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