple documents none of this. These are the LS* SMC keys on the M5 Ultra Studio and what writing them did to the LED:

Key Stock Write LED
LSLN ffff 0000...ffff smooth dimmer; 0000 is dim, not off
LS0S b366 0000 very dim
LS0C ffff 0000 dim
LS0P 66b3 0000 bright
LSOF, LSLB, LSLF no visible change
LS1* little or no change (probably a second channel the Studio doesn't wire up)

Some writes push the LED brighter than stock, but which key does it isn't pinned down yet. LS0P and LS0S are byte-swapped copies of each other, so they probably work as a pair. Keys such as LSOO and LSSB fail on read. They drove the sleep light on Intel Macs, and they may be write-only here.

Tools

File What it does
studio-led the daemon
smcdump [prefix] prints every SMC key with that prefix, with its type and value; no root
smcwrite KEY HEX writes raw bytes to one key and reads it back; root
led-probe.sh steps through the LS* keys one at a time and restores each one; root

smc.c and smc.h come from mactop (MIT).

Contributors

STRML

Issues