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# Monitor the Battery Voltages

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#### ONT

##### Newbie level 3
I need a circuit with 3 LEDs to indicate the battery voltage level.

LED 1---LOW = 3 V
LED 2---Medium = 3.4 - 3.7 V
LED 3---High or Full = 4.0 V - 4.1 V (battery full or battery maximum voltage)

Supply voltages must be less than 4.2 V.

You can use lm311 voltage comparator, only thing you should do is obtain three voltages (3v, 3.5v, and 4v) you can use voltage divider (by 2-resistor) then connect the outputs of the comparator to the leds. **broken link removed**

You can use lm311 voltage comparator, only thing you should do is obtain three voltages (3v, 3.5v, and 4v) you can use voltage divider (by 2-resistor) then connect the outputs of the comparator to the leds. **broken link removed**

I finally manage to design circuit that works for me

Low = 3 V
Med = 3.4 V
High = 4

I simulate this on Multisim with OP09 as there is not footprint of LM339. So can I use the same circuit with all these values with LM339? Kindly rectify if there is any fault in the circuit.

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But I did this with hit and trial method....So anyone kindly explain me this with voltage divider formula. Actually I forget this.

Voltage divider can calculate shortly like in figure I added.

But your references are not your limit-voltages. You should calculate them correctly. For example U1:A-ref-voltage = 3.9*8500/8720=3.8V.

For simplicity it's hard to beat the diode drop method. It takes advantage of built-in volt thresholds.

Screenshot:

By choosing resistors properly, no led will be exposed to more than 20mA.

The led's turn on gradually, so you won't see a distinct change between levels. OTOH if you use the comparator method, the led's will turn on more abruptly.

sorry I couldn't understand, so if you don't mind, kindly make above circuit and also mention the calculation of resistance values. Thanks in advance.

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