IC 555 Low Battery Indicator Circuit

The article explains a simple low battery indicator circuit using the IC 555 and a few resistors only.

Circuit Concept

Many electronic circuits such as emergency lights,  battery chargers, UPS systems, flashlights etc essentially require a low battery indication feature in order to avoid over discharge of the involved battery. An over discharge could mean a permanent damage to the battery.

A novel little low battery indicator circuit can be learned here, which incorporates just a single IC555 and a few resistors, it's a simple "plug and watch" kind of circuit.

Circuit Operation

The circuit functioning may be understood with the following points:

We all know regarding the basic characteristic of the IC 555 when it's being used in the comparator mode: if pin#2 is subjected to a potential lower than 1/3rd of the Vcc, the output pin#3 goes high.

The above fact also indicates that pin#2 responds with reference to the supply voltage applied at pin#8 of the IC, which implies this voltage at pin#8 should be clamped to some constant level.

Therefore in the proposed design the supply pin of the IC is fixed at some reference level using a zener diode.

The battery voltage is allowed to reach pin#2 of the IC via the preset, which must be manually set such that voltage at pin#2 just falls below the 1/3rd of the zener voltage when the battery voltage reaches the specified lower threshold.

The above setting can be done manually by applying a sample voltage to the circuit imitating the lower threshold level.

Suppose, the specified lower threshold is 11.4V for a 12V battery, the applied sample voltage can be fixed at 11.4V and applied to the circuit.

Next, the preset should be adjusted such that the LED just lights up. Now the preset may be glued by some permanent adhesive for preventing the setting from getting disturbed.

The set circuit is now ready to be attached with the battery in question, whenever the battery voltage reaches the 11.4V mark, the LED would light up, providing the required low battery information.

Simple low battery indicator circuit using IC 555 Diagram is shown below:

Parts List

R1,R3 = 10K
R2 = 100K
IC1 = 555
P1 = 100K preset
Z1 = zener diode, having voltage lower than the battery voltage.

IC 555 Pinout

555 Low Battery Indicator with Flasher

This 555 low voltage indicator circuit is designed to activate an LED when the power supply voltage exceeds a specific threshold and to make it flash when the batteries require replacement.

The provided component values are optimized for a 9 V operation. The flashing function initiates at approximately 7.5 V, while the LED won't illuminate at all below 2 V.

The circuit comprises a 555 timer configured in the bistable mode, serving as the driving mechanism for the LED.

The key element here involves connecting a zener diode between pin 7 of the 555 timer and the ground.

During standard operation, pin 7 oscillates between one-third and two-thirds of the supply rail voltage (Vcc). If the voltage at 2/3 Vcc exceeds the zener voltage, it inhibits the 555 timer's functionality.

To adapt this circuit to your specific needs, determine the minimum voltage necessary for proper equipment operation and multiply it by 2/3 to obtain the required zener voltage.

Additionally, you may need to adjust the resistor (R) to match the particular LED you intend to use.

In its original configuration, this indicator circuit drew 7 mA of current at 9 V, decreasing to 5 mA at 7 V.

If these values are potentially problematic for your application, consider using the CMOS variant of the 555 timer, the 7555.