diff --git a/src/CollarRx.cpp b/src/CollarRx.cpp
index 205ad32..10b10af 100644
--- a/src/CollarRx.cpp
+++ b/src/CollarRx.cpp
@@ -79,6 +79,10 @@ const char *CollarRx::mode_to_str(collar_mode mode)
 
 void CollarRx::rx_start()
 {
+  #ifdef ISR_MONITOR_PIN
+   pinMode(ISR_MONITOR_PIN,OUTPUT); 
+  #endif 
+
   _instance = this;
   pinMode(_rx_pin, INPUT_PULLUP);
   attachInterrupt(digitalPinToInterrupt(_rx_pin), CollarRx::s_isr, CHANGE);
diff --git a/src/collar.h b/src/collar.h
index 7611688..4b53732 100644
--- a/src/collar.h
+++ b/src/collar.h
@@ -5,6 +5,9 @@
 #include <stdint.h>
 #include "Arduino.h"
 
+// show running ISR on defined pin for debug
+#define ISR_MONITOR_PIN 5
+
 enum collar_mode { SHOCK=1, VIBE=2, BEEP=3 };
 enum collar_channel { CH1=0, CH2=1, CH3=2 };
 
diff --git a/src/type1/CollarRxType1.cpp b/src/type1/CollarRxType1.cpp
index 0d6c4a4..aca9806 100644
--- a/src/type1/CollarRxType1.cpp
+++ b/src/type1/CollarRxType1.cpp
@@ -55,6 +55,9 @@ bool CollarRxType1::is_message_valid(const uint8_t buffer[5])
 
 void CollarRxType1::isr()
 {
+  #ifdef ISR_MONITOR_PIN
+    digitalWrite(ISR_MONITOR_PIN,HIGH);
+  #endif
   static unsigned long rx_micros =0;
   static unsigned int high_pulse_len =0;
   static unsigned int low_pulse_len=0;
@@ -66,6 +69,9 @@ void CollarRxType1::isr()
   { //falling edge
     high_pulse_len = micros() - rx_micros;
     rx_micros = micros(); //start measurement of pulse length for low state
+    #ifdef ISR_MONITOR_PIN
+      digitalWrite(ISR_MONITOR_PIN,LOW);
+    #endif
     return;
   } 
   else 
@@ -82,6 +88,9 @@ void CollarRxType1::isr()
         byte_position = 0;
         bit_position = 0;
         memset(buffer, 0, sizeof(buffer));
+        #ifdef ISR_MONITOR_PIN
+          digitalWrite(ISR_MONITOR_PIN,LOW);
+        #endif
         return;
     }
     else
@@ -111,6 +120,9 @@ void CollarRxType1::isr()
                 buffer_to_collar_message(buffer, &_rx_msg);
                 _cb(&_rx_msg, _userdata);
                 byte_position=-1; //done, wait for new start
+                #ifdef ISR_MONITOR_PIN
+                  digitalWrite(ISR_MONITOR_PIN,LOW);
+                #endif
                 return;
               }
             }
@@ -120,9 +132,16 @@ void CollarRxType1::isr()
         {
           //transmission error, wait for new start
           byte_position=-1;
+          #ifdef ISR_MONITOR_PIN
+             digitalWrite(ISR_MONITOR_PIN,LOW);
+          #endif
           return;
         }
       }
     }
   }
+  #ifdef ISR_MONITOR_PIN
+      digitalWrite(ISR_MONITOR_PIN,LOW);
+  #endif
+  
 }