The post describes a simple battery 4 step LED DC voltage monitor circuit using the versatile IC LM324. The idea was requested by Ms. Piyali.

Technical Specifications

I've a project, if you could help me out:
1. basically its a battery voltage detector cum indicator circuit.
2. the output from a transformer is 6V, 12V, 24V resp., depending on the supplied input. O/p is A.C.
3. by converting it into D.C. I've to design a circuit which will detect and indicate the voltage o/p by colored LED lamps. Such as,
Blue LED - 6V
Green LED - 12V
Red LED - 24V
4. Circuit should be compact in nature as much as possible.
1. should we be using comparator circuit ?
2. how to detect the diff. voltage levels ?
3. Is relay required ?
Please consider at earliest.

Thanks & Regards

The Design

The proposed battery voltage status monitor circuit using 4 LEDs makes use of comparators in the form of opamps from the IC LM324.

This IC is much versatile than the other opamp counterparts due to its higher voltage tolerance level and due to the quad opamps in one package.

In the proposed LED battery voltage monitor/indicator circuit all the four opamps have been used, although a few of them may be eliminated in case they are not required or depending on the specs of the individual users.

As can be seen the circuit diagram, the configuration is simple yet the outcome too effective.

Here the inverting pins of all the four opamps are clamped to a fixed reference level determined by the value of the zener diode which is not critical and can be any value close to the suggested one in the parts list.

The non-inverting pins of the oipamps are configured as the sensing inputs and are terminated with variable resistors or the presets.

How to Adjust the Thresholds

The preset should be adjusted in the following manner:

Initially keep all the presets slider arm shifted toward the ground end so that the potential at the non inverting pins become zero.

Using a regulated variable power supply apply the first voltage to be monitored starting from the lowest value to the circuit.

Adjust P1 such that at the above level the white LED just lights up. Fix P1 with some glue.

Next apply the second higher voltage or increase the voltage to the next level which is to be monitored and adjust P2 such that the yellow LEDs just switches ON. This should instantly shut OFF the white LED.

Similarly proceed with P3 and P4. Seal of all the presets after they are set.

The shown battery indicator circuit is configured in the "dot" mode meaning only one LED glows at any instant indicating the relevant voltage level.

If you want to make it respond in a "bar graph" mode, simply disconnect the cathodes of all the LEDs from the existing points and connected them all with the ground or the negative line.

Circuit Diagram

Parts List for the battery status monitor circuit

R1---R4 = 6K8
R5 = 10K
P1---P4 = 10k presets
A1----A4 = LM 324
z1 = 3.3V zener diode
LEDs = 5mm, color as per individual preference.

Modifying the above 4 status Battery Indicator with Flashing LEDs

The above explained 4 LED battery status indicator can be modified appropriately for enabling it with flashing LED indicators, as shown in the following diagram:

flashing battery indicator circuit 4 LED

R1 = 2k2

R2 = 100 ohms

LED = 20mA 5mm type

C1 = 100uF to 470uF depending on flashing rate preference

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