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CAR_BATTERY_VOLTMETER_WITH_BAR_GRAPH_DISPLAY

2016-05-11 11:50  
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CAR_BATTERY_VOLTMETER_WITH_BAR_GRAPH_DISPLAY
This uses an LM3914 bar-graph display, IC1, adapted to measure its own supply, so you simply wire it directly across the 12-V supply voltage (right way round-watch pins 2 and 3 carefully!). Ten LEDs, D1 to D10, will indicate the applied battery voltage, ranging from 1.5 to 15 V. The IC contains 10 internal comparators connected totem-pole-like, each sinking current through an LED. They compare an internal reference voltage against the chip's input voltage, at pin 5. Set the IC reference voltage with resistors R3 and R4 to just under 5 V. This means that one LED will light for every 500 mV (5 V reference/10 stages) increase in the signal. To enable this to be used to read a 12-V battery (the chip's own supply rail, in this case), resistors R1 and R2 are included as a divider: An input of roughly +15 V will cause D1 to light. When the voltage gradually falls, LEDs D2 to D10 will progressively illuminate. The circuit is set as a moving dot display. Connect pin 9 to the positive rail for a bar-graph display (not recommended because of the current consumption). Because the 10 outputs are effectively constant-current sinks, the LEDs will glow at a level independent of the changes in the supply rail; they won't dim when the rail drops. However, the first two or three display LEDs (D8 to D10) are superfluous in this application, because the LM3914 won't operate correctly below a rail of about +5 V.