iv used this site for a long time
on the site it will show you how to do anything you can think of

Diodes................http://www.the12volt.com/diodes/diodes.asp
Ohm's Law...........http://www.the12volt.com/ohm/ohmslaw.asp
Relays.................http://www.the12volt.com/relays/relays.asp
Resistors.............http://www.the12volt.com/resistors/resistors.asp

have a look at whats on the site ,, its got so much for you to look at
it will show you so much