diff --git a/ActivityObserver/ActivityObserver.hpp b/Activity/Observer.hpp
similarity index 90%
rename from ActivityObserver/ActivityObserver.hpp
rename to Activity/Observer.hpp
index 48115e374..6cf99095e 100644
--- a/ActivityObserver/ActivityObserver.hpp
+++ b/Activity/Observer.hpp
@@ -1,16 +1,18 @@
 //
-//  Header.h
+//  ActivityObserver.h
 //  Clock Signal
 //
 //  Created by Thomas Harte on 07/05/2018.
 //  Copyright © 2018 Thomas Harte. All rights reserved.
 //
 
-#ifndef Header_h
-#define Header_h
+#ifndef ActivityObserver_h
+#define ActivityObserver_h
 
 #include <string>
 
+namespace Activity {
+
 /*!
 	Provides a purely virtual base class for anybody that wants to receive notifications of
 	'activity' — any feedback from an emulated system which a user could perceive other than
@@ -19,7 +21,7 @@
 	So: status LEDs, drive activity, etc. A receiver may choose to make appropriate noises
 	and/or to show or unshow status indicators.
 */
-class ActivityObserver {
+class Observer {
 	public:
 		/// Announces to the receiver that there is an LED of name @c name.
 		virtual void register_led(const std::string &name) = 0;
@@ -44,4 +46,6 @@ class ActivityObserver {
 
 };
 
-#endif /* Header_h */
+}
+
+#endif /* ActivityObserver_h */
diff --git a/Machines/ActivitySource.hpp b/Activity/Source.hpp
similarity index 60%
rename from Machines/ActivitySource.hpp
rename to Activity/Source.hpp
index 2747ebf00..9db6b7a53 100644
--- a/Machines/ActivitySource.hpp
+++ b/Activity/Source.hpp
@@ -9,13 +9,13 @@
 #ifndef ActivitySource_h
 #define ActivitySource_h
 
-#include "../ActivityObserver/ActivityObserver.hpp"
+#include "Observer.hpp"
 
-namespace ActivitySource {
+namespace Activity {
 
-class Machine {
+class Source {
 	public:
-		virtual void set_activity_observer(ActivityObserver *receiver) = 0;
+		virtual void set_activity_observer(Observer *observer) = 0;
 };
 
 }
diff --git a/Analyser/Dynamic/MultiMachine/MultiMachine.cpp b/Analyser/Dynamic/MultiMachine/MultiMachine.cpp
index 6343bcc1c..5f7a92f29 100644
--- a/Analyser/Dynamic/MultiMachine/MultiMachine.cpp
+++ b/Analyser/Dynamic/MultiMachine/MultiMachine.cpp
@@ -22,7 +22,7 @@ MultiMachine::MultiMachine(std::vector<std::unique_ptr<DynamicMachine>> &&machin
 	crt_machine_.set_delegate(this);
 }
 
-ActivitySource::Machine *MultiMachine::activity_source() {
+Activity::Source *MultiMachine::activity_source() {
 	return nullptr; // TODO
 }
 
diff --git a/Analyser/Dynamic/MultiMachine/MultiMachine.hpp b/Analyser/Dynamic/MultiMachine/MultiMachine.hpp
index 9b4124cca..f1a8ea845 100644
--- a/Analyser/Dynamic/MultiMachine/MultiMachine.hpp
+++ b/Analyser/Dynamic/MultiMachine/MultiMachine.hpp
@@ -50,7 +50,7 @@ class MultiMachine: public ::Machine::DynamicMachine, public MultiCRTMachine::De
 		static bool would_collapse(const std::vector<std::unique_ptr<DynamicMachine>> &machines);
 		MultiMachine(std::vector<std::unique_ptr<DynamicMachine>> &&machines);
 
-		ActivitySource::Machine *activity_source() override;
+		Activity::Source *activity_source() override;
 		ConfigurationTarget::Machine *configuration_target() override;
 		CRTMachine::Machine *crt_machine() override;
 		JoystickMachine::Machine *joystick_machine() override;
diff --git a/Machines/DynamicMachine.hpp b/Machines/DynamicMachine.hpp
index 2b51c5e60..a8f2595b5 100644
--- a/Machines/DynamicMachine.hpp
+++ b/Machines/DynamicMachine.hpp
@@ -10,7 +10,7 @@
 #define DynamicMachine_h
 
