Make your second light mirror power, brightness, and color temperature from the first one — reliably and fast. Tuned for Matter lights that may expose Kelvin or mireds.

TL;DR 🧾

  • L2 is the source, L3 is the target.
  • Mirrors on/off, brightness, and CCT (prefers Kelvin, falls back to mireds).
  • Uses numeric guards to avoid bad states.
  • mode: restart keeps things snappy during slider drags.

Why this approach? 💡

Light groups mirror power well, but attributes (brightness/CCT) can be flaky across vendors. This automation enforces deterministic one‑way syncing with Kelvin‑first logic and solid guards — a pattern consistent with the style used across practical, step‑by‑step posts on vahac.com.

Requirements ✅

  • Home Assistant 2024.x+
  • Two light entities (here named):
    • Source (L2): light.matter_living_light_2
    • Target (L3): light.matter_living_light_3
  • Source exposes at least one of: color_temp (mireds), color_temp_kelvin, or kelvin.
Tip: Kelvin is more intuitive (e.g., 2700 K = warm, 6500 K = cool). Mireds are the reciprocal scale.

Full YAML (copy‑paste) 📦

Replace entity IDs to match your setup.
alias: Sync power + brightness + color temp
sequence:
- data:
entity_id: light.matter_living_light_3
kelvin: >-
{{ (state_attr('light.matter_living_light_2','color_temp_kelvin')
or state_attr('light.matter_living_light_2','kelvin')) | int }}
action: light.turn_on
default:
- condition: template
value_template: >-
{{ state_attr('light.matter_living_light_2','color_temp') is number }}
- data:
entity_id: light.matter_living_light_3
color_temp: >-
{{ state_attr('light.matter_living_light_2','color_temp') | int }}
action: light.turn_on


- conditions:
- condition: template
value_template: "{{ trigger.id == 'brightness' }}"
sequence:
- condition: state
entity_id: light.matter_living_light_2
state: "on"
- condition: template
value_template: "{{ trigger.to_state.attributes.brightness is number }}"
- data:
entity_id: light.matter_living_light_3
brightness: "{{ trigger.to_state.attributes.brightness | int }}"
action: light.turn_on


- conditions:
- condition: template
value_template: "{{ trigger.id == 'ct_mireds' }}"
sequence:
- condition: state
entity_id: light.matter_living_light_2
state: "on"
- condition: template
value_template: "{{ trigger.to_state.attributes.color_temp is number }}"
- data:
entity_id: light.matter_living_light_3
color_temp: "{{ trigger.to_state.attributes.color_temp | int }}"
action: light.turn_on


- conditions:
- condition: template
value_template: "{{ trigger.id in ['ct_kelvin','kelvin'] }}"
sequence:
- condition: state
entity_id: light.matter_living_light_2
state: "on"
- choose:
- conditions:
- condition: template
value_template: >-
{{ trigger.to_state.attributes.color_temp_kelvin is number }}
sequence:
- data:
entity_id: light.matter_living_light_3
kelvin: >-
{{ trigger.to_state.attributes.color_temp_kelvin | int }}
action: light.turn_on
default:
- condition: template
value_template: "{{ trigger.to_state.attributes.kelvin is number }}"
- data:
entity_id: light.matter_living_light_3
kelvin: "{{ trigger.to_state.attributes.kelvin | int }}"
action: light.turn_on
mode: restart

How it works (short & sweet) 🛠️

  • Power OFF → light.turn_off(L3).
  • Power ON → light.turn_on(L3) → copy brightness (if numeric) → copy Kelvin if present, else mireds.
  • While ON: attribute‑specific updates listen to brightness, color_temp (mireds), or color_temp_kelvin/kelvin and forward only that field.

Guards that matter:

  • ... is number avoids errors on unknown/unavailable.
  • condition: state == on prevents changes while the source is off.
  • mode: restart keeps the last change winning during rapid slider moves.

Attribute primer 🌈

  • Brightness: HA service data uses 0–255 (even if UI shows %).
  • Kelvin: 2700 K (warm) ↔ 6500 K (cool). Preferred when available.
  • Mireds (color_temp): reciprocal scale. Lower mireds = cooler.

Testing checklist ✅🧪

  • Toggle L2 on/off → L3 mirrors instantly.
  • Drag brightness on L2 → L3 tracks smoothly.
  • Set Kelvin on L2 (e.g., 2700/4000/6500) → L3 matches.
  • If Kelvin isn’t exposed, adjust mireds → L3 matches via color_temp.
  • While L2 is off, changing sliders should not affect L3; turning L2 on snaps L3 to the latest look.

Troubleshooting 🔍

  • Brightness doesn’t update → Check Developer Tools → States: ensure L2 brightness is numeric while ON.
  • CCT doesn’t update → Verify which attribute L2 exposes (color_temp, color_temp_kelvin, kelvin) and that L3 accepts the same field you’re sending.
  • Flicker/lag → Device may rate‑limit; mode: restart already helps. Consider adding a small for: delay to the brightness trigger if needed.
  • No loops → Only L2 → L3 is wired. Don’t add automations that write L3 → L2 unless you isolate them.

FAQ ❓

Q: Why prefer Kelvin over mireds?
A: Many Matter integrations surface Kelvin and users find it more intuitive; the fallback keeps compatibility with mired‑only devices.

Q: Will this mess with effects or scenes?
A: If L2 changes due to a scene, L3 will mirror. Use a helper to pause syncing when you want L3 independent.

Conclusion 🧩

A single, guarded automation keeps two bulbs visually identical across vendors — power, brightness, and CCT, with Kelvin‑first logic. Clean, fast, dependable.