Input terminal VIN is connected to the +V line of tbe circuit that the indicator is to monitor, and the grounds of botb circuits are connected togetber. The position of potentiometer Rl`s wiper determines Q1`s base voltage. As long as the transistor gets enough bias voltage to remain on, the low voltage at tbe collector will keep the SCR from firing.
As tbe battery voltage starts to fall, the transistor"s base voltage will fall as well. When Ql turns off (VIN drops), tbe collector voltage increases. That voltage provides enough gate drive to tum on the SCR, which turns on tbe LED. The LED could also be a buzzer or almost any otber type of warning device.