Simple Low Battery Indicator Circuit Using Two Transistors

This post shows you how to build a tiny helper circuit for your battery powered devices.

It uses just a few parts including 2 transistors to light up an LED when the battery is getting low.

This way, you will know when it is time to replace the battery before your device stops working.

Circuit Working:

Parts List:

Circuit Concept:

Previously, we explored low battery indicator circuits using a 741 IC and a 555 IC both renowned for their ability to detect and signal low battery voltage thresholds.

However, this post introduces another similar circuit that is more economical employing just a pair of NPN transistors to generate the necessary low battery indications.

Advantage of Transistor over IC:

The primary benefit of the proposed two transistor low battery indicator circuit lies in its significantly lower current consumption when compared to IC counterparts, which tend to consume relatively higher currents.

While an IC 555 consumes approximately 5mA and an IC741 around 3mAthe current consumption of the present circuit is just around 1.5mA.

This makes the present circuit more efficient, particularly in situations where standby current consumption is a concern such as in devices relying on low current battery supplies like a 9V PP3 battery.

Circuit Can Operate at 1.5V:

Another advantage of this circuit is its ability to function even at voltages around 1.5V providing a distinct advantage over IC based circuits.

Illustrated in the circuit diagram below the two transistors are configured as a voltage sensor and an inverter.

The first transistor on the left detects the threshold voltage level based on the setting of the 47k preset.

As long as this transistor conducts the second transistor on the right remains switched OFF keeping the LED turned off.

When the battery voltage drops below the set threshold level, the left transistor can no longer conduct.

This scenario promptly activates the right hand side transistor, allowing the LED to illuminate.

The LED turning ON provides the necessary indication of a low battery warning.

Formula:

A resistive divider and two transistors are used in a low battery indication circuit, where the resistive dividers main function is to supply a reference voltage against which the battery voltage may be compared.

The resistive divider operates using the following fundamental formula:

Formula for Resistive Divider:

The voltage divider formula is used to determine the resistive dividers output voltage V_out:

V_out​ = V_Battery​ × R2 / R1 + R2​

where:

• V_Battery​ is the battery voltage.
• R1 is the resistor connected to the battery.
• R2 is the resistor connected to ground

You may use two transistors to create an efficient low battery indication circuit by employing the resistive divider formula and comprehending how the circuit works.

Based on the needs of your particular application and the properties of the battery, change the resistor values and component selections.

How to Build:

To build the low battery indicator circuit described using two NPN transistors you should go through the following steps:

• Connect the positive terminal of the battery to the circuit.
• Connect the negative terminal of the battery to the ground.
• Connect the collector of the first NPN transistor to the positive terminal of the battery.
• Connect the emitter of the first NPN transistor to the base of the second NPN transistor.
• Connect the collector of the second NPN transistor to the positive terminal of the battery.
• Connect the emitter of the second NPN transistor to the ground.
• Connect the wiper of the preset to the base of the first NPN transistor.
• Connect one end of the 47k resistor to the base of the first NPN transistor.
• Connect the other end of the 47k resistor to the ground.
• Connect the resistor for the LED in series with the LED between the collector of the second NPN transistor and the positive terminal of the battery.
• Adjust the preset to set the threshold voltage level for the low battery indication.

Note:

This circuit operates based on the voltage threshold set by the 47k preset.

When the battery voltage falls below this threshold the LED will light up indicating a low battery condition.

Adjust the preset to set the desired voltage threshold for your application.

Conclusion:

This type of low battery indicator circuit is commonly used in various electronic devices to alert users when the battery voltage is approaching a critical level.

Adjusting the 47k preset allows users to customize the circuit for different battery voltage thresholds.

References:

Low battery level circuit – Changing “trigger” level