diff --git a/ScienceJournal/ActionArea/ActionAreaBarButtonItem.swift b/ScienceJournal/ActionArea/ActionAreaBarButtonItem.swift
index 0fd1ec9..b66a286 100644
--- a/ScienceJournal/ActionArea/ActionAreaBarButtonItem.swift
+++ b/ScienceJournal/ActionArea/ActionAreaBarButtonItem.swift
@@ -43,6 +43,7 @@ extension ActionArea {
     private(set) var title: String
     private(set) var image: UIImage?
     private(set) var action: () -> Void // TODO: Consider passing the AA controller here.
+    private(set) var enabler: FeatureEnabler?
 
     /// Designated initializer.
     ///
@@ -50,10 +51,16 @@ extension ActionArea {
     ///   - title: The title for the item.
     ///   - accessibilityHint: The accessibility hint for the item.
     ///   - image: The image for the item.
+    ///   - enabler: A feature enabler to handle changes in an observed Bool value.
     ///   - action: A block to execute when the item is tapped.
-    init(title: String, accessibilityHint: String?, image: UIImage?, action: @escaping () -> Void) {
+    init(title: String,
+         accessibilityHint: String?,
+         image: UIImage?,
+         enabler: FeatureEnabler? = nil,
+         action: @escaping () -> Void) {
       self.title = title
       self.image = image
+      self.enabler = enabler
       self.action = action
       super.init()
       self.accessibilityHint = accessibilityHint
diff --git a/ScienceJournal/ActionArea/ActionAreaBarViewController.swift b/ScienceJournal/ActionArea/ActionAreaBarViewController.swift
index 33708c7..06261cf 100644
--- a/ScienceJournal/ActionArea/ActionAreaBarViewController.swift
+++ b/ScienceJournal/ActionArea/ActionAreaBarViewController.swift
@@ -429,6 +429,12 @@ extension ActionArea {
         (stackView.arrangedSubviews.count ..< 4).forEach { _ in
           stackView.addArrangedSubview(UIView())
         }
+
+        items.forEach { (item) in
+          item.enabler?.observe({ (enabled) in
+            self.setButtonFor(item: item, enabled: enabled)
+          })
+        }
       }
     }
 
@@ -448,8 +454,12 @@ extension ActionArea {
       fatalError("init(coder:) has not been implemented")
     }
 
+    func setButtonFor(item: BarButtonItem, enabled: Bool, animated: Bool = true) {
+      if let index = items.index(of: item) {
+        buttons[index].setEnabled(enabled, animated: animated)
+      }
+    }
   }
-
 }
 
 // MARK: - ActionArea.BarButton
@@ -527,8 +537,20 @@ extension ActionArea {
       button.backgroundColor = customTint.secondary
     }
 
+    func setEnabled(_ enabled: Bool, animated: Bool = true) {
+      button.isEnabled = enabled
+      let updatedAlpha: CGFloat = enabled ? 1.0 : BarViewController.Metrics.Bar.disabledAlpha
+      if animated {
+        UIView.animate(withDuration: 0.5) {
+          self.button.alpha = updatedAlpha
+          self.label.alpha = updatedAlpha
+        }
+      } else {
+        self.button.alpha = updatedAlpha
+        self.label.alpha = updatedAlpha
+      }
+    }
   }
-
 }
 
 // MARK: - Debugging
diff --git a/ScienceJournal/UI/ObserveViewController.swift b/ScienceJournal/UI/ObserveViewController.swift
index f814e97..5ed6130 100644
--- a/ScienceJournal/UI/ObserveViewController.swift
+++ b/ScienceJournal/UI/ObserveViewController.swift
@@ -176,6 +176,8 @@ open class ObserveViewController: ScienceJournalCollectionViewController, ChartC
     return self.observeDataSource.availableSensorIDs
   }
 
+  @objc dynamic private(set) var unusedSensors: Bool = false
+
   // MARK: - Public
 
   /// Designated initializer.
@@ -197,6 +199,7 @@ open class ObserveViewController: ScienceJournalCollectionViewController, ChartC
 
     let flowLayout = MDCCollectionViewFlowLayout()
     flowLayout.minimumLineSpacing = SensorCardCell.cardInsets.bottom
+
     super.init(collectionViewLayout: flowLayout, analyticsReporter: analyticsReporter)
 
     observeDataSource.delegate = self
@@ -757,6 +760,10 @@ open class ObserveViewController: ScienceJournalCollectionViewController, ChartC
     sensorCard.toneGenerator.stop()
   }
 
+  private func updateUnusedSensors() {
+    unusedSensors = observeDataSource.items.count < sensorController.availableSensors.count
+  }
+
   private func removeSensorCardCell(_ cell: SensorCardCell) {
     guard let indexPath = collectionView?.indexPath(for: cell) else {
       return
@@ -780,7 +787,7 @@ open class ObserveViewController: ScienceJournalCollectionViewController, ChartC
       }
     }, completion: { (_) in
       self.updateSensorPickersIfNeeded()
-      // TODO: Show/Enable the "Add Sensor" button if needed
+      self.updateUnusedSensors()
     })
   }
 
@@ -809,7 +816,7 @@ open class ObserveViewController: ScienceJournalCollectionViewController, ChartC
         self.collectionView?.deleteSections(IndexSet(integer: previousFooterIndexPath.section))
       }
     }, completion: { (_) in
-      // TODO: Hide/Disable the "Add Sensor" button.
+      self.updateUnusedSensors()
     })
     collectionView?.scrollToItem(at: IndexPath(item: newItemIndexPath.item, section: 0),
                                  at: .top,
@@ -1028,6 +1035,7 @@ open class ObserveViewController: ScienceJournalCollectionViewController, ChartC
     addInitialSensorCardIfNeeded(andAddListener: andAddListeners)
 
     collectionView?.reloadData()
+    updateUnusedSensors()
   }
 
   /// Sets the available sensors for the current experiment.
diff --git a/ScienceJournal/UI/UserFlowViewController.swift b/ScienceJournal/UI/UserFlowViewController.swift
index 4d5ebd9..5a295b1 100644
--- a/ScienceJournal/UI/UserFlowViewController.swift
+++ b/ScienceJournal/UI/UserFlowViewController.swift
@@ -962,7 +962,9 @@ class UserFlowViewController: UIViewController, ExperimentsListViewControllerDel
       let addSensorItem = ActionArea.BarButtonItem(
         title: String.actionAreaButtonAddSensor,
         accessibilityHint: String.actionAreaButtonAddSensorContentDescription,
-        image: UIImage(named: "ic_action_area_add_sensor")
+        image: UIImage(named: "ic_action_area_add_sensor"),
+        enabler: FeatureEnabler(target: experimentCoordinator.observeViewController,
+                                keyPath: \.unusedSensors)
       ) {
         experimentCoordinator.observeViewController.observeFooterAddButtonPressed()
       }