 #include "../Configurable/Configurable.hpp"
-#include "ActivitySource.hpp"
+#include "../Activity/Source.hpp"
 #include "ConfigurationTarget.hpp"
 #include "CRTMachine.hpp"
 #include "JoystickMachine.hpp"
@@ -26,7 +26,7 @@ namespace Machine {
 struct DynamicMachine {
 	virtual ~DynamicMachine() {}
 
-	virtual ActivitySource::Machine *activity_source() = 0;
+	virtual Activity::Source *activity_source() = 0;
 	virtual ConfigurationTarget::Machine *configuration_target() = 0;
 	virtual CRTMachine::Machine *crt_machine() = 0;
 	virtual JoystickMachine::Machine *joystick_machine() = 0;
diff --git a/Machines/Electron/Electron.cpp b/Machines/Electron/Electron.cpp
index 9b4c18762..103c2f855 100644
--- a/Machines/Electron/Electron.cpp
+++ b/Machines/Electron/Electron.cpp
@@ -8,7 +8,7 @@
 
 #include "Electron.hpp"
 
-#include "../ActivitySource.hpp"
+#include "../../Activity/Source.hpp"
 #include "../ConfigurationTarget.hpp"
 #include "../CRTMachine.hpp"
 #include "../KeyboardMachine.hpp"
@@ -47,7 +47,7 @@ class ConcreteMachine:
 	public CPU::MOS6502::BusHandler,
 	public Tape::Delegate,
 	public Utility::TypeRecipient,
-	public ActivitySource::Machine {
+	public Activity::Source {
 	public:
 		ConcreteMachine() :
 			m6502_(*this),
@@ -480,7 +480,7 @@ class ConcreteMachine:
 			return selection_set;
 		}
 
-		void set_activity_observer(ActivityObserver *observer) override {
+		void set_activity_observer(Activity::Observer *observer) override {
 			activity_observer_ = observer;
 			if(activity_observer_) {
 				activity_observer_->register_led(caps_led);
@@ -578,7 +578,7 @@ class ConcreteMachine:
 		// MARK: - Caps Lock status and the activity observer.
 		const std::string caps_led = "CAPS";
 		bool caps_led_state_ = false;
-		ActivityObserver *activity_observer_ = nullptr;
+		Activity::Observer *activity_observer_ = nullptr;
 };
 
 }
diff --git a/Machines/Electron/Plus3.cpp b/Machines/Electron/Plus3.cpp
index 95178291a..1338bd11d 100644
--- a/Machines/Electron/Plus3.cpp
+++ b/Machines/Electron/Plus3.cpp
@@ -53,3 +53,8 @@ void Plus3::set_motor_on(bool on) {
 	// writing state, so plenty of work to do in general here.
 	get_drive().set_motor_on(on);
 }
+
+void Plus3::set_activity_observer(Activity::Observer *observer) {
+	drives_[0]->set_activity_observer(observer, "Drive 1", true);
+	drives_[1]->set_activity_observer(observer, "Drive 2", true);
+}
diff --git a/Machines/Electron/Plus3.hpp b/Machines/Electron/Plus3.hpp
index 71b8b1c4b..d5bf1a7c9 100644
--- a/Machines/Electron/Plus3.hpp
+++ b/Machines/Electron/Plus3.hpp
@@ -10,6 +10,7 @@
 #define Plus3_hpp
 
 #include "../../Components/1770/1770.hpp"
+#include "../../Activity/Observer.hpp"
 
 namespace Electron {
 
@@ -19,6 +20,7 @@ class Plus3 : public WD::WD1770 {
 
 		void set_disk(std::shared_ptr<Storage::Disk::Disk> disk, int drive);
 		void set_control_register(uint8_t control);
+		void set_activity_observer(Activity::Observer *observer);
 
 	private:
 		void set_control_register(uint8_t control, uint8_t changes);
diff --git a/Machines/Utility/TypedDynamicMachine.hpp b/Machines/Utility/TypedDynamicMachine.hpp
index 07084b035..db3632733 100644
--- a/Machines/Utility/TypedDynamicMachine.hpp
+++ b/Machines/Utility/TypedDynamicMachine.hpp
@@ -25,8 +25,8 @@ template<typename T> class TypedDynamicMachine: public ::Machine::DynamicMachine
 			return *this;
 		}
 
