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. The circuit is powered by regulated 5V DC available from the USB socket of the PC.
LDR1 acts as a light-dependent switch to turn on the lamp (LED2 through LED7) when the ambient light in the room drops below the preset level. Transistors T1 and T2 (BC547) are used to switch on the lamp. The base of transistor T1 is connected to the voltage divider comprising LDR1 and preset VR1.
When light in the room is sufficient, the resistance of LDR1 is low. This results in a high voltage at the base of T1, driving it into saturation. When transistor T1 conducts, transistor T2 is cut off. This disconnects the power supply to all the white LEDs (LED2 through LED7). LED1 (green LED) glows as it is forward-biased, indicating the standby mode.
When it is dark, or the ambient light in the room is lesser than the pre-determined level set by VR1, transistor T1 is cut off and T2 conducts. All the white LEDs glow with sufficient brightness as these are connected to the power supply through series dropper resistors R2 through R7. These resistors are used to limit the current through white LEDs to a safe level.
LDR1 acts as a light-dependent switch to turn on the lamp (LED2 through LED7) when the ambient light in the room drops below the preset level. Transistors T1 and T2 (BC547) are used to switch on the lamp. The base of transistor T1 is connected to the voltage divider comprising LDR1 and preset VR1.
When light in the room is sufficient, the resistance of LDR1 is low. This results in a high voltage at the base of T1, driving it into saturation. When transistor T1 conducts, transistor T2 is cut off. This disconnects the power supply to all the white LEDs (LED2 through LED7). LED1 (green LED) glows as it is forward-biased, indicating the standby mode.
