12V car battery indicator

Hi, I have been trying to design a circuit to monitor my 12V leisure car battery, I'm thinking this should be fairly simple, but its been years since I did any real electronics.

So the basic idea: Use the circular bargraph to indicate the level of the battery when a momentary push button is pressed
the circular LED bar graph: https://www.sparkfun.com/products/11492

So the range of the battery would be something like 10.5V to 13.5V, there are 32 LEDs in pairs, so essentially 16.

1. So if we take the first LED pair, this should illuminate when less than 10.5V, so that should be easy, as long as there is some life in the battery it should illuminate

2. The next LED should only illuminate at the next step, 16 LEDs, 3V range 16/3= steps of 0.2V So at the following steps:
10.5, 10.7, 10.9, 11.1, 11.3, 11.5, 11.7, 11.9, 12.1, 12.3, 12.5, 12.7, 12.9, 13.1, 13.3, 13.5.
So for 10.7V I guess I should use resistor before each LED.

3. The LEDs should light up after a momentary push switch is triggered, I would like this to be graceful, so the ring lights up smoothly one LED at a time as the voltage increases, I guess capacitors between the resistors and LEDs?

This is the simplest way I can think of doing it, I have seen circuits which use a LM339 chip, but that can only monitor 4 voltages, perhaps 4 of these in series could do the job? http://www.electroschematics.com/7068/lm339-lm239-lm2901-datasheet/ I think this is the way to go rather than using inline resistor/ capacitor pairs?

So I am a clear novice when it comes to electronics (but I'm enthusiastic), so if anyone can help me with this I would be more than happy to put together an instructables how to, something similar has been done before: https://www.instructables.com/id/Car-battery-tester/
In that example only one bar is lit, I want the other bars to stay lit.
I can use Eagle to make the PCB diagram and get it printed, but I need to figure out what components I need and how to do it!

Thanks for any help.


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Get the original 3914 datasheet: there is a worked example with cascaded devices.
garynobles (author) 3 years ago
Ok, so with extra searching I have come across this http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/108340-lm3914-voltmeter.html

So I can use a LM3914 for 10 LEDs, but I have 16, so two of these chips?
And put them in Bar Display mode.
I'm working through this tutorial: http://www.youtube.com/watch?v=iIKGvHjDQHs
This is much easier to do with a processor like the Arduino. You can make the scale suit precisely what you want - its impossible to make it unequal with a 3914
garynobles (author)  steveastrouk3 years ago
Thanks, I only want equal steps of 0.2V so unequal scales are not required so the 3914 should be best suited once the reference voltage is adjusted
garynobles (author) 3 years ago

Attached is a partial circuit, I got stuck.

So I've gone with the LM3914 IC set to Bar mode (pin 9)
On the wiring diagram I saw the LEDs were connected cathode to anode from the IC, with this ring LED setup they share 4 common cathodes, so how do I get around this?

I am returning to electronics after 15 years and a very bad teacher, so I'm learning as I go, any obvious problems?
The capacitors are going to be used to create a fade out effect, I guess this is where they should go?
You will notice I have only concentrated on the right side LEDS, I will use a second IC for them once I have this side working.

Thanks for any input

garynobles (author) 3 years ago
Oh yes, here is a resistor/capacitor/transistor solution indicating the fading effect: http://www.pcbheaven.com/circuitpages/LED_Fade_In_Fade_Out_Dimmer/