-		ActivitySource::Machine *activity_source() override {
-			return get<ActivitySource::Machine>();
+		Activity::Source *activity_source() override {
+			return get<Activity::Source>();
 		}
 
 		ConfigurationTarget::Machine *configuration_target() override {
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 6a98f57b7..cab4b4feb 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -697,8 +697,6 @@
 		4B055ABE1FAE98000060FFFF /* MachineForTarget.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = MachineForTarget.cpp; sourceTree = "<group>"; };
 		4B055ABF1FAE98000060FFFF /* MachineForTarget.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = MachineForTarget.hpp; sourceTree = "<group>"; };
 		4B055AF01FAE9C080060FFFF /* OpenGL.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = OpenGL.framework; path = System/Library/Frameworks/OpenGL.framework; sourceTree = SDKROOT; };
-		4B055C1E20A113B00014E5AB /* ActivityObserver.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = ActivityObserver.hpp; sourceTree = "<group>"; };
-		4B055C1F20A1166A0014E5AB /* ActivitySource.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = ActivitySource.hpp; sourceTree = "<group>"; };
 		4B0783591FC11D10001D12BB /* Configurable.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = Configurable.cpp; sourceTree = "<group>"; };
 		4B08A2741EE35D56008B7065 /* Z80InterruptTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Z80InterruptTests.swift; sourceTree = "<group>"; };
 		4B08A2761EE39306008B7065 /* TestMachine.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = TestMachine.h; sourceTree = "<group>"; };
@@ -865,6 +863,8 @@
 		4B5073051DDD3B9400C48FBD /* ArrayBuilder.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = ArrayBuilder.cpp; sourceTree = "<group>"; };
 		4B5073061DDD3B9400C48FBD /* ArrayBuilder.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = ArrayBuilder.hpp; sourceTree = "<group>"; };
 		4B5073091DDFCFDF00C48FBD /* ArrayBuilderTests.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = ArrayBuilderTests.mm; sourceTree = "<group>"; };
+		4B51F70920A521D700AFA2C1 /* Source.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = Source.hpp; sourceTree = "<group>"; };
+		4B51F70A20A521D700AFA2C1 /* Observer.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = Observer.hpp; sourceTree = "<group>"; };
 		4B54C0BB1F8D8E790050900F /* KeyboardMachine.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = KeyboardMachine.cpp; sourceTree = "<group>"; };
 		4B54C0BD1F8D8F450050900F /* Keyboard.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; name = Keyboard.cpp; path = Oric/Keyboard.cpp; sourceTree = "<group>"; };
 		4B54C0BE1F8D8F450050900F /* Keyboard.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; name = Keyboard.hpp; path = Oric/Keyboard.hpp; sourceTree = "<group>"; };
@@ -1498,15 +1498,6 @@
 			path = ../SDL;
 			sourceTree = "<group>";
 		};
-		4B055C1D20A113B00014E5AB /* ActivityObserver */ = {
-			isa = PBXGroup;
-			children = (
-				4B055C1E20A113B00014E5AB /* ActivityObserver.hpp */,
-			);
-			name = ActivityObserver;
-			path = ../../ActivityObserver;
-			sourceTree = "<group>";
-		};
 		4B0CCC411C62D0B3001CAC5F /* CRT */ = {
 			isa = PBXGroup;
 			children = (
@@ -1972,6 +1963,16 @@
 			path = 1540;
 			sourceTree = "<group>";
 		};
+		4B51F70820A521D700AFA2C1 /* Activity */ = {
+			isa = PBXGroup;
+			children = (
+				4B51F70920A521D700AFA2C1 /* Source.hpp */,
+				4B51F70A20A521D700AFA2C1 /* Observer.hpp */,
+			);
+			name = Activity;
+			path = ../../Activity;
+			sourceTree = "<group>";
+		};
 		4B55CE551C3B7D360093A61B /* Documents */ = {
 			isa = PBXGroup;
 			children = (
@@ -2708,7 +2709,7 @@
 			isa = PBXGroup;
 			children = (
 				4BC76E6A1C98F43700E6EF73 /* Accelerate.framework */,
-				4B055C1D20A113B00014E5AB /* ActivityObserver */,
+				4B51F70820A521D700AFA2C1 /* Activity */,
 				4B8944E2201967B4007DE474 /* Analyser */,
 				4BB73EA01B587A5100552FC2 /* Clock Signal */,
 				4BB73EB51B587A5100552FC2 /* Clock SignalTests */,
@@ -2812,7 +2813,6 @@
 			isa = PBXGroup;
 			children = (
 				4B54C0BB1F8D8E790050900F /* KeyboardMachine.cpp */,
-				4B055C1F20A1166A0014E5AB /* ActivitySource.hpp */,
 				4BA9C3CF1D8164A9002DDB61 /* ConfigurationTarget.hpp */,
 				4B046DC31CFE651500E9E45E /* CRTMachine.hpp */,
 				4BBB709C2020109C002FE009 /* DynamicMachine.hpp */,
diff --git a/Storage/Disk/Drive.cpp b/Storage/Disk/Drive.cpp
index d62654171..8f5648b12 100644
--- a/Storage/Disk/Drive.cpp
+++ b/Storage/Disk/Drive.cpp
@@ -88,6 +88,14 @@ bool Drive::get_is_ready() {
 
