Single Transistor Lead Acid Battery Monitor
Using a dual-colour LED and few surrounding components, we can make a very simple battery monitor. This battery monitor circuit is suitable for monitoring your car battery, enrich your dashboard with cool and functional accessory.
Orange colour is produced by LED when the red and green LEDs are illuminted. The red LED illuminates below 10.5v, green LED illuminates above 11.6v, and orange LED illuminates between 10.5v and 11.6v. [Source: talkingelectronics.com]