Light brightness changed

The Light brightness changed trigger fires after the brightness of a light entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more] changes by a meaningful amount. Use it to react to fine-grained dimming, like adjusting a fan speed as you brighten the room, or logging changes to a dashboard graph.

The threshold field tells Home Assistant how big a change counts. The trigger only fires when the light’s brightness moves by at least that much, which keeps it from firing on every tiny tick from a smooth fade.

Using this trigger from the user interface

If you prefer building automations visually, Home Assistant walks you through this trigger step by step. You pick what to watch, tweak a few options, and save. No YAML knowledge required.

To use this trigger in an automation:

1. Go to Settings > Automations & scenes.
2. Open an existing automation, or select Create automation > Create new automation.
3. In the When section, select Add trigger.
4. From the search box, search for and select Light: Light brightness changed.
5. Under Targets, select the light entity, an area, a floor, or a label.
6. Under Threshold type, set how much the brightness has to change before the trigger fires.
7. Select Save.

Options in the UI

Using this trigger in YAML

If you work directly in YAML, or you want to know exactly what Home Assistant does under the hood, this section has the technical reference. It lists the field names you use in YAML, their types, and which ones are required.

In YAML, refer to this trigger as light.brightness_changed. A basic example looks like this:

trigger: light.brightness_changed
target:
  entity_id: light.living_room
options:
  threshold: 10

This fires whenever the living room light’s brightness changes by at least ten percent.

Options in YAML

YAML sometimes provides additional options for more complex use cases that are not available through the UI.

Targets

This trigger supports targets. A target tells Home Assistant what the trigger should watch. You can point it at a single entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more], a device, an area, a floor, or a label, and Home Assistant watches every matching light entity behind that target.

• Entity: one specific light entity, such as light.living_room.
• Device: every light entity that belongs to a device.
• Area: every light entity in a room or area.
• Floor: every light entity on a floor.
• Label: every light entity that shares a label.

You can also mix target types in one trigger. For example, combine a specific entity with an area to watch both at once.

Good to know

• The trigger uses absolute percentage change, so a jump from 20% to 30% is the same size as a drop from 80% to 70%.
• To react only when brightness crosses a fixed line in one direction, use Light brightness crossed threshold instead.
• The trigger only fires when the light is on. It does not fire when the light transitions from off to on.

Try it yourself

Ready to test this? Go to Settings > Automations & scenes, create a new automation, and add this trigger. Save the automation, then change the state of the targeted entity to watch the trigger fire on your actual entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup.

Automation: sync a ceiling fan speed to the ceiling light

When you dim the ceiling light down, slow the fan down too. A classic “scene mood” automation that keeps the room coordinated.

• Trigger: Light brightness changed
• Target: Living room ceiling light
• Threshold type: 10
• Action: Fan: Set speed

alias: "Match fan to ceiling light"
triggers:
  - trigger: light.brightness_changed
    target:
      entity_id: light.living_room_ceiling
    options:
      threshold: 10
actions:
  - action: fan.set_percentage
    target:
      entity_id: fan.living_room
    data:
      percentage: "{{ state_attr('light.living_room_ceiling', 'brightness_pct') | int }}"

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with the trigger you’re using and what you expected to happen, or share on our subreddit /r/homeassistant.

Related triggers

These triggers work well alongside this one:

• Light brightness crossed threshold - Triggers after the brightness of one or more lights crosses a threshold.

• Light turned on - Triggers after one or more lights turn on.