Car- Automotive - Motocycle Circuit page 01 :
LED Voltmeter for car battery by LM324
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The circuit, is a comparator, can measure with step of 1Volt, the voltage of battery of car. The clue of voltage become after comparison of voltage of battery, that is applied in the inverting inputs of amplifiers, with voltages of reference that are produced by a Zener D1,the value of which is such so that it present good thermic stability. With the RV1, we regulate the gradation of voltage that we want.
The optical clue become from four Led.
R1=1K2 R6=10K D2-3-4-5=LED
R2-3-4=680R R7-8-9-10=1K IC1=LM324
R5=15K D1=5V6 /0.5W Zener RV1=10K trimmer
Source :: http://users.otenet.gr/~athsam/voltmeter_with_led_for_car_battery.htm