Back to the base project... The Electronic Referee
So if you have not watched the first video, go do that now and then come back to this one, you can find it here Driving BIG loads with your micro controller - no isolation
This video extends what you will have learned from the previous blog and adds an OPTO Isolator into the mix, for added safety and to protect the microcontroller from EMF and other bad things
The OPTO isolator provides over 4KV of isolation and is easily driven by a 3V3 or 5V microcontroller
using an OPTO Isolator also simplifies creating a circuit for HIGH side or LOW side switching, it has the same components in each case but simply connects them in a different way... very cool
Circuits have been tested on a breadboard and built into a prototype Launchpad Booster pack for my electronic Referee project The Electronic Referee
use as you like, enjoy 
This is a link to the OPTO Isolator I used http://www.vishay.com/docs/83621/ild55.pdf
and this to the P Channel FET: http://media.digikey.com/pdf/Data%20Sheets/Toshiba%20PDFs/TPC8114.pdf
This is the final circuit used for the electronic referee
here is the next video, How to read BIG things using OPTO Isolators Sense BIG things with your Launchpad or Arduino, OPTO Isolated inputs.