The aim of this circuit is to demonstrate how we can utilise solar light to electrify the remote areas, i.e., how we can store the solar energy and then use it for small-scale lighting applications.
The DC voltage from the 10W, 12V solar panel is used to charge the battery and control the relay. Capacitor C connected in parallel with a 12V SPDT relay coil remains charged in daytime until the relay is activated. Capacitor C is used to increase the response time of the relay, so switching occurs moments after the voltage across it falls below 12V. The higher the value of the capacitor, the more the delay in switching.
During daytime, relay RL energises, the positive terminal of the battery is connected to the output of regulator IC 7808 (a 3-terminal, 1A, 8V regulator) via diode D and normally-open (N/O) contacts of relay RL1. Here we have used a 6V, 4.5Ah maintenance-free, lead-acid rechargeable battery. It requires a constant voltage of approx. 7.3 volts for its proper charging.
Even though the output of the solar panel keeps varying with the light intensity, IC 7808 (IC1) is used to give a constant output of 8V. Diode D causes a drop of 0.7V, so we get approx. 7.3V to charge the battery.
At night, there will be no generation of electricity. The relay will not energise and charging will not take place. The solar energy stored in the battery can then be used to light up the lamp. A 3W lamp glows continuously for around 6 hours if the battery is fully charged. Instead of a 3W lamp, you can also use a parallel array of serially connected white LEDs and limiting resistors to provide sufficient light for even longer duration.