# LED VoltMeter Circuit

Here is a Simple LED Voltmeter to Monitor the charge level in Lead Acid Battery or Tubular battery. The terminal voltage of the battery is indicated through a four level LED indicators.

The nominal terminal voltage of a Lead Acid battery is 13.8 volts and that of a Tubular battery is 14.8 volts when fully charged. The LED voltmeter uses four Zener diodes to light the LEDs at the precise breakdown voltage of the Zener diodes. Usually the Zener diode requires 1.6 volts in excess than its prescribed value to reach the breakdown threshold level. When the battery holds 13.6 volts or more, all the Zener breakdown and all LEDs light up. When the battery is discharged below 10.6 volts, all the LEDs remain dark. So depending on the terminal voltage of the battery, LEDs light up one by one or turns off.

## Schematic of the LED VoltMeter Circuit

##### Related Tutorials

• gus

I’ll try this simple circuit to m

• Flipperjesus

This is how i did it. I am going to make only one calculation for the D1 circuit.
Vin= Maximum input voltage used
Vout= Zener diode voltage rating
Iout= Maximum current that you want to pass thru to feed the Led(mA), + 5mA for the zener diode.

Lets say that we are going to use a 9V zener diode and the maximum output voltage is going to be 14.8V and that you want to pass 20mA of current to feed the Led and additional 5mA for the zener diode , this is how you calculate the limiting resistor:

R= Vin – Vout/ Iout
R= 14.8 – 9 / 25mA
R= 5.8 / 0.025
R= 232 Ohm (choose the nearest value)

Watch out for the power dissipation! (i burned a resistor when playing with completely different voltages and currents, but the zener diode made it) Unfortunatley i could not find the maximum current allowed through the zener diode (the ones i used anyway)in the datasheet.

P=I x V
P=0,025mA x 5.8V
P=0.145W

Do the same thing for the rest of the diode circuits and it should work great.

It worked for me, if i have done any errors, please tell me, i am still a newbie.

• Flipperjesus

I wonder what the calculations looks like, how do you calculate the values for the resistors?

I confess, i am a total newbie in electronics, that is why i would like to know all details to understand everything from the start, i think this would be the perfect tutorial for reverse biased diodes, but i just dont get where all the numbers comes from. If i would like to make a circuit like this but with a range starting from about 4 volts up to about 40 volts, how do i calculate the values for the resistors? I like the circuit because of the simplicity.
Please explain, not just what happens but also why it happens.

It seems that all cool circuits i find almost always explains what happens but nothing else, i like to see the math of it.

• Kong

I installed this circuit to my DIY desulfator but I can’t
light up last LED of 12V zener at 13.6V. It always turn on 14.1~14.2.
I used IN5242 and found it’s breakdown voltage is between 11.4 and 12.6.
According to the data, their 12V breakdown voltage is typically 12V but some of it is minimum 11.4 or maximum 12.6. Unfortunately, I think purchased one would be maximum 12.6V. so total 12.6+1.6=14.2V!. It makes ligh up last LED at that voltage.
So I changed various zener LED manufacture and finally IN4742 from
Vishay is the best 12V zener by replacing bad IN5242B.
hope it will help.

• richard

good morning can i use this circuit connected to my baterry while charging?its ok to steady connected while charging my baterry tnx

• Jim Keith

This circuit will work if you do the following:

12V: Use 9.1 or 10V zener
9V: 6.2 or 6.8V zener
6V: 3.3, 3.6 or 3.9V zener
3V: 2 or 3 series diodes (1N4004)

For sharper thresholds for 3 and 6V, add parallel resistors across the LEDs –e.g. 1K –experiment

My suggestion: For experimentation, obtain all standard voltage zeners in the 1N47xx series up to 11V.
(1N4728A to 1N4741A)
nxp.com/documents/data_sheet/1N4728A_SER.pdf

• Jim Keith

Also, the resistors in series with the LEDs are altogether too low in resistance. Experiment with higher values to obtain reasonable brightness without making the low voltage indicators too bright when connected to 12V.

• vasudha

sir,
i forgot to mention we have to make the circuit using IC741.

• Jim Keith

I do not normally help readers this much, but I think I gave you a bum steer (bad info). This, I think, will work better:

http://electroschematics.com/6956/led-dc-voltage-indicator/

• Jim Keith

electroschematics.com/6868/12v-battery-level-indicator-circuit/

• Jim Keith

Use this circuit and replace the 4 sections of the 339 with (4) 741’s. Since you are into engineering, you can recalculate the resistor values for your voltages.

Because the common mode range of the 741 does not go close enough to the negative rail, make R8 = R7 = 10K = ½Ein.

Use RED led for 3V because it has a lower threshold voltage than other colors and has a chance of turning on.

Good luck!

• vasudha

sir,
i have to design a circuit that can measure 3V, 6V, 9V and 12V at the output and display the result by turning ON the LED for the corresponding voltage at the output. however i am quite new to this concept and requre guidance . please help.

• sahana

dear sir..i want the circuit for.. led which works at 3v/10mA is to be lightened using a +5vdc regulated supply derived from a 230v ac suppy..

• Christian

Hi,

I am considering making a similar circuit for a ‘led voltmeter’ to check the health of a 9 volt battery, for a university project.

Could the above circuit be adapted with different resistor values and zener values, to give an indication of the 9v battery voltage? Also, why is the capacitor being used in the circuit?

I would be very grateful for any help as soon as possible,

Many thanks

• Udiarto

Dear D.mohankumar,
My name Udiarto from Indonesia, your article is very helpful and I’ve tried with success.
There are a few questions:
1. How many watts of resistance using ½ or ¼ watts.
2. Is each resistance can use 1K
Thank you so much for your explanation.

My best regards,
Udiarto

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