diff --git a/NightLight.Core.Tests/FakeHome.fs b/NightLight.Core.Tests/FakeHome.fs
new file mode 100644
index 0000000..df2da1b
--- /dev/null
+++ b/NightLight.Core.Tests/FakeHome.fs
@@ -0,0 +1,81 @@
+namespace NightLight.Core.Tests
+
+open System
+open System.Text.RegularExpressions
+open NightLight.Core.Models
+open FsToolkit.ErrorHandling
+open FSharp.Data
+
+type HumanInteraction =
+    | LightTurnedOn of Light
+    | LightTurnedOff of Light
+
+type Interaction =
+    | HumanInteraction of HumanInteraction
+    | TimeChanged of DateTime
+
+type LightState =
+    | Off
+    | On of Brightness: byte
+
+type FakeLight(light: Light) =
+    let mutable hasPower = false
+    let mutable brightness: byte = 255uy
+
+    member _.LightWithState = light, if hasPower then On brightness else Off
+
+    member _.TurnOn() = hasPower <- true
+
+    member _.TurnOff() = hasPower <- false
+
+    member _.SetBrightness(newBrightness: byte) =
+        if hasPower then
+            brightness <- newBrightness
+
+type FakeHome(now: DateTime) =
+    let nightLightStateMachine = NightLightStateMachine now
+
+    let friendlyNameToFakeLight =
+        lights
+        |> Seq.map (fun light -> light.FriendlyName, FakeLight light)
+        |> Map.ofSeq
+
+    let processCommand command =
+        option {
+            let! friendlyName =
+                let m = Regex.Match(command.Topic, "^zigbee2mqtt/(.+)/set$")
+                if m.Success then Some m.Groups.[1].Value else None
+
+            let! fakeLight = Map.tryFind friendlyName friendlyNameToFakeLight
+
+            let parsedPayload = JsonValue.Parse command.Payload
+
+            match parsedPayload.TryGetProperty "brightness" with
+            | Some(JsonValue.Number newBrightness) -> fakeLight.SetBrightness(byte newBrightness)
+            | None -> ()
+            | value -> failwith $"Unexpected brightness value {value}"
+        }
+        |> ignore
+
+    member _.LightStates = friendlyNameToFakeLight.Values |> Seq.map _.LightWithState
+
+    member _.Interact(interaction: Interaction) : Result<unit, ParseEventError> =
+        result {
+            match interaction with
+            | HumanInteraction(LightTurnedOn light) ->
+                friendlyNameToFakeLight[light.FriendlyName].TurnOn()
+
+                do!
+                    { Topic = "zigbee2mqtt/bridge/event"
+                      Payload =
+                        $@"{{
+                            ""type"": ""device_announce"",
+                            ""data"": {{ ""friendly_name"": ""{light.FriendlyName}"" }}
+                          }}" }
+                    |> nightLightStateMachine.SendMessage
+            | HumanInteraction(LightTurnedOff light) -> friendlyNameToFakeLight[light.FriendlyName].TurnOff()
+            | TimeChanged time -> do! nightLightStateMachine.ChangeTime time
+
+            nightLightStateMachine.TransmittedCommands |> Seq.iter processCommand
+            nightLightStateMachine.ClearTransmittedCommands()
+        }
diff --git a/NightLight.Core.Tests/NightLight.Core.Tests.fsproj b/NightLight.Core.Tests/NightLight.Core.Tests.fsproj
new file mode 100644
index 0000000..4979a7a
--- /dev/null
+++ b/NightLight.Core.Tests/NightLight.Core.Tests.fsproj
@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <TargetFramework>net9.0</TargetFramework>
+        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <Compile Include="NightLightStateMachine.fs" />
+        <Compile Include="FakeHome.fs" />
+    </ItemGroup>
+
+    <ItemGroup>
+	<PackageReference Include="coverlet.collector" Version="6.0.2" />
+	<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+	<PackageReference Include="xunit" Version="2.9.2" />
+	<PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+	<PackageReference Include="FsCheck" Version="3.3.0" />
+	<PackageReference Include="FsCheck.Xunit" Version="3.3.0" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="../NightLight.Core/NightLight.Core.fsproj" />
+    </ItemGroup>
+
+</Project>
diff --git a/NightLight.Core.Tests/NightLightStateMachine.fs b/NightLight.Core.Tests/NightLightStateMachine.fs
new file mode 100644
index 0000000..0379a3b
--- /dev/null
+++ b/NightLight.Core.Tests/NightLightStateMachine.fs
@@ -0,0 +1,28 @@
+namespace NightLight.Core.Tests
+
+open System
+open System.Collections.Generic
+open FsToolkit.ErrorHandling
+open NightLight.Core.Models
+open NightLight.Core.Core
+
+type NightLightStateMachine(now: DateTime) =
+    let mutable state = { Time = now }
+
+    let transmittedCommands = new List<Message>()
+
+    let sendEvent event =
+        result {
+            let! newState, commands = onEventReceived state event
+            state <- newState
+            transmittedCommands.AddRange commands
+        }
+
+    member _.TransmittedCommands = transmittedCommands.AsReadOnly()
+
+    member _.SendMessage message =
+        ReceivedZigbeeEvent message |> sendEvent
+
+    member _.ChangeTime time = TimeChanged time |> sendEvent
+
+    member _.ClearTransmittedCommands() = transmittedCommands.Clear()
diff --git a/NightLight.sln b/NightLight.sln
index 0918513..7985b44 100644
--- a/NightLight.sln
+++ b/NightLight.sln
@@ -4,6 +4,8 @@ Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "NightLight", "NightLight\Ni
 EndProject
 Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "NightLight.Core", "NightLight.Core\NightLight.Core.fsproj", "{FE406BDE-B6C1-4BDA-A29D-54D50A7828A9}"
 EndProject
+Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "NightLight.Core.Tests", "NightLight.Core.Tests\NightLight.Core.Tests.fsproj", "{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -38,6 +40,18 @@ Global
 		{FE406BDE-B6C1-4BDA-A29D-54D50A7828A9}.Release|x64.Build.0 = Release|Any CPU
 		{FE406BDE-B6C1-4BDA-A29D-54D50A7828A9}.Release|x86.ActiveCfg = Release|Any CPU
 		{FE406BDE-B6C1-4BDA-A29D-54D50A7828A9}.Release|x86.Build.0 = Release|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Debug|x64.Build.0 = Debug|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Debug|x86.Build.0 = Debug|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Release|Any CPU.Build.0 = Release|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Release|x64.ActiveCfg = Release|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Release|x64.Build.0 = Release|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Release|x86.ActiveCfg = Release|Any CPU
+		{23C7B106-B1B8-49A8-B2CF-22C078C8DDB3}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE