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Don’t get caught with a flat battery
Car Battery
Monitor
This little Car Battery Monitor provides an
early warning of battery problems. It has
seven LEDs to indicate the battery voltage
and a piezo buzzer to warn when the voltage
drops below 12V.
By ALAN BONNARD
A
CLOSE CALL on the road can
really focus your mind on the
importance of having a battery monitor
in a car. I had been enjoying a pleasant
week of travelling around the countryside at a leisurely pace and taking
in the beautiful scenery each day. It
wasn’t until the final day, with the
big rush to return home, that I had to
drive at night.
My home is deep in the country and
on the road I was travelling the closest
petrol station may be 80km away. I was
travelling through an area that is full of
open-cut coal mines and large heavily
loaded semi-trailers constantly pound
the roads, travelling at quite high
speeds. It was around 8pm at night and
everything was very dark – no street
lights or house lights anywhere.
Just as I was going up a hill, the
lights began to dim and the engine
coughed. A large semi-trailer loomed
in the rear-vision mirror as I pushed
the clutch in and tried to restart. My
Where To Buy The Kit
This project is copyright to Futurlec. It is priced at $12.90 plus
$4 for postage and packing. It
can only be purchased from:
www.futurlec.com
74 Silicon Chip
speed was falling rapidly and my lights
were blacked out – I was like a sitting
duck in the middle of the road, as
the semi-trailer came rapidly bearing
down on me.
I just managed to pull the car off the
road, as the semi-trailer came screaming past, missing me by inches!
After calling for assistance from
the NRMA, the problem was found
to be a fault in the alternator, which
was failing to charge the battery. The
battery voltage had been falling under the heavy load of the lights and
at the worst possible time, there was
not sufficient power for the lights or
the motor.
After the initial shock wore off, I put
on my thinking cap to come up with
a PIC-based solution to the problem.
What was really needed was a display and a buzzer, to get my attention
should the voltage fall outside a specified range. So my design criteria was
set, a series of LEDs could indicate
the voltage and a buzzer would also
be used to warn of problems.
Circuit details
The circuit is based on PIC16F819
18-pin microcontroller which has an
analog-to-digital (A/D) input to monitor the battery voltage and outputs
capable of driving LEDs directly, to
keep the component count down.
There are seven LEDs in all, giving
a good range of voltage indication.
The topmost LED, LED1, comes on for
voltages above 14V which will occur
when the battery is fully charged.
LED2 indicates for voltages between
13.5V and 14V while LED3 indicates
between 13V and 13.5V. Normally, one
of these LEDs will be on. LED4 covers
12.5V to 13V while LED5 covers 12V
to 12.5V.
LED6 covers from 11.5V to 12V
while LED7 comes on for voltages below 11.5V. These two LEDs are backed
up by the piezo chime which beeps for
voltages between 11.5V and 12V and
becomes more insistent for voltages
below 11.5V.
That might seem fairly conservative.
After all, most cars will start with no
troubles, even though the battery voltage might be a touch below 12V, won’t
they? Well, no. Some modern cars will
happily crank the motor at voltages below 11V but their engine management
will not let the motor start unless the
voltage is above 11V.
So don’t think that a modern car will
always start reliably. This little battery
monitor could easily prevent a very
inconvenient failure to start!
So let’s describe the rest of the
circuit. The incoming supply is connected via diode D1 which provides
protection against reverse polarity
while zener diode ZD1 provides protection from spike voltages.
A standard 7805 3-terminal regulator is then used to provide a stable
5V to the microcontroller. The battery
voltage is sensed via a voltage divider
using 33kΩ and 100kΩ resistors. This
brings the voltage down to within the
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Main Features
•
•
•
•
Fig.1: the circuit is based on a PIC16F819 microcontroller. This provides
A/D conversion of the battery voltage and drives the indicator LEDs.
0-5V range for the A/D input of the
PIC16F819. Port B (RB0 to RB7) of the
microcontroller is then used to drive
the various LEDs, with current limiting
provided via the 330Ω resistor network. RB7, pin 13, drives a switching
transistor for the piezo buzzer.
Software
For the software, the design follows
the basic template for a PIC microcontroller. Port A and its ADC (analog-to
-digital converter) function are set up
while port B functions as the output
for the LEDs and buzzer. Once the
set-up is complete, a reading will be
taken at port RA2, the input for the
A/D convertor. This reading is then
compared with a series of values to determine the range of the voltage. This
is similar to a series of “if” statements
in Basic language.
If the voltage is found to be within
a certain range, the relevant port B
pin will be turned on. If the voltage is
below 12V, the buzzer will be turned
on for a brief period, to signal a low
www.siliconchip.com.au
battery condition. As the voltage falls
below 11.5V, the frequency of the
beeps will increase, to signal increased
urgency.
Building it
All the parts are mounted on a
small PC board measuring 46 x 46mm
(available from Futurlec). The starting
point should be the IC socket for the
PIC16F819, as this is easiest to mount
Fig.2: here’s how to install the
parts on the small PC board.
Visual indication of battery
voltage
Audible warning when voltage
becomes low
Screw terminals for easy
connection
Simple and easy to build
while the board is bare. The next item
can be the PC terminal block. The resistors and capacitors can then follow.
Make sure the electrolytics are inserted
with correct polarity.
Make sure that you do not confuse
the zener (ZD1) with the diode when
you are installing them; the diode is
the larger package of the two.
Even more important, don’t get the
78L05 3-terminal regulator and the
2N3906 transistor mixed up; they
come in identical packages. The 78L05
will be labelled as such while the
2N3906 will be labelled “3906”. And
make sure you insert them the correct
way around.
The buzzer must also be installed
with the correct polarity. The 330Ω
current limiting resistors are all in a
10-pin in-line package.
There are four green LEDs, two
yellow and one red. They need to be
installed in line and with the correct
orientation.
Testing
Before you insert the PIC16F819
microcontroller, do a voltage check.
Connect a 12V source and check for
the presence of 5V between pins 14 &
5 OF IC1. If 5V is not present, check
the polarity of regulator REG1 and the
polarity of the diode D1.
If these tests are OK, insert the IC
and test the unit over a range of voltage
between 9V and 15V. Make sure that
all LEDs come on in sequence and
the piezo buzzer beeps for voltages
below 12V.
Now it is matter of installing the unit
in your car. It is preferable to install the
unit in a visible position for the driver.
However, it should not obscure any
other instruments. The unit should be
connected to the car’s 12V supply after
the ignition switch. This will turn the
unit off with the other instruments and
prevent battery drain while the motor
SC
is not running.
December 2003 75
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