Help with Circuit

[Art]

Hi Guys,
Not PBP specific, but I'm trying to make a pic powered voltmeter using an LCD display and Pic 16F876.
and then hope to go on to log voltages, display averages, predict battery behavior, etc.

A couple of the panel voltmeters sold at Jaycar are powered from the line being measured.
I'm wondering if they go about this by regulating the line with a 7805 or similar to power the device,
and then also feed the unregulated side of the 7805 through a voltage divider,
and then straight into one of the ADC channels without being regulated???

Also.. I have an ADC variable being readout to LCD for experimentation.
The LCD shows zero when the ADC is grounded, and 255 when connected to +5 Volts.
Is this what I should be using for a voltmeter? (once some calculation is done)
Cheers, Art.