LEDs that light up progressively as the
voltage increases:
- Red: Power Connected (0%)
- Yellow: Greater than 10.5V (25%)
- Green 1: Greater than 11.5V (50%)
- Green 2: Greater than 12.5V (75%)
Of course, you may select your own
colors if desired.
12 Volts Battery Level Indicator
Circuit Schematic
Operation of the battery level
indicator
D1 is the voltage reference zener. Tied
to this is a string of divider resistors
(R2-6) that set the various fixed
voltage levels. R7 & 8 form a voltage
divider to that reduces the battery
voltage by a factor of 3. U1 is an
LM339 quad comparator that
compares the various voltages from
the two dividers. The comparator
sections have open collector outputs
that simply function as switches to
operate the LEDs. D7 protects against
reverse battery connection.
The LEDs are biased to operate at
about 4mA which is quite bright if
modern LEDs are used. This current
can be adjusted simply by varying the
series resistors (R9 through R13). The
overall current drain as shown is
about 25mA which tends to be
wasteful for continuous operation. For
energy conservation, connect to
battery via a pushbutton (Push to
Test).
Printed Circuit Board
I did a www.expresspcb.com SMT
layout using 0805 size components,
1N753 zener and SOIC-14 IC. D7 is in
a SOT-23 package. These components
are about as small as I like to work
with. The layout has not yet been
carefully checked or built. Note that
surprises abound when constructing
prototypes.
The circuit board measures only 0.5” x
1.5”.
download the PCB layout
More recently, I located an
inexpensive SOT-23 zener with a 2%
voltage tolerance—this has not yet
been incorporated.
I have had good results with 0805 size
LEDs purchased from China on eBay.
They are both inexpensive and
BRIGHT!
The 12v battery level indicator unit in
the photo has no reverse polarity
diode and R2 is the calibration
potentiometer.
Source: Elektro symetrics
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