1  # Light Switch
  2  
  3  [Public Overview – Microsoft Learn](https://learn.microsoft.com/en-us/windows/powertoys/light-switch)
  4  
  5  ## Quick Links
  6  
  7  * [All Issues](https://github.com/microsoft/PowerToys/issues?q=is%3Aissue%20state%3Aopen%20label%3AProduct-LightSwitch)
  8  * [Bugs](https://github.com/microsoft/PowerToys/issues?q=is%3Aissue%20state%3Aopen%20label%3AProduct-LightSwitch%20label%3AIssue-Bug)
  9  * [Pull Requests](https://github.com/microsoft/PowerToys/pulls?q=is%3Apr+is%3Aopen+label%3AProduct-LightSwitch)
 10  
 11  ## Overview
 12  
 13  The **Light Switch** module lets users automatically transition between light and dark mode using a timed schedule or a keyboard shortcut.
 14  
 15  ## Features
 16  
 17  * Set custom times to start and stop dark mode.
 18  * Use geolocation to determine local sunrise and sunset times.
 19  * Apply offsets in sunrise mode (e.g., 15 minutes before sunset).
 20  * Quickly toggle between modes with a keyboard shortcut (`Ctrl+Shift+Win+D` by default).
 21  * Choose whether theme changes apply to:
 22  
 23    * Apps only
 24    * System only
 25    * Both apps and system
 26  
 27  ## Architecture
 28  
 29  ### Main Components
 30  
 31  * **Shortcut/Hotkey**
 32    Listens for a hotkey event. Calling `onHotkey()` flips the theme flags.
 33  
 34    > **Note:** Using the shortcut overrides the current schedule until the next transition event.
 35  
 36  * **LightSwitchService.cpp**
 37    is the heart beat of the module. Controls ticking every minute and depending on user actions (manual override, settings changing, etc) triggers the state manager to perform the corresponding operation.
 38    
 39  * **LightSwitchStateManager.cpp**
 40    handles updating the state based on the signals sent by LightSwitchService.
 41    
 42  * **SettingsXAML/LightSwitch**
 43    Provides the settings UI for configuring schedules, syncing location, and customizing shortcuts.
 44  
 45  * **Settings.UI/ViewModels/LightSwitchViewModel.cs**
 46    Handles updates to the settings file and communicates changes to the front end.
 47  
 48  * **modules/LightSwitch/Tests**
 49    Contains UI tests that verify interactions between the settings UI, system state, and `settings.json`.
 50  
 51  ### Data Flow
 52  
 53  1. User configures settings in the UI (default: manual mode, light mode from 06:00–18:00).
 54  2. Every minute, the service checks the time.
 55  
 56     * If it’s not a threshold, the service sleeps until the next minute.
 57     * If it matches a threshold, the service applies the theme based on settings and returns to sleep.
 58  3. At **midnight**, when in *Sunrise to Sunset* mode, the service updates daily sunrise and sunset times.
 59  4. If the machine was asleep during a scheduled event, the service applies the correct settings at the next check.
 60  
 61  ## User Interface
 62  
 63  The module’s settings are exposed in the PowerToys Settings UI. Options include:
 64  
 65  * Shortcut customization
 66  * Mode selection (Manual or Sunrise to Sunset)
 67  * Manual start/stop times (manual mode only)
 68  * Automatic sunrise/sunset calculation (location-based)
 69  * Time offsets (sunrise mode)
 70  * Target scope (system, apps, or both)
 71  
 72  ## Development Environment Setup
 73  
 74  ### Prerequisites
 75  
 76  * Visual Studio 2019 or later
 77  * Windows 10 SDK
 78  * PowerToys repository cloned from GitHub
 79  
 80  ### Building and Testing
 81  
 82  1. Clone the repo:
 83  
 84     ```sh
 85     git clone https://github.com/microsoft/PowerToys.git
 86     ```
 87  2. Initialize submodules:
 88  
 89     ```sh
 90     git submodule update --init --recursive
 91     ```
 92  3. Build the solution:
 93  
 94     ```sh
 95     msbuild -restore -p:RestorePackagesConfig=true -p:Platform=ARM64 -m PowerToys.slnx
 96     ```
 97  
 98     > Note: This may take some time.
 99  4. Set `runner` as the startup project and press **F5**.
100  5. Enable Light Switch in PowerToys Settings.
101  6. To debug the service:
102  
103     * Press `Ctrl+Alt+P` or go to **Debug > Attach to Process**.
104     * Select `LightSwitchService.exe` and click **Attach**.
105     * You can now set breakpoints in the service files.
106  7. To debug the Settings UI:
107  
108     * Set the startup project to `PowerToys.Settings` and press **F5**.
109     * Note: Light Switch settings will not persist in this mode (they depend on the service executable).
110     * Alternatively, you can attach `PowerToys.Settings.exe` to the debugger while `runner` is running to test the full flow with breakpoints.