Battery voltage monitor circuit by LM339

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 a LM339 Quad Comparator 14pin IC is 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.

  • 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 pass the current to limit in save level  for the 6V  Zener diode at pin 4 (inverting). It has the reference voltage. Both ways is compared the voltage inside IC1.

The LED1 and Buzzer is indicator of 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 sound buzzer.  Because each their legs  are connected to positive suppy 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 adjust sensitivity  of 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 this circuit on breadboard and them will operate from 9V to 12V now we take it to check 9-volts battery. Which a good available battery will have voltage about 8V.

Here is video of lectronicsNmore that make this project worked very great. Thanks a lot.

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

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I always try to make Electronics Learning Easy.

This Post Has 12 Comments

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

  3. Ok, i found the false…

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

  5. 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

  6. Hi,Jonas Strauch
    Thanks for your feedback.

  7. 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.


  8. 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

  9. 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!!

  10. 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

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

  11. 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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