Use optocouplers.
And you could just have the 3 optocouplers conneted in series on the PIC-
side and thereby get the AND gate function as a little bonus !

Btw, just out of curiosity... do you have 3 phases and a 0-line and also
protective ground as we do here in Sweden ? (total 5 wires)

/PJ

> I have designed a 16f628 based timer for 3 phase loads (using a contactor).
> Now, here in India, its quite a common occurance that 1 or 2 phases in the
> 3 phase supply fail. And it could quite easily kill the 3 Phase Motors.
> I want to design a system for detecting phase failiure and to signal
> the PIC about this. I've got only one Input pin left on the PIC (MCLR!). Any
> suggestions or Ideas ?
> I thought of using a 3 input AND gate fed by each of the phase via a
> 1 meg resistor and a 1N4007 diode. and connecting the output of the AND
> gate to the PIC. Is this ok, or do I need to use 3 opto couplers for isolation?
> Jeethu Rao

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