Added deep sleep to save battery -- but this solution is not clean, as deep sleep is possible for an hour only and peripherals (display) is not turned off.

src/main.cpp

@@ -16,6 +16,7 @@
 #include "radio.h"
 
 #define AUTOSTART_CLOCK_AFTER_ms 10000
+#define POWER_OFF_PIN 0
 
 //define your default values here, if there are different values in config.json, they are overwritten.
 //length should be max size + 1
@@ -134,9 +135,12 @@ void setupWifiConnection() {
 }
 
 void setup() {
+  // if coming from deep sleep, we just go to sleep again
+
   // put your setup code here, to run once:
   Serial.begin(115200);
   Serial.println("Starting *** FastclockMasterESP8266");
+  Serial.println(ESP.getResetReason());
   display.begin();
   display.showLog();
 
@@ -205,6 +209,7 @@ void setup() {
 
   radio.begin();
   fastclock.begin();
+  pinMode(POWER_OFF_PIN, INPUT);
 }
 
 bool requestToRegisterSent = false;
@@ -221,6 +226,24 @@ void loop(void)
     return;
   }
 
+  static unsigned long powerOffButtonPressed_ts = 0;
+  static bool powerOffMessagePrinted = false;
+  if (!digitalRead(POWER_OFF_PIN)) {
+    if (!powerOffMessagePrinted) {
+      Serial.println("Switching off? Keep button for 2 seconds to turn off.");
+      powerOffMessagePrinted = true;
+    }
+    if (millis() - powerOffButtonPressed_ts > 2000) {
+      // pressed for longer than 2 seconds, now turn off
+      ESP.deepSleep(ESP.deepSleepMax());
+      delay(200);
+    }
+  } else {
+    powerOffButtonPressed_ts = millis();
+    if (powerOffMessagePrinted) Serial.println("Power off cancelled.");
+    powerOffMessagePrinted = false;
+  }
+
   #if defined(AUTOSTART_CLOCK_AFTER_ms)
     #if AUTOSTART_CLOCK_AFTER_ms > 0
       static unsigned long autostart_ms = 0;