Use a voltage regulator from the car battery down to 5V. Now you have a
stable reference to which you can compare the battery voltage.

Bob Ammerman
RAm Systems

----- Original Message -----
From: "Tal Bejerano - AMC" <kooter@ZAHAV.NET.IL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, May 22, 2002 6:34 PM
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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