CIRCUIT ON REQUEST
Here is a cute White LED Lamp that automatically switches on when the ambient light in the room decreases. It is too small and powered by the Mobile phone battery. The circuit uses only a few components and can be assembled in 10 minutes.
LDR is used as the light sensor. When the room light is normal, LDR conducts and provides base current to T1 and it forward biases. When T1 conducts, base of T2 will be grounded so that T2 remains in the cut off state and LEDs off. When the room light decreases below the present level adjusted by VR, T1 cease to conduct and its collector voltage raises enough to forward bias T2. When T2 conducts, LEDs get current and they glow.
White LED requires 3 volts and 30 mA current for glowing in full brightness. The Lithium battery used in Mobile phone can deliver 3.7 volts and around 750 mA current. Alternately, two 2532 number Lithium button cells can be used. Using the Mobile charger, the mobile battery can be charged through the contacts shown in the circuit.