MonitorKeys

A small macOS menu-bar app for external monitors on Apple Silicon:

• Brightness: one slider per display. It drives the monitor's backlight over DDC/CI, then dims further below the backlight's minimum with a dark overlay, which leaves colour apps such as f.lux alone. The overlay doesn't dim the pointer, the screen saver, the lock screen, or a game that takes over the whole display.
• Brightness keys: F1/F2 control the external display under the pointer, or the one with the active window. The built-in display stays with macOS.
• Monitor speakers: a volume slider and mute, plus volume and mute keys, while the Mac plays through the monitor (DisplayPort/HDMI audio that macOS can't control).
• Day schedule: brightness follows sunrise and sunset, with Day, Evening and Night levels and a bedtime. A manual change holds until the next transition. Resume returns to the schedule, and Use … as the Day/Evening/Night level makes the held brightness the current period's level.
• Open at Login: a menu toggle that registers the app as a macOS login item.

Install

1. Download MonitorKeys.zip from Releases.
2. Unzip it and move MonitorKeys.app to /Applications.
3. The app isn't notarized, so clear the download flag once before opening it:

xattr -dr com.apple.quarantine /Applications/MonitorKeys.app
open /Applications/MonitorKeys.app

Requires macOS 14+ on Apple Silicon.

Build

Requires Swift 6.

scripts/make-signing-cert.sh   # optional, once: local signing so Accessibility permission survives rebuilds
scripts/build-app.sh           # → build/MonitorKeys.app
scripts/make-icon.sh           # only after editing scripts/make-icon.swift: redraws Resources/AppIcon.icns
open build/MonitorKeys.app
swift test

The keyboard features need Accessibility permission (System Settings → Privacy & Security → Accessibility).

Diagnostics

.build/release/MonitorKeys --probe                    # list displays, read backlight and speaker levels
.build/release/MonitorKeys --probe --ddc-set 50       # write raw luminance
.build/release/MonitorKeys --probe --volume-set 10    # write raw speaker volume
.build/release/MonitorKeys --probe --ddc-queue 50     # write luminance as the app does: read back, resent if dropped

Bus timing can be overridden with the MONITORKEYS_SENDS, MONITORKEYS_SETTLE_MS, MONITORKEYS_REPLY_MS and MONITORKEYS_READ_SENDS environment variables, and MONITORKEYS_VERIFY=0 turns off the read-back. The menu's Advanced submenu has the same knobs.

Advanced settings

Setting	Default	What it does
Slider response	Linear	"Finer at the dark end" moves the backlight and dimming along brightness² instead of brightness, so low levels get more slider travel. Software dimming still starts where "Software dimming below" says. Apple panels stay linear: their scale is already perceptual.
Darkest software dimming	15%	How dark the overlay gets at 0: 15%, 10% or 5%. Never black, because the overlay also covers the menu bar.
Software dimming below	30%	Where the backlight reaches its floor and the overlay takes over.
Backlight floor	0%	Lowest backlight level ever sent, for monitors whose low levels flicker.
DDC sends per change, settle time	Twice, 20 ms	Bus timing for monitors that drop or garble commands.
Check that DDC changes took	On	Once a change settles (250 ms), reads the monitor back and sends the value again, twice, if it didn't take.
Follow changes made on the monitor	On	Reads DDC monitors every 30 s (needs "Check that DDC changes took"). A level changed with the monitor's buttons or another app holds the schedule until its next transition, as on Apple panels, and shows on the slider.

After the Mac wakes, each DDC monitor is sent its level again, since some come back at a level of their own.

Notes

• Commands are sent twice by default. Some monitors silently drop a single DDC write, and setting a value twice is harmless.
• Monitor support varies. The monitor must support DDC/CI, and not every monitor exposes speaker volume (VCP 0x62) or mute (0x8D).
• Tested on an M5 MacBook Pro with an LG UltraWide, macOS 26.