Battery level indicator is basically a design of light column voltmeter. It displays a column of light which height is proportional to the voltage. In the practical sense of view battery level indicator is used to show the voltage level of battery.
Working principle of this circuit is battery voltage compared with the reference voltage. This circuit is constructed by using 741 op-amp ICs. Op-amp is differential amplifier which output will be positive or negative, depends on the difference of input voltages.
When the input voltage difference is positive with respect to reference voltage and battery voltage, then the output is positive and the LED will emit light. If negative LED cannot emit light. It is the main principle of battery level indicator.
Voltage level indicated by the column of LEDs. For easy of reading LED rearranged in a vertical array. The top four LEDs indicates the battery voltage is high. Sequentially the lower level indicates the battery voltage is low and bottom four LEDs indicates very low voltage of the battery.
So, if the battery voltage continues to decline then LEDs will die in sequence from top to bottom.
In this circuit 1mA current is adjusted for reference voltage. Vcc is divided by the total number of resistor for finding the value of resistor R.
On the other hand, the resistor value of R1 is calculated by the equation,
1. Op-amp IC 741
2. R = 3kΩ
3. R1 =220kΩ
The voltage drop against the each resistor is 3 volt.
So when the Vin = 0v or less than 3v the output of all op-amp are at
–Vsat . The silicon diodes protect the light emitting diodes against excessive reverse bias voltage.
When Vin is increased to a value 3v to 6v, only the output of op-amp#1 goes positive to light which indicate “Very Low”.
When Vin is increased to a value 3v and 6v, the output of op-amp#1 and op-amp#2 are at +Vsat and goes positive to light which indicate “Low”.
When Vin is increased to a value 6v and 9v, the output of op-amp#1, op-amp#2 and op-amp#3 are at +Vsat and goes positive to light which indicate “Medium”.
Similarly, when Vin increased to 12v or higher, the output of all op-amps are at +Vsat and all op-amps output goes positive to emit light of all LEDs which indicate “Full” .