If an LED indicator is present in battery powered gadgets such as Emergency lamps, it will consume power even if the gadget is not using. This will reduce the battery voltage since the LED takes around 2 volts. So it is necessary to charge the battery continuously to keep the battery voltage level. This circuit eliminates this and the LEDs turn on only in two conditions. That is in the over charged and over discharged conditions only.
The circuit is basically a voltage controlled switch using Zener diodes. Two state LED indication is provided using a Bicolour LED. Zener diode ZD1 and the PNP transistor T1 forms the over discharge indicator switch. When the battery voltage is above the breakdown point of ZD1 (around 5 volts), it conducts and keeps T1 out of conduction. So the Red half of the bicolur LED remains off. When the battery voltage reduces below 5 volts, Zener turns off allowing T1 to conduct and Red LED turns on. This indicates that the battery is going to the over discharged state.
Zener diode ZD2 and NPN transistor T2 forms the Over charge indicator switch. When the battery voltage is below 6.8 volts (maximum voltage level), ZD cease to conduct and T2 remains off. So that the Green half of the LED also remains off. When the battery voltage increases above 7 volts due to overcharging, ZD2 conducts followed by T2 and Green LED turns on. This is the over charged state. In short, if the battery voltage is between 5 and 7 volts, both LEDs remain off. This reduces the chance of power consumption.
A variable power supply is necessary for the calibration. Provide 5 volts and adjust VR1 till Red LED turns on. At this point, Green LED remains off. Increase the voltage to 7volts and adjust VR2 till Green LED turns on. At this point, Red LED should remain off. Reduce the voltage to 6 volts. Both LEDs should be in the off state.