Battery voltage monitor circuit by LM339

Last updated on July 12th, 2021 at 09:45 pm

This is a simple low voltage tester circuit. Even it can be a battery voltage monitor for others voltage sources that have problems.  By showing on  LED display and alarm sound. Which this circuit holds to checks a voltage of 9-volts battery.

Battery voltage monitor description

We use an LM339 Quad Comparator 14pin IC as the base of this circuit and a few parts (6 pcs.) include two resistors, LEDs, 10K-potentiometer without capacitors.

When connects the probe input into the source, 9-volt battery. The current get in the circuit. Two pins are important to see right now.

  • The positive supply pin is pin 3
  • Negative supply pin is pin 12

At the same time, the battery current will flow through to a VR1 to the input pin 5 (non-inverthing) of IC1.
Then, another way, R1 passes the current to limit in save level for the 6V  Zener diode at pin 4 (inverting). It has the reference voltage. Both ways are compared the voltage inside IC1.

The LED1 and Buzzer is an indicator of the circuit.  R2 controls the proper current through them.

When the input voltage is higher than 6-volts. It will have a high voltage state at output. But LED1 will still not works and not a sound buzzer.  Because each their legs are connected to the positive supply and output pin.

But when the input voltage is lower than 6-volts. Then, it makes LED1 glow brightly and the buzzer emit sound. Which we can set the level voltage input with adjusted sensitivity of the circuit by control VR1.

The component list

R1,R2: 1K 1/4W Resistors
VR1: 5K Potentiometer
IC1: LM339 Voltage comparator IC
ZD1: Zener Diode 6V 0.5W no. 1N5233
LED1: any color you like.
BZ1: Piezo Buzzer.
P1, P2: Probes (See Notes)
Perforated board, wires, socket for IC

Application of  battery voltage monitor

I experiment with this circuit on a breadboard and them will operate from 9V to 12V now we take it to check the 9-volts battery. Which a good available battery will have voltage of about 8V.

Here is a video of electronics more that makes this project worked very great. Thanks a lot.

I made a divided voltage as ladder level with normal diodes in series as Figure 2. A LED is used as load and R-resistors act as limiter of LED.


Figure 2 The voltage dropper using diodes

We use volts meter to measure the voltage at the input probe. And take it to the 8.3V.  Then, adjust VR until LED1 glow and buzzer emit.

Then apply the probe to lower level dropper voltage.

Please watch a video below for better understand.

‘Keep reading: Simple level voltage detector »


I always try to make Electronics Learning Easy.


  1. Jonas Strauch

    Sorry, but may it be the false pins in the image of the Voltage Indicator?
    It’s the false way round, 5&4, 7&6 and so on…
    The Voltage of reference should be on the -Pin and the Voltage you want to comparate on the +Pin, shouldn’t it?
    Please give me a answer if it’s like i sad, or if not tell me why, because if i build it like in the picture all the LEDs flash!

  2. Jonas Strauch

    Oh, sorry i’ve looked somewhere else it’s right, but why isn’t it working?

  3. atewele elvis

    i like what you allays polished. i will like some projects that you have already design

  4. Sanjay

    It works great, but when kept connected continuously to 9 v battery, battery is draining fast , can you suggest what can be done to avoid this circuit not to drain battery fast when kept connected on continuous on to battery

  5. chromedragon

    Hi, I was very interested in your info, however, I had difficulty with grammar. Is it possible to correct the following: When the input voltage is higher than 6-volts will cause high at output but LED1 will still not works and not soud buzzer sine each leg them are connected to positive suppy and output pin. But when the input voltage is lower than 6-volts then will makes LED1 glow brightly and buzzer emit sound. Which we can set the level voltage input with adjust sentility of circuit by control VR1

    Only bbecause I want to understand the LM339 and how the correct the voltage setting using VR1.


  6. Shayan

    Hi , how to change this circuit to alarm when voltage reaches 11.8 volts and alarm goes off when voltages reaches 12.4 volts
    thank you

  7. Hi!, I’m looking for the materials to build this circuit and I can’t find here in Argentina the 1N5233, Do you think that can be replaced with the 1N753? The spec are very similar 6.2V 0.4W

    Thanks for your help!!

  8. I would like a more definitive identity for the LM339 chip used in the 12v lead acid battery low voltage alarm. When I looked at Mouser there were a dozen LM339s with different labels. Which one should be used for this project. Thanks. Tedd Lovell

    • Hi Tedd Lovell,
      Thanks for your question.
      I think you may use LM339N better because it is cheaper. I like it.

  9. Bob Baker

    What component values are needed for 24 volt system? I need to monitor a 24 volt system with voltage alarm at approximately 22 volts Thanks

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