Replacing R3 with a variable resistor in series with a 1K resistor enables the brightness of the LEDs to be easily changed for example for indoor or outdoor use. Increasing the value reduces the brightness (and also reduces current consumption), decreasing the value increases the brightness (and also increases the current consumption). To change the brightness of the LEDs, R3 can be changed. In BAR mode the current consumption would be increased by an amount depending on the number of LEDs turned on up to around 100mA when the battery is full. In DOT mode the circuit shown above would draw under 10mA. With BAR mode – achieved by connecting pin 9 to the incoming positive voltage – all LEDs up to the measured voltage are on at the same time like a bar graph. With DOT mode (as used in our example above) only one LED is on at a time. With the resistor values used, you will just need to check that the first red LED turns off as it should at 11.89V. To calibrate the monitor, connect it to 12.65V and adjust the 10K potentiometer (VR1) until the last green LED turns on. The other LEDs will turn on one by one as the measured voltage increases through the range. the first red LED (D1) will light up at a voltage of 11.89V which is a virtually empty 12V lead acid battery, and the last green LED (D10) will light up at a voltage of 12.65V corresponding to a full 12V lead acid battery. The values of the resistors are R1 = 56K, R2 = 120K + 47K, R3 = 3K9 to give us an 11.89V to 12.65V voltage range – i.e. Above is shown the schematic for our 12 Volt battery monitor. First, we need to define the LCD Library and specify the pins we are going to use with the Arduino.
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