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Voltage Detector Electronic Circuit Project
This circuit is using a LM339 quad comparator as its operation. Four variable resistor or potentiometers are used to allow individual settings for each section of the comparator. Four LEDs are connected to the output to indicate the status of the input level. If the circuit is duplicated, and the inputs connected together, the resolution of the voltage detector will be double and more readings can be set. The LM339 consists of four independent precision voltage comparators with an offset voltage specification as low as 2 mV max for all four comparators. These were designed specifically to operate from a single power supply over a wide range of voltages. Operation from split power supplies is also possible and the low power supply current drain is independent of the magnitude of the power supply voltage. These comparators also have a unique characteristic in that the input common-mode voltage range includes ground, even though operated from a single power supply voltage. Further detail of the specifications can be obtained from
Electronic Circuit Project National Semiconductor website. 
This circuit can be used as a bargraph voltmeter with each potentiometer set for a specific voltage.
Source : Extracted from Popular Electronics Oct 1996, By Charles D. Rakes
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