A battery level indicator circuit is depicted in the schematic diagram below. This circuit is designed for 9V battery operation, as this circuit will start dimming below 7V and will be completely turned off at 6V. If you want the LED indicator not to dimming but just abruptly turns off at below 6V, then you can remove R3, and change R1 to 330k.
[Circuit's schematic diagram source: National Semiconductor Application Note]
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