The USB-Powered PC Lamp circuit is powered by regulated 5V DC available from the USB socket of the PC. This White LED Lamp lets you use your PC or laptop at night without disturbing others’ sleep. It produces a soft white light just enough to see the keyboard in darkness as well as when the ambient light is poor during daytime. 

LDR acts as a light-dependent switch to turn on the lamp led when the ambient light in the room drops below the preset level. USB-Powered PC Lamp Transistors T1 and T2 (BC547) are used to switch on the lamp. The base of transistor T1 is connected to the voltage divider comprising LDR and preset (variable resistance).