 void Drive::set_motor_on(bool motor_is_on) {
 	motor_is_on_ = motor_is_on;
+
+	if(observer_) {
+		observer_->set_drive_motor_status(drive_name_, motor_is_on_);
+		if(announce_motor_led_) {
+			observer_->set_led_status(drive_name_, motor_is_on_);
+		}
+	}
+
 	if(!motor_is_on) {
 		ready_index_count_ = 0;
 		if(disk_) disk_->flush_tracks();
@@ -265,3 +273,19 @@ void Drive::end_writing() {
 		invalidate_track();
 	}
 }
+
+void Drive::set_activity_observer(Activity::Observer *observer, const std::string &name, bool add_motor_led) {
+	observer_ = observer;
+	announce_motor_led_ = add_motor_led;
+	if(observer) {
+		drive_name_ = name;
+
+		observer->register_drive(drive_name_);
+		observer->set_drive_motor_status(drive_name_, motor_is_on_);
+
+		if(add_motor_led) {
+			observer->register_led(drive_name_);
+			observer->set_led_status(drive_name_, motor_is_on_);
+		}
+	}
+}
diff --git a/Storage/Disk/Drive.hpp b/Storage/Disk/Drive.hpp
index be32071e7..e10ab4099 100644
--- a/Storage/Disk/Drive.hpp
+++ b/Storage/Disk/Drive.hpp
@@ -14,6 +14,7 @@
 #include "Track/PCMPatchedTrack.hpp"
 
 #include "../TimedEventLoop.hpp"
+#include "../../Activity/Observer.hpp"
 #include "../../ClockReceiver/Sleeper.hpp"
 
 #include <memory>
@@ -122,6 +123,10 @@ class Drive: public Sleeper, public TimedEventLoop {
 		// As per Sleeper.
 		bool is_sleeping();
 
+		/// Adds an activity observer; it'll be notified of disk activity.
+		/// The caller can specify whether to add an LED based on disk motor.
+		void set_activity_observer(Activity::Observer *observer, const std::string &name, bool add_motor_led);
+
 	private:
 		// Drives contain an entire disk; from that a certain track
 		// will be currently under the head.
@@ -189,6 +194,11 @@ class Drive: public Sleeper, public TimedEventLoop {
 
 		void setup_track();
 		void invalidate_track();
+
+		// Activity observer description.
+		Activity::Observer *observer_ = nullptr;
+		std::string drive_name_;
+		bool announce_motor_led_ = false;
 };