Introduction: LED Running on USB

Hi, guys, Today I am going to show you a circuit which is very beneficial for the people who work on Laptop, you need , sometimes, light to work on laptop keyboard.

For this circuit you need any spare USB cable, one LED, 1 resistor. & tools.

Step 1: Connections of the Circuit...

USB pin diagram

Connect resistor one end to the longer pin(Anode) of LED with the help of solder (you can also connect them by twisting both the pins), Now we have two pins spare, one pin of resistor and another one short leg(Cathode) of LED.

Step 2: Connections of the Circuit

Now connect the resistor spare end to the red wire of USB cable, and LED's to the Black wire of USB cable.

Step 3: Circuit Testing...

Now connect the USB cable to

Laptop or Desktop.

Is it glowing?



So we have glowing LED now,

but the question is, we choose the resistor of 100 Ohm, why?

Step 4: Calculation of Resistance

So we

calculate the value.

There is a simple and most significant formula, Ohm's Law.

According to Ohm's law

I = V/R,


V= I*R

I= Current (A) (current flowing through the resistor),

V= Voltage (V) (voltage across the resistor),

R= Resistance (Ohm).

In our circuit, we know…

The voltage supply voltage = 5V.

Voltage across LED = 3V. (Normally LED 5mm runs on 3V).

Voltage dropped by resistance= 5-3=2V.

Current to be flowed through LED = 20mA = 0.020 Amp.

Resistance= ?

Resistance= V/I = 2/0.020 = 100 Ohm.

Thanks for being on my Instructable.