That's what I do on my standby generator's PLC, but should work on a 
PIC. A wallwart with 12 VAC out, rectifier bridge, and resistor voltage 
divider with a pot for for calibration adjustment. All I need is voltage 
within a few volts accuracy, to verify the generator is actually putting 
out current, or the power line is down.

Michael Rigby-Jones wrote:
>   
>> -----Original Message-----
>> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
>>     
> Behalf
>   
>> Of Dario Greggio
>> Sent: 08 March 2010 10:18
>> To: Microcontroller discussion list - Public.
>> Subject: [PIC] measuring 220V mains
>>
>> I am to use a 24FJxxGA106 to (along other things) measure 220V mains
>> voltage with some coarse accuracy, say 1-2%
>>
>> I have a small 220->6V transformer, whose inputs goes via a partitor
>> (still have to decide, but let's assume it 4K7 - 2K2) to AN2 of the
>>     
> PIC.
>   
>> So, more or less 2Vrms or 3Vpp circa when mains is nominal 220V.
>>     
>
> Sorry if this is being obvious, but you will either need to rectify the
> output of the transformer or apply a suitable voltage offset in order to
> prevent any negative voltages on the PIC pin.
>
> Mike
>
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