Calculating necessary transistor for LED switching
Start with a type of transistor, for your application a N chanel Mosfet is the best to use

To chose a transistor you have to define some parameters like 

Vth : in your case you have to chose one around 1 - 3 volt (to be sure that the Arduino 5V will be enogh to sature it)

Vgsmax : you will use Arduino to have margin, take a transistor with at least 8V 

Rdson : Not critical in your case (just to minimise loses take the one with the minimum Rdson)

Id : the current in your circuit, in your case it is 20mA so you have to chose one with support a greater current (attention do not take the value in the first page, generally it is given for a specific Vgs and Vds, verifyyour condition, or go and find this value in the abacus
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