This LDR controlled Adjustable charger can be used to charge the Rechargeable battery of portable devices. It charges the battery only during day time. This saves energy and prevents overcharging of the battery. The output voltage from the charger varies from 6 – 12 volts which can be adjusted. Current flow depends on the ambient light in the room.
Charging current is obtained from a 0-12 volt 500 mA step-down transformer, a full wave rectifier D1through D4 and smoothing capacitor C1. LED indicates the power on status. LDR is used as a light sensitive switch for controlling the charging process. During day time, LDR offers less resistance and provides sufficient base bias to T1. When T1 conducts, output will be available from the emitter of T1.Pot VR1 controls the output voltage. Current flow depends on the resistance of LDR which in turn depends on the light falling on LDR. In bright day light, resistance of LDR reduces to 100 K or less and more current flows into the base of T1. When the LDR is in dark, its resistance increases to 10 Meg Ohm and prevents the base current to T1. T1 then turns off and output ceases.
Note: Keep the LDR/charger in a place where day light is available. Adjust VR1 till sufficient output voltage is obtained. Measure the output using a multimeter. Output current can be between 50 mA to 300 mA depending on the illumination. If continuous charging is required, replace LDR with a 470 ohms resistor.