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This circuit uses the popular and easy
to find LM3914 IC. This IC is very simple to drive,
needs no voltage regulators (it has a built in voltage
regulator) and can be powered from almost every source.
This circuit is very easy to explain:
When the test button is pressed, the Car
battery voltage is feed into a high impedance voltage
divider. His purpose is to divide 12V to 1,25V (or lower
values to lower values). This solution is better than
letting the internal voltage regulator set the 12V
sample voltage to be feed into the internal voltage
divider simply because it cannot regulate 12V when the
voltage drops lower (linear regulators only step down).
Simply wiring with no adjust, the regulator provides
stable 1,25V which is fed into the precision internal
resistor cascade to generate sample voltages for the
internal comparators. Anyway the default setting let you
to measure voltages between 8 and 12V but you can
measure even from 0V to 12V setting the offset trimmer
to 0 (but i think that under 9 volt your car would not
start). There is a smoothing capacitor (4700uF 16V) it
is used to adsorb EMF noise produced from the ignition
coil if you are measuring the battery during the engine
working. Diesel engines would not need it, but i'm not
sure. If you like more a point graph rather than a bar
graph simply disconnect pin 9 on the IC (MODE) from
power. The calculations are simple (default)
For the first comparator the voltage is
: 0,833 V corresponding to 8 V
* * * * * voltage is : 0,875 V
corresponding to 8,4 V
for the last comparator the voltage is :
1,25 V corresponding to 12 V
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