You can make a simple LED light to read and type in Computer or Laptop during night without disturbing other’s sleep. It is also useful as a mini Emergency light in the event of a power failure. It is too simple and powered from the USB port. USB lights are available in the market, but costs around Rs.150. You can make this USB light at the cost of just Rs.5.
USB ( Universal Serial Bus ) is the port that gives 5 volts and around 400 milli ampere current. We can use this power to light the 0.5 watt White LED.
Procure the USB male from a discarded Pen drive or USB cable. Remove the wires or the Pendrive circuit connected to it.It has four pins. The pins at the ends are + ( +5 V ) and – ( Ground ) while the middle pins are Data In ( D + ) and Data Out ( D – ). We need only the + and – pins to power the LED.
Connect 100 Ohms ¼ Watt resistor to the + pin and the other end of resistor to the Anode of LED. Connect the Cathode of LED to the – pin. Note that, first identify the + and – pins of USB male by connecting it in the USB port and checking with a multimeter or simply using an LED with 1K resistor.
How much current can be provided to 0.5 W White LED.
The 0.5 W White LED works well between 50 mA to 250 mA current. As the current increases, brightness also increases. But if the LED takes more than 100 mA current from USB port, it will over load the USB hub. In the PC, all the USB ports are sharing the same current of 350-500 mA. Mouse and other devices are usually connected to ports. If LED draws more current, over loading occurs and the devices connected to the ports will not work. So use minimum current of 50- 100 mA from USB port for the LED. You can apply Ohms law to fix the LED current. First fix LED current and select Resistor using Ohms law.
I = 50 mA or 0.05 A
V = 5 Volt
R = V / I = 5 / 0.05 = 100 Ohms
Other resistors and current at 5 V
82 Ohms – 60 mA
68 Ohms – 73 mA
47 Ohms – 106 mA
Note – In the diagram, 47 Ohms resistor is shown and it gives 106 mA current to LED. In the Prototype, 100 Ohms resistor is used which gives only 50 mA current to LED.