diff --git a/Expanding-Sonoffs.md b/Expanding-Sonoffs.md
index 37f51693..a5e8d6ed 100644
--- a/Expanding-Sonoffs.md
+++ b/Expanding-Sonoffs.md
@@ -155,7 +155,7 @@ The ESP8266 has one SPI connection available to the user, referred to as HSPI. I
 
 ### GPIO overview
 
-NodeMCU Labelled Pin | GPIO | 	Function | 	State	 | Restrictions
+[NodeMCU Labelled Pin](https://techtutorialsx.com/2017/04/02/esp8266-nodemcu-pin-mappings/) | GPIO | 	Function | 	State	 | Restrictions
 -|-|-|-|-
 D3 | 0 | 	Boot mode select	 | 3.3V	 | No Hi-Z
 D10 | 1 | 	TX0 | 	-	 | Not usable during Serial transmission