2 Battery voltage monitor (12V and 6V)

The purpose of this circuit is similar to simple battery voltage indicator that provides visual indication of the battery voltage or charge.  When LED2 (red) is on, it means that the battery needs charging. On the other hand, when LED1 (green) is on, it means that the battery is in good condition. Green LED is on when battery voltage is 12V (12V version) and 6V (6V version).

schematic diagram

2N2222 and NPN transistor pinout

Part List: 6V version
R1 - 1 kilo ohms 1/4W
R2, R3, R4 - 560 ohms 1/4W
Q1, Q2 - 2N2222, CS9013, or similar NPN transistor
LED1 - green LED
LED2 - red LED
Dz - 1N5226B, or any 3.3V zener diode

Part List: 12V version
R1, R2, R3, R4 - 1 kilo ohms 1/4W
Q1, Q2 - 2N2222, CS9013, or similar NPN transistor
LED1 - green LED
LED2 - red LED
Dz - 1N5239B, or any 9.1V zener diode

Other voltage indicator circuits:
Simple Battery Voltage Indicator - uses only 1 transistor
Battery discharge indicator - improve version, uses comparator IC