One of the choices is to put your 'F84 in sleep and then wake-up with an interrupt. In power-down mode also an "old" F84 has a very-very low current (datasheet speaks < 0.5 uA at 2V, however is always low).  
If your application permits, another chance is running the PIC at 32Khz speed (again, datasheet speaks 15 uA at 2V) or in R/C mode as variable frequency RC Oscillator: see Microchip Appnote for many suggestions, for example one of these is AN606).
With these current values you can run AA cells for years, obviously when door bell isn't active ; 

regards
Marco



---------- Initial Header -----------

>From      : piclist-bounces@mit.edu
To          : "Microcontroller discussion list - Public." piclist@mit.edu
Cc          : 
Date      : Mon, 29 Aug 2005 19:41:46 -0700 (PDT)
Subject : RE: [PIC] 16F84A power savings....







> I have currently completed a door bell by using 16F84A
> MCU. Now comes the question of power....
> It is possible to run 16F84a in low power for a door
> bell solution? How long would it run on batteries? 
> Is there any pic with PDIP that can run longer?
> This is my first real project........ Please bear with
> me.
> 
> Thanks,
> John Chung
> 
> 
> 		
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