About: Hi, This is ArunS,hobbiyst of Electronics,Aeronautics,Automation.

Hi Everyone,

This project sounds alarm when the battery voltage falls below the user defined (low) voltage .It is used in 12 volt battery (It may be any type of Battery- Lead acid ,Lithium -ion, Lithium - Polymer etc.,) and it will produce audio and visual indication to us. It can be connected directly to Inverter Battery.

It uses IC LM358 as main component,which is dual operational-amplifier,used in comparator mode, with some active and passive components.The Battery low voltage can be set by user by adjusting Variable Resistor. For instance, Lead acid battery 10.7 Volt is considered as safe low voltage. Here I am setting the voltage of 10.6 volt for demonstration purpose.

Besides ,Viewers with some basic knowledge in electronics can do this Project.

The total cost of this project would around 3 USD.

The componets used in this project are given below:

1. LM358 IC

2. 1Kilo Ohm Resistor------------------------2Nos.

3. 4.7Kilo Ohm Resistor----------------------1Nos.

3. 10Kilo Ohm Variable Resistor-----------1Nos.( I am using Multi-turn Pot)

4. 1N5234B (6.2V ) ZENER DIODE-------1No.

5. 1N4007 DIODE-----------------------------1No.

6. BC 547 (or) 548 NPN Transistor--------1no.

7. LED (Small -3mm red) -------------------1No.

8. Buzzer 12 volt small ----------------------1No.

9. Two pin screw Header -------------------1No.

10. General Purpose PCB -----------------1No.(or) use my PCB which is 5cm x 3.7cm.(approx.)

Step 1: Assembly

The schematic circuit diagram is shown in step 1. You can use this circuit and you can make PCB circuit as your own or you can use my PCB layout readily available. In my Pcb use jumper wire at J-J. Make Print out and transfer it to copper sheet and do etching as regular process.Then pcb is ready. Make holes by drilling using 0.6 to 1.0mm bits as necessary.etc.,After choosing the PCB, solder all the components. check for any short circuit.

Note: After discussion in this forum, revised schematic, PCB,Components files were updated.

Step 2: Calibration / Setting Low Voltage

Using DC-DC step down converter or variable voltage source , set the voltage to 10.59 volts. Then connect this 10.59 volt to the Battery low alarm circuit. Adjust R3 multi turn pot in Alarm circuit , clockwise or anticlockwise until buzzer beeps and LED on.Hold on R3. Then increase the votage in DC-DC step down converter or variable voltage source to 10.65volts. Now you can see the Buzzer and LED stops.

Now the Battery Low voltage is set for 10.60 volts.

Step 3: Testing

Connect the Low Battery Circuit to a variable power supply and adjust the voltage slowly upto to 10.75 volts .Here I am connecting 3cell Li-Po battery to dc-dc step down converter . The output of li-po is around 11.1v , which is reduced by step down converter to around to 10.75 volts and it is fed into the Low Battery Circuit. since the set value is 10.6 volts and battery voltage is around 10.75 volts, Alarm doesn't work .Slowly reduce the voltage to 10.61 volts and when voltage reaches 10.59 volts the buzzer and LED becomes ON.


you can use this circuit in many fields . In automotive cars, almost all vehicles up to 24 volts, Inverters,UPS Burglary Alarms, and many more fields , etc.,
Thanks for reading. Any questions/suggestions/improvements in respect of this project, you can mail to



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14 Discussions

hi, really like this project. Want to get started but you said an update will come soon. I want to make the printed board but need update. Can you post it ?

4 replies

The Schematic and PCB presented here is Revised /updated one.You can use the PCB to make your project.

Thank you for quick response. Just one more question
Some circuits use 12 to 20 ma or so standby current. Eventually it will drain the battery down just sitting there.
Do you know what the standby current is for your circuit?


As per calculation, the standby current for the above circuit is less than 2mA.


How so?

Current through R1: (12V-6.2V) / 1k = 5.8mA

Current through R3 12V/10k = 1.2mA

Op-Amp current: ~1mA

Total stand-by current: ~8mA

(More with noise and oscillations of the unused and untied op-amp)


A neat and useful instructable. Thank you.


9 months ago

Could even use SMD on a lot smaller PC board to make it even smaller in size. That way you can actually install in on the back of the buzzer if you like.

1 reply

It is better idea. But micro soldering station is costlier when compared to ordinary 35 watt soldering iron.Moreover skill is required for soldering SMD components, which is somewhat difficult to ordinary people.

Thanks for pointing out missing of R3 in Schematic . The value is 100 Kilo Ohm which is mentioned in the component list.Soon it will be updated in the Project.

Thanks for sharing :)