diff --git a/include/ExternalSensor.hpp b/include/ExternalSensor.hpp
index fd36bb0..02110fe 100644
--- a/include/ExternalSensor.hpp
+++ b/include/ExternalSensor.hpp
@@ -5,6 +5,7 @@
 
 #include <sdbusplus/asio/object_server.hpp>
 
+#include <chrono>
 #include <string>
 #include <vector>
 
@@ -13,19 +14,49 @@
     public std::enable_shared_from_this<ExternalSensor>
 {
   public:
-    ExternalSensor(const std::string& objectType,
-                   sdbusplus::asio::object_server& objectServer,
-                   std::shared_ptr<sdbusplus::asio::connection>& conn,
-                   const std::string& sensorName,
-                   const std::string& sensorMeasure,
-                   std::vector<thresholds::Threshold>&& thresholds,
-                   const std::string& sensorConfiguration,
-                   const double& maxReading, const double& minReading,
-                   const PowerState& powerState);
+    ExternalSensor(
+        const std::string& objectType,
+        sdbusplus::asio::object_server& objectServer,
+        std::shared_ptr<sdbusplus::asio::connection>& conn,
+        const std::string& sensorName, const std::string& sensorMeasure,
+        std::vector<thresholds::Threshold>&& thresholdsIn,
+        const std::string& sensorConfiguration, double maxReading,
+        double minReading, double timeoutSecs, const PowerState& powerState,
+        std::function<void(std::chrono::steady_clock::time_point now)>&&
+            writeHookIn);
     virtual ~ExternalSensor();
 
+    // Returns true if sensor has external Value that is subject to timeout
+    bool isAliveAndPerishable(void) const;
+
+    // Returns true if AliveAndPerishable and timeout has not yet happened
+    bool
+        isAliveAndFresh(const std::chrono::steady_clock::time_point& now) const;
+
+    // Marks the time when Value successfully received from external source
+    void writeBegin(const std::chrono::steady_clock::time_point& now);
+
+    // Marks sensor as timed out, replacing Value with floating-point "NaN"
+    void writeInvalidate(void);
+
+    // Returns amount of time elapsed since last writeBegin() happened
+    std::chrono::steady_clock::duration
+        ageElapsed(const std::chrono::steady_clock::time_point& now) const;
+
+    // Returns amount of time remaining until sensor timeout will happen
+    std::chrono::steady_clock::duration
+        ageRemaining(const std::chrono::steady_clock::time_point& now) const;
+
   private:
     sdbusplus::asio::object_server& objServer;
 
+    std::chrono::steady_clock::time_point writeLast;
+    std::chrono::steady_clock::duration writeTimeout;
+    bool writeAlive;
+    bool writePerishable;
+    std::function<void(const std::chrono::steady_clock::time_point& now)>
+        writeHook;
+
     void checkThresholds(void) override;
+    void externalSetTrigger(void);
 };