# Electronic Circuits and Projects Forum

1. ## 12V & 24V battery monitor using LM3914

Hi
New to this forum, and returning to electronics as a hobby (but spent the last 15 years in electronic manufacture, until it moved to a "low Cost country"

I want to build a battery monitor unit, to be built into a model locomotive to show battery condition, and re-charge. I want the make a PCB & circuit which will work for 12v and 24v with a few component changes. The LED display will be a few green, yellow and red LEDs, to show battery condition.

I tried the circuit which Roff produced back in February 2006, on a bread baord, as this looked nice, (used a LM3914, and LM742 ot TL071 op amp, & a LM317 volt reg)
Sorry, not sure how to get a link to this.

but could not get it to work properly, then tried a few other simpler designs, using the LM3914, just to check I understood the diagrams, and that I did not damage the LM3914 chip, and these work. I am starting to understand the chip and the datasheet.

I have downloaded the LM3914 calculator, which has also helped me get my old brain working again. so have worked out the resisitor network arround pin 7&8.
I like the idea of using a voltage reference, so want to use a fixed 9V volt reg, (with a couple of caps, to cover noise etc) to drive the LED's & the LM3914.
I was think of a voltage divider to get the input to pin 5, the signal pin, at arround 4 to 5 volts, plus I guess a decoupling cap.
I think I then need to supply "V Low" (pin 4) with a voltage arround 4 volts,(bottom of the display, and this is this just a resistor and pot, to 0V to set the voltage at pin 4 to 4ish volts?
The a similar arrangement around pin 6, "V High" to set the upper point?
Does pin 6 and 7 link together?

Does this sound right?
or
Is there a better way?

Sorry, I have no way of drawing this on a computer, to show my ramblings.

Neil

2. hi Neil,

Ref your PM, I have just added a circuit option for a 12V monitor on my Blog.

It uses the battery supply, no 9V regulator.

Eric

http://www.electro-tech-online.com/a...rystatusv1.gif

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