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i got a circuit which detect earth fault, i.e if earthing is provided or not. it indicates fault using LED. Now i want to use this circuit with arduino so that i can send alert remotely using gsm. not getting any idea how to proceed. below is the link--- Earth Fault Detector
Just replace the indicator LED [LED1] with an opto isolator. The transistor in that will be switched on with 50Hz pulses any time the LED would have illuminated.
However, note that it's a rather primitive circuit and will only sense an earth disconnection if that causes a difference in voltage between neutral and earth. That's quite possible but in no way guaranteed.
Note that R1 will get hot, it definitely needs to be a 1W or possibly 2W resistor; it's average dissipation is about 0.7W so rather dependant on construction and airflow as to what's actually required for safety.