diff --git a/sonoff/_changelog.ino b/sonoff/_changelog.ino
index 4d6cb27aa..0071e38d1 100644
--- a/sonoff/_changelog.ino
+++ b/sonoff/_changelog.ino
@@ -1,4 +1,7 @@
-/* 6.5.0.10 20190513
+/* 6.5.0.11 20190517
+ * Add command SetOption64 0/1 to switch between "-" or "_" as sensor index separator impacting DS18X20, DHT, BMP and SHT3X sensor names (#5689)
+ *
+ * 6.5.0.10 20190513
  * Enable ADC0 by default in my_user_config.h (#5671)
  * Add user configurable ADC0 to Module and Template configuration compatible with current FLAG options (#5671)
  * Add support for Shelly 1PM Template {"NAME":"Shelly 1PM","GPIO":[56,0,0,0,82,134,0,0,0,0,0,21,0],"FLAG":2,"BASE":18} (#5716)
diff --git a/sonoff/settings.h b/sonoff/settings.h
index 4c56fe62b..c08c3e041 100644
--- a/sonoff/settings.h
+++ b/sonoff/settings.h
@@ -77,7 +77,7 @@ typedef union {                            // Restricted by MISRA-C Rule 18.4 bu
     uint32_t button_switch_force_local : 1;// bit 11 (v6.3.0.16) - SetOption61 - Force local operation when button/switch topic is set
     uint32_t no_hold_retain : 1;           // bit 12 (v6.4.1.19) - SetOption62 - Don't use retain flag on HOLD messages
     uint32_t no_power_feedback : 1;        // bit 13 (v6.5.0.9)  - SetOption63 - Don't scan relay power state at restart
-    uint32_t spare14 : 1;
+    uint32_t use_underscore : 1;           // bit 14 (v6.5.0.12) - SetOption64 - Enable "_" instead of "-" as sensor index separator
     uint32_t spare15 : 1;
     uint32_t spare16 : 1;
     uint32_t spare17 : 1;
diff --git a/sonoff/sonoff_version.h b/sonoff/sonoff_version.h
index 4dea3a792..ce4413124 100644
--- a/sonoff/sonoff_version.h
+++ b/sonoff/sonoff_version.h
@@ -20,6 +20,6 @@
 #ifndef _SONOFF_VERSION_H_
 #define _SONOFF_VERSION_H_
 
-const uint32_t VERSION = 0x0605000A;
+const uint32_t VERSION = 0x0605000B;
 
 #endif  // _SONOFF_VERSION_H_
diff --git a/sonoff/support.ino b/sonoff/support.ino
index 953297dbf..317d25065 100644
--- a/sonoff/support.ino
+++ b/sonoff/support.ino
@@ -437,6 +437,21 @@ char* NoAlNumToUnderscore(char* dest, const char* source)
   return dest;
 }
 
+char IndexSeparator()
+{
+/*
+  // 20 bytes more costly !?!
+  const char separators[] = { "-_" };
+
+  return separators[Settings.flag3.use_underscore];
+*/
+  if (Settings.flag3.use_underscore) {
+    return '_';
+  } else {
+    return '-';
+  }
+}
+
 void SetShortcut(char* str, uint8_t action)
 {
   if ('\0' != str[0]) {     // There must be at least one character in the buffer
diff --git a/sonoff/xsns_05_ds18x20.ino b/sonoff/xsns_05_ds18x20.ino
index 7a67f80a9..cee07f1d7 100644
--- a/sonoff/xsns_05_ds18x20.ino
+++ b/sonoff/xsns_05_ds18x20.ino
@@ -380,7 +380,7 @@ void Ds18x20Name(uint8_t sensor)
   }
   GetTextIndexed(ds18x20_types, sizeof(ds18x20_types), index, kDs18x20Types);
   if (ds18x20_sensors > 1) {
-    snprintf_P(ds18x20_types, sizeof(ds18x20_types), PSTR("%s-%d"), ds18x20_types, sensor +1);
+    snprintf_P(ds18x20_types, sizeof(ds18x20_types), PSTR("%s%c%d"), ds18x20_types, IndexSeparator(), sensor +1);
   }
 }
 
