Low Voltage Alert Circuit for 9V Battery

Battery power is very important in electronic circuits.

But many times a battery voltage becomes low without notice.

As a result, devices stop working suddenly and therefore, a low voltage alarm is needed.

This Low Voltage Alert Circuit for 9V Battery gives warning when a 9V battery goes low.

It uses LM339 comparator IC and also uses few simple available components which can be build easily.

The circuit is with low cost and reliable and hence, it is useful for beginners and hobby projects.

Circuit Working:

Components	Value	Quantity
Resistors	1k	2
	Potentiometer 5k	1
Semiconductors	IC LM339 Comparator	1
	LED any color	1
	Zener Diode 6V	1
	Buzzer 5V to 9V DC	1
	Probes / Clips for battery terminals	2
	Battery 9V	1

First, 9V battery is connected to the circuit.

Then the Zener diode creates fixed 6V reference voltage.

This reference voltage is given to non-inverting input of LM339.

Meanwhile, battery voltage is divided using VR1 and R1.

This divided voltage goes to inverting input of LM339.

Now, when battery voltage is normal the inverting input voltage is higher than reference voltage.

Therefore, output of LM339 remains LOW and as a result a LED and buzzer remain OFF.

However, when battery voltage drops the divided voltage also drops.

After that, it becomes lower than reference voltage and therefore, LM339 output becomes HIGH.

Now LED glows and buzzer sounds and thus a low battery condition is indicated clearly.

Reference voltage from Zener diode Vz is 6V

Voltage divider formula:

Vout = Vin × VR1 / (VR1 + R1)

As per circuit the battery voltage:

Vout = 9 × 5 / (5 + 1)
Vout = 9 × 5 / 6
Vout = 7.5V

When battery voltage reduces to around 7V the divider voltage becomes less than 6V reference.

So, comparator switches output.

To build a Low Voltage Alert Circuit for 9V Battery follow the below connection steps:

First start collecting all the parts as shown in circuit diagram.
Pin 2 of IC is the output pin and is connected to LED, resistor R2.
Pin 3 of IC is the Vcc pin and is connected to positive supply.
Pin 4 of IC is the -input1 pin and is connected to positive supply through resistor R1.
Pin 5 of IC is the +input1 pin and is connected to center pin of VR1 pot.
Pin 12 of IC is the GND pin and is connected to negative supply of battery.
Zener diode cathode is connected from pin 4 of IC and anode is connected to negative of battery.
Two probes are connected to 9V battery source.
Buzzer one end is connected to positive supply and other end is connected to junction R2 and pin 2 of IC.

This Low Voltage Alert Circuit for 9V Battery is very simple.

It is easy to make and easy to test as it uses very few components.

So, beginners can try this circuit easily.

LM339 IC gives stable voltage comparison and therefore, alarm works properly every time.

Thus, this circuit helps to protect battery powered devices.