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'Another Battery Monitor Sch'
1997\10\17@123915 by Walter Lenk

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         Subject:   Another Battery Monitor Scheme

On a recent 9 Volt battery powered project, I implemented a battery level
monitor scheme by measuring the voltage drop across the low dropout 5V
regulator (LT1121CZ-5).  This was implemented with a single PNP transistor
(2N4403) and 4 resistors as follows : Q1's emitter was connected to the 5V
regulator's input terminal ; R1 (15K) was connected from Q1's emitter to it's
base ; R2 (33K) was connected from Q1's base to the 5V regulator's output
terminal ; R3 (47K) was connected from Q1's collector to ground ; R4 (100K)
was connected from Q1's collector to the PIC 12C508's GP3 input.  As long as
the battery's voltage is greater than about 6.7 Volts, R4 tries to pull GP3
hi (Gp3 is also connected to the output of a LTC1298 serial A2D convertor).
The software polls GP3 once a second for the battery level information, and
the user is then notified of battery condition by the pilot LED flash pattern
: one 20 mS flash every Second for a good battery, two successive 20 mS
flashes every 2 Seconds for a low battery.
This scheme works pretty well, but it has several restrictions : 1) the post
regulator current drawn by the load must be at least .11 mA to offset the
regulator bypass current sourced by R1 & R2, and 2) the whole circuit needs
to have a mechanical or electrical on/off switch before it (you can't use the
shutdown pin on the regulator and still have the circuit turn completely
off).

I hope this is useful,
Walter Lenk    Cambridge, MA

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