How to make USB operated light?

Material: One LED, 5mm size, one resistor of 300 Ohm, one USB Connector wire, and other tools like soldering iron, cutter, wire etc.



Steps
1-      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.


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





3-     Now connect the USB cable to Laptop or Desktop. 



Is it glowing?

Yes?

Great!

So we have glowing LED now,

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

So we calculate the value.

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

According to Ohm's law
I = V/R,
R=V/I,
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.

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