For a good answer you need to provide more information.

But based on what you stated, you could do this with two diodes to 
switch power and another diode-R-C network to tell the PIC that AC power 
has gone bye-bye.  The alarm can then be whatever you wish: LED, buzzer, 
siren, etc.

If this is a one-off application, you could buy a computer 
battery-backup power supply (often called UPS - Uninterruptible Power 
Supply) which will do everything you want.

Miguel Reznicek wrote:
> Friends:
> 
> Does someone know of a PIC chip circuit that will monitor the 220 VAC mains
> and use them to run, but that can automatically switch to a battery power
> when the mains fails?  I need to make an alarm for when the power fails.
> 
> Thanks in advance,
> 
> Miguel Reznicek
> mreznicek@pretensa.com
> 
> 

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