Three State Voltage Indicator Circuit

This is an ideal circuit to monitor the Voltage level in 12 volt Lead Acid batteries used in Inverters .For the perfect operation of Inverters, the battery voltage should be between 12 and 13.5 volts. A fully charged battery shows 13.5 volts. If the voltage reduces below 10 volts, the Inverter will not give back up. Moreover, if the Inverter is continuously charging without attaining 13.5 volts, it indicates that the battery is not accepting and holding charge.

The circuit is too simple and uses only a few components. It shows three indications.

1. If the battery has 13.5 volts, Green half of the bicolor LED lights.
2. If the battery has 12 volts(Normal) LED will be Off
3. If the battery voltage reduces below 10 volts, Red half of bicolour LED lights.

About the Semiconductors

The important components of the circuit are a two lead type bicolour LED and the Operational Amplifier CA3140.

Bicolour LED

The Bicolour LED used in the circuit is a Two Lead type. That is, it can give Red or Green lights based on the polarity of the leads. If the leads are connected in one direction, it gives one colour and if the leads are reversed, it gives the second colour. Thus there is no Anode- Cathode arrangement. Both the leads can function as Anode and Cathode. The internal arrangement of the LED automatically changes the direction of current based on the polarity. Here the LED used gives Red and Green colours.

BIcolour LED Datasheet


This Operational Amplifier is a 4.5 MHz BiMOS Op Amp with gate protected MOSFET inputs and Bipolar output. Its inputs have very high input impedance (1.5T Ohms).It require very low input current (10pA) and hasvery high speed of performance. The IC can operate between 4 – 36 volts with single or dual power supply.CA3140 is internally phase compensated for stable operation.

About the Three State Voltage Indicator Circuit

IC1 is designed as a voltage comparator with its non inverting input connected to the ground. Resistor R1, R2 and the Preset VR1 forms a potential divider to give variable voltage to the inverting input of IC. Resistor R3 gives necessary feed back to the inverting input. Resistors R4 and R5 are current limiters for the Green and Red halves of the bicolour LED. When the voltage in the battery is 13.5, the inverting input of IC1 gets a higher voltage so that its output goes low. Then current flows through the Green half of the LED into the IC and it emits Green light. When the battery voltage is 12 volts, both the Green and Red half of LED will not conduct and it remains off. When the voltage level reduces below 10 volts, output of IC1 becomes high and Red half of LED lights. Thus the LED remains off in normal 12 volt and it lights Green when the voltage is 13.5 and Red when the voltage is below 10 volts.

Three State Voltage Indicator circuit

Setting of the Three State Voltage Indicator

After assembling the circuit on a common PCB ,adjust VR1 using a variable power supply. Connect the points A and B to the variable power supply and provide 10 volts. Adjust VR1 till the LED gives Red colour. Then give 13.5 volts. At this state, LED lights Green. The circuit is ready to use. Connect it to the 12 volt battery using Crocodile Clips. Observe polarity while connecting to the battery.


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