Posted on Aug 22, 2012 6502
If the battery voltage exceeds about 11 V, current flows through R1, D1, and R2. The voltage produced as a result of current flow through R2 is sufficient to bias transistor Q1 into conduction. That places the collector voltage of Q1 virtually at ground. Therefore, Q2, driven from the collector of Q1, is cut off, LED1 and current-limiting resistor R4 are connected in the collector circuit of Q2. With Q2 in the cut-off state, the LED does not light.
Should Q1"s base voltage drop below approximately 0.6 V, Q1 turns off, biasing Q2 on and illuminating LED 1 to indicate that the battery voltage has fallen below the 11 V threshold level.