diff --git a/sonoff/xsns_05_ds18x20_legacy.ino b/sonoff/xsns_05_ds18x20_legacy.ino
index d92843393..b31085e8b 100644
--- a/sonoff/xsns_05_ds18x20_legacy.ino
+++ b/sonoff/xsns_05_ds18x20_legacy.ino
@@ -197,7 +197,7 @@ void Ds18x20Show(bool json)
 #endif  // USE_KNX
 #ifdef USE_WEBSERVER
       } else {
-        snprintf_P(stemp, sizeof(stemp), PSTR("%s-%d"), ds18x20_types, i +1);
+        snprintf_P(stemp, sizeof(stemp), PSTR("%s%c%d"), ds18x20_types, IndexSeparator(), i +1);
         WSContentSend_PD(HTTP_SNS_TEMP, stemp, temperature, TempUnit());
 #endif  // USE_WEBSERVER
       }
diff --git a/sonoff/xsns_06_dht.ino b/sonoff/xsns_06_dht.ino
index aecdb18f9..7615dbc6c 100644
--- a/sonoff/xsns_06_dht.ino
+++ b/sonoff/xsns_06_dht.ino
@@ -187,7 +187,7 @@ void DhtInit(void)
     Dht[i].lastresult = 0;
     GetTextIndexed(Dht[i].stype, sizeof(Dht[i].stype), Dht[i].type, kSensorNames);
     if (dht_sensors > 1) {
-      snprintf_P(Dht[i].stype, sizeof(Dht[i].stype), PSTR("%s-%02d"), Dht[i].stype, Dht[i].pin);
+      snprintf_P(Dht[i].stype, sizeof(Dht[i].stype), PSTR("%s%c%02d"), Dht[i].stype, IndexSeparator(), Dht[i].pin);
     }
   }
 }
diff --git a/sonoff/xsns_09_bmp.ino b/sonoff/xsns_09_bmp.ino
index 82b681cc4..12252e3b6 100755
--- a/sonoff/xsns_09_bmp.ino
+++ b/sonoff/xsns_09_bmp.ino
@@ -542,7 +542,7 @@ void BmpShow(bool json)
       char name[10];
       strlcpy(name, bmp_sensors[bmp_idx].bmp_name, sizeof(name));
       if (bmp_count > 1) {
-        snprintf_P(name, sizeof(name), PSTR("%s-%02X"), name, bmp_sensors[bmp_idx].bmp_address);  // BMXXXX-XX
+        snprintf_P(name, sizeof(name), PSTR("%s%c%02X"), name, IndexSeparator(), bmp_sensors[bmp_idx].bmp_address);  // BMXXXX-XX
       }
 
       char temperature[33];
diff --git a/sonoff/xsns_14_sht3x.ino b/sonoff/xsns_14_sht3x.ino
index 7ead33fcf..17ac313ee 100755
--- a/sonoff/xsns_14_sht3x.ino
+++ b/sonoff/xsns_14_sht3x.ino
@@ -104,7 +104,7 @@ void Sht3xShow(bool json)
         dtostrfd(t, Settings.flag2.temperature_resolution, temperature);
         char humidity[33];
         dtostrfd(h, Settings.flag2.humidity_resolution, humidity);
-        snprintf_P(types, sizeof(types), PSTR("%s-0x%02X"), sht3x_sensors[i].types, sht3x_sensors[i].address);  // "SHT3X-0xXX"
+        snprintf_P(types, sizeof(types), PSTR("%s%c0x%02X"), sht3x_sensors[i].types, IndexSeparator(), sht3x_sensors[i].address);  // "SHT3X-0xXX"
 
         if (json) {
           ResponseAppend_P(JSON_SNS_TEMPHUM, types, temperature, humidity);