Car battery Indicator




This is a really useful instrument for your car. It uses three LED’s to give a visual indication of the state of charge of your car battery. When the red LED is on the battery voltage is low, green is the OK signal and orange is a warning that the battery is overcharging. The whole circuit is accommodated on a small p.c. board and can be fitted easily anywhere in the dashboard.


  • Working voltage: 12 V DC
  • Max. current: 40 mA

How it works

The principle behind the circuit is very simple. The first LED D1, is connected in series with R2 and D4 and will light when the battery voltage is below 11.5 V. If the voltage is above 12 V the zener Z3 will bias the transistor TR1 sufficiently to turn it on and this will light the second LED D2 which is green and indicates that the battery voltage is normal. As the collector to emitter voltage drop is less than the forward voltage drop of D4 the first LED does not get sufficient voltage across it to light and is turned off. If now the battery voltage rises above 13.5 volts then in a similar way the transistor TR2 is turned on and the third LED lights to indicate potential trouble. The zener diodes are used to provide the reference voltages for greater accuracy.




R1, R4 10K
R2 470Ω
R3 100Ω
R5 680Ω
D1 red LED
D2 green LED
D3 orange LED
Z1,Z2 6,8V zener diode (1/2W)
Z3 11V Zener diode (1/2W)
TR1, TR2 BC548 – BC547 NPN Transistor


Please follow and like us:
Pin Share


Notify of

Inline Feedbacks
View all comments
Anis ur Rehman

I have designed this project but In how many voltage does the red light gets on because i have put 11.5 ,13.5 , 9V also and 25 also my green and orange light is working put not red plz help


What is D4??



Samrat pushpesh

Car batteries current ratings is almost 30Ah then how will your 40mA rating circuit will work . Will it burn out??


TOP PCB Companies