LM3914 Battery Level Meter

Adjustable Li-Ion Battery Level Indicator

I needed a lithium-ion battery level indicator for my hybrid tube amplifier project. The goal was to show the battery levels of batteries at once and needed the min/max voltage to be adjustable.

After searching the internet for ages, I came across a LM3914 circuit that looked promising, the result came from an image search of LM3914, it appeared to come from an obscure Chinese website which I can no longer find. The circuit diagram was very poor quality and the writing was not in English.

So I looked up the LM3914 specs again and noticed different types of circuits in the document and was able to guess some of the component values.

The end result was a working battery level indicator based on the LM3914 that was powered from the battery being measured.

 

The Components

For stability reasons four electrolytic capacitors were added as per the IC spec sheets. Other than these capacitors, the only components are resistors, LED bargraphs, the LM3914 IC and trimmer pots.

Due to the choice of the LED bargraph I found out the order of LEDs were reversed (1 = 10, 2 = 9 etc), this means the design of this circuit and PCB only works with this particular LED bargraph.

The Schematic

The circuit that does have many components, as four batteries need to be measured at once, the circuit is multipled by four. Of course you can redesign this to measure only one battery, the circuit will be a lot smaller.

The circuit is adjusted using precision multi-turn potentiometers. Each LM3914 has two pots, one is used to set the lowest LED by supplying the lowest voltage you want to measure and trimming the pot, the same process is applied for the max voltage measurement. Battery connection is done via JST PH connectors.

Quad LM3914 Battery Level Meter

 

The circuit be used to measure higher voltages, up to the max of the LM3914 supply voltage.

The PCB

The PCB was made as small as possible, all the trace widths are the minimum, as four circuits were combined onto the PCB, the ground was not common, so I added an area to join them, this was a silly idea, as I could have just joined them in the EasyEDA designer and ignored the DRC errors.

The join area is located next to each JST connector (the six small pads all close to each other).

Top of PCB

Bottom of PCB

3D top side

3D bottom side

Bill of Materials

Name	Designator	Qty	Manufacturer Part	Manufacturer	Supplier	Supplier Part
SW_PUSH_DPDT	BATT1-2,BATT3-4	2	XKB8080-W	XKB Connectivity	LCSC	C318861
PH-2AK	BATT1,BATT2,BATT3,BATT4	4	PH-2AK	BOOMELE	LCSC	C8886
10-segment led bar display	BATTERY1, BATTERY2, BATTERY3,  BATTERY4	4			AliExpress	Ali Express 32877938733
10u	C1,C2,C3,C4	4	UMW1E100MDD	Nichicon	Mouser	647-UMW1E100MDD
3k	R1_1,R1_2,R1_3,R1_4	4	RC0805FR-073KL	YAGEO	LCSC	C126350
LM3914N	U1,U2,U3,U4	4	LM3914N-1/NOPB	TI	LCSC	C9408
5K	VR1_1,VR1_2,VR1_3,VR1_4	4	3296X-1-502LF	BOURNS	LCSC	C111778
50K	VR2_1,VR2_2,VR2_3,VR2_4	4	3296X-1-503LF	BOURNS	LCSC	C111780

Links

EasyEDA Project Page

https://easyeda.com/Ratti3/lm3914n-li-ion-adjustable-battery-level-meter 

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