I did this recently with a PIC 16F872.  A 5V regulator drops the ~12V
down to the operating voltage of the PIC, as well as providing the reference
voltage.  The input is simply a voltage divider on the ~12V supply to limit
the voltage going into the PIC.  Actually I set it up so 25.5V at the input
will divide down to 5V.  This way, 25.5V reads 255 on the top 8 bits of
the A/D and I saved myself some math that way.  But I do lose a bit of
resolution (not a big deal).  Also have upper and lower alarms settable
with a single-button interface.

Cheers,
-Neil.



-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
Behalf Of Tal Bejerano - AMC
Sent: Wednesday, May 22, 2002 5:35 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: Voltage Compare


Hi All

I want to measure and get a warning if car battery is drop under 11.5v.
look at some circuits that use op-amps/comparators and read the explanations
and I wonder, how can it work? if the circuit use the battery voltage to
operate and the voltage drops, then all the adjustments are not correct. as
I understand it I need 2 voltage sources: one is stable and the other is the
battery I monitor. ???????

Regards

Tal Bejerano
AMC - ISRAEL

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