LED Circuits

Low-Cost Battery Voltage Indicator

A 12-volt battery voltage indicator which fits into a conventional cigarette lighter socket utilizes light emitting and Zener diodes to display three levels of battery voltage. Three light emitting diodes are arranged so that two lights indicate normal voltage, three lights indicate overvoltage, and one light indicates low voltage.

As shown schematically in the figure, the light emitting diodes D₁, D₂, and D₃ turn on, respectively, at 11.5, 13.5, and 14.5 volts. The "turn on" voltage is determined by the drop across the Zener diodes D₄, D₅ and D₆ and the common resistor R₄. Resistors R₁, R₂ and R₃ limit the current through each light emitting diode. R₄ is used to maintain relatively constant brightness from each LED as it turns on, as the voltage ranges from the lowest "turn on" voltage up to 16 volts.