Simple Low Battery Indicator Circuit using Two Transistors

This article discusses how to construct a small assistance circuit for the battery powered devices.

When the battery is running low it uses only a few components including two transistors to light up an LED.

In this way it will be able decide when to change the battery before the device goes out.

Circuit Diagram:

Parts List:

Idea of a Circuit:

A 741 IC and a 555 IC both of which are well known for their capacity to detect and interact with low battery voltage thresholds were used in earlier studies on low battery indicator circuits.

To provide the required low battery signals this post presents an alternative, more cost effective circuit that uses only two NPN transistors.

Transistor advantages against IC

The two transistor low battery indicator circuits main advantage is that it uses a lot less current than its IC counterparts which often use slightly greater currents.

The current usage of the this circuit is just around 1.5mA whereas an IC 555 uses about 5mA and an IC741 uses about 3mA.

This increases the efficiency of the circuit especially in devices that use low current battery sources such a 9V PP3 battery or in situations where standby current usage is an issue.

Circuit Can Operate at 1.5V:

An further benefit of this circuit is that it can operate at voltages as low as 1.5V which gives it a clear edge over circuits based on integrated circuits.

The circuit design above shows how the two transistors are configured up as an inverter and a voltage sensor.

The 47k preset setting controls the threshold voltage level which is detected by the first transistor on the left.

The second transistor on the right stays turned OFF keeping the LED OFF as long as this transistor is conducting.

The left transistor stops conducting when the battery voltage falls below a certain threshold.

The right hand side transistor gets quickly activated in this situation allowing the LED to light up.

The essential signal of a low battery warning is provided by the LED being lit.

Formula:

The circuit for a low battery indicator, makes use of a resistive divider and two transistors.

The main function of the resistive dividers is to supply a reference voltage for monitoring the battery voltage.

The resistive divider operates using the following basic formula:

Resistive Divider Formula:

The output voltage Vout of the resistive divider is calculated using the voltage divider formula:

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

An efficient low battery indicator circuit using two transistors can be constructed by using the resistive divider formula and understanding how the circuit works.

Modify component selections and resistor values to the needs of the particular application and the characteristics of the battery.

How to Build:

To build the Simple Low Battery Indicator Circuit using Two Transistors one should go through the following steps:

• Charge the battery by connecting its positive terminal to the circuit.
• Insert the batteries negative connection into the ground.
• The first NPN transistors collector should be connected to the batteries positive terminal.
• The base of the second NPN transistor should be connected to the emitter of the first NPN transistor.
• Connect the second NPN transistors collector to the batteries positive terminal.
• The second NPN transistors emitter should be connected to ground.
• Join the base of the first NPN transistor to the wiper of the preset.
• Connect the 47k resistors one end to the first NPN transistors base.
• The 47k resistors opposite end should be connected to ground.
• Between the second NPN transistors collector and the batteries positive terminal connect the LEDs resistor in series with the LED.
• To set the threshold voltage level for the low battery indicator modify the preset.

Note:

• The 47k presets voltage threshold decides how this circuit functions.
• The LED will turn ON to signal a low battery status when the battery voltage drops below the set level.
• To find the best voltage threshold for the application modify the preset.

Conclusion:

A common function of many electronic devices is the low battery indicator circuit which warns users when the battery voltage is about to drop extremely low.

Users can modify the circuit to fit different battery voltage thresholds by adjusting the 47k preset.

References:

Low battery level circuit – Changing “trigger” level