Basic Electronic Exercises

How to startup LED

LED (Light Emiting Diode) is a semiconductor element
that emits light at the current flow. It's a current
consumer and you have to keep it in mind. It is not 
important the voltage to which you are connecting 
but the current flowing through is.

How to limit current, ie calculate serial resistance:
If the EMS is source voltage,UR is voltage drop on 
the resistor and UD is a voltage drop on the diode. 
EMS = UR + UD UR = EMS - UD the current is constant
so it can be written as: I × R = EMS - UD ...
and finally what we need is the value of resistance:
R = (EMS - UD)/I  A Practical Example: 
We want to connect a red LED to a 5V (USB output 
on a computer) R = (5V -2V)/ 0,02A R =150 Ohm. 
It is not recommended to use the LEd diode at full 
power. Most of them were based on germanium so that 
the peak temperature was 90 ° C. Insert the 220 Ohm
resistor and you will not even notice the difference
in the intensity of the light. I = (EMS -UD) / R 
(do not run away from Ohm's law :)) I = (5 - 2)/ 220,
 I = 0,014A ili I=14mA It works great and very long.

Serial conection of LEDs and resistor

The most used LE diodes made for a current up to 20mA, which means 
that the serial resistor should be dimensioned to limit the current
below the limit value, as I already said "less than 20mA". 
Characteristics of individual LED you will find on the net, 
so-called Datasheet. The voltage drop depends on the type and color
of the LED. In principle it is about 1.5 to 4V.

For example: Standard1
20 mA LEDs (ranging from approximately 40 mW to 90 mW) at around:
1.9 to 2.1 V for red, orange, yellow, and traditional green
3.0 to 3.4 V for pure green and blue
2.9 to 4.2 V for violet, pink, purple and white

1.https://en.wikipedia.org/wiki/Light-emitting_diode