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# battery voltage indicator circuit

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#### binaryninja

##### Member level 3
Hi all, I am trying to design a circuit that will indicate battery charge levels for a 12V battery. I have seen many designs online, but they all use diodes to determine the voltage thresholds or work like a meter. I need it to be somewhat precise and turn off/on leds.

I need to indicate when the voltage is between 11.9 and 12.3, and when it's below 11.8, with only one led on at a time.

I know there are people on this forum that are good with op amps/ comparators that can help me put a circuit together that meets these requirements. Please help me.

#### uoficowboy

##### Full Member level 3
Just to be clear - you want, LED 1 to light when the voltage is between 11.9 and 12.3V, and LED 2 to light when the voltage is below 11.8V, right?

And no LEDs to be lit for 11.8V-11.9V, and greater than 12.3V?

binaryninja

### binaryninja

Points: 2

#### binaryninja

##### Member level 3
Well, it's easy for Vbat >12.3, because I can make that come on only when its above 12.3 without affecting the other voltages. That's why I didn't include it in my problem statement.

I didn't think about 11.8-11.9, I suppose I should make it: (1) 11.8 <= Vbat <=12.3 and (2) Vbat<11.8.

#### Shehata

##### Member level 3
yes u can use comparators for sure but i think you will need to find very accurate resistances to obtain these thresolds
I personally prefer to build cheap ADC cct for that job

binaryninja

### binaryninja

Points: 2

#### binaryninja

##### Member level 3
I'm able to get the correct thresholds for the voltages in a meter style indicator using 3 comparators and a resistor network. However, like I said, I need to have only one LED on at time because that's how I have to design it. How can I block out the other voltages?

I thought about using logic, such as XOR gates to decide which one to turn on, but I can't find an XOR IC that's capable of high input voltages. When the LED is off the output of the comparator sees the battery voltage minus the LED voltage drop. Obviously, this can be maximum of about 15V.

Unfortunately the circuit is on my computer at work so I can't post it on here until tomorrow, but it doesn't really matter because they are all over the internet.

Example:

This one is actually a bit different from mine. I suppose I could change mine so that the LEDs aren't connected to Vbat, such as in the example. This may allow me to use XOR logic...

I realize that this could be super easy with a pic but I can't use a microcontroller/ microprocessor.

I thought maybe there could be some different configuration with op amps/ comparators that would allow me to do something with at least the two lower voltage ranges.

Any ideas will be greatly appreciated :-D

Last edited:

#### IanP

How can I block out the other voltages?
You can use NPN transistor to block LED indicating lower level by voltage from one level up, see attached picture ..

IanP
:wink:

#### Attachments

• car-battery-voltmeter-with-LED-indicator -blockedLED.GIF
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Points: 2

### binaryninja

Points: 2

#### binaryninja

##### Member level 3
Thanks IanP!!! I really overlooked that. I was able to use a second NPN to block the remaining voltages so only one is on at a time, so I have all three LEDs in the same circuit! :-D

-Gave you 20 of my pts ;-)

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