0 Simple Battery Voltage Indicator for 12V Lead Acid

This simple circuit will help us monitor the voltage of a 12V lead acid battery in our vehicles. It uses only one transistor and no comparator needed unlike the battery discharge indicator circuitry.

Indicator circuit

Summary of the operation:
1. Only Red Led is on when voltage is less than 11.2V
2. Red and Green on (like orange) voltage is between 11.2V to 11.7V
3. Only Green Led is on when voltage is more than 11.7V


Part List:
D1 - 11V zener minimum 1/2W
D2 - 9.1V zener minimum 1/2W
R1, R3 - 1.2 kilo ohms 1/4W
R2, R4 - 120 ohms 1/4W
LED1 - high efficiency Red LED or any Red with 2V forward voltage
LED2 - any Green LED with 2.1V forward voltage
Q - 2N2222, BC547, CS9013 or similar NPN transistor

NPN transistor pinout

Note: You can use dual-color LED. See LED basic info to know more about the voltage drop of different LED color.