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Simple Solar LED Lantern

This solar LED lantern can be used as an emergency light. Its 6V battery can be charged either from 230V, 50Hz AC mains or a 12V, 10W solar panel. Two LED indicators have been provided—red LED (LED1) indicates battery charging and green LED (LED2) indicates fully-charged battery.

You can choose to charge the battery either from the mains power or the solar panel by using the single-pole, double-throw (SPDT) switch. Capacitor C1 (1000µF, 35V) removes ripples from the power supply and regulator IC LM7809 (IC1) provides regulated 9V DC to the emitter of pnp transistor T1 (TIP127/BD140) and pin 7 of op-amp IC CA3140 (IC2), which is configured in comparator mode.

Solar LED Lantern
The reference voltage of 6.3V at pin 2 of IC2 is obtained through the combination of resistor R7 (1-kilo-ohm) and zener diode ZD1 (6.3V). The comparator controls charging of the battery. Pin 3 of IC2 is connected to the positive terminal of the battery to be charged through resistor R5. When the battery is fully charged, it stops charging and the green LED (LED2) glows to indicate the full-charge status.

When the battery voltage is low, diode D1 (1N4007) forward-biases and the
battery connects (through resistor R3) to the collector of T1 for charging (indicated by the glowing of red LED1). Three high-wattage white LEDs (LED3 through LED5), such as KLHP3433 from Kwality Photonics, are used for lighting. These are switched on using switch S3.

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