hi,
1. If V1 is greater than voltage required to start D8,D9,D10 conducting then Q3 is switched ON and the GREEN LED is ON.
2. As Q3 is ON, D3 is connected to the +V rail, so the ORANGE LED is shorted out and is OFF.
3. If V1 is greater than the voltage required to start D4,5,6 conducting then the ORANGE LED would glow, BUT as Q3 is ON, the ORANGE LED is OFF.
4. As Q2 is ON then Q1 is OFF, so the RED LED is OFF.
5. If V1 now falls to where Q3 is OFF, but is greater than voltage to make D4,5,6 conduct the ORANGE LED is ON and the GREEN LED is OFF. [the RED LED is still OFF]
6. If V1 falls below the conduction voltage of D4,5,6, then Q2 switches OFF and the ORANGE LED goes OFF.
7. As Q2 is OFF, then Q1 switches ON and the RED LED is ON.
Its a V1 voltage status indicator..
Do you follow this.?