Making a laser wipe effect

I was trying to make it laser effect trace where at first you didn’t see anything and then all of a sudden you see a letter or logo come out to be drawn by a little laser pointer. I tried to do this using motion guide and masking. No luck. I want to be able to have the laser trace out like say the letter “B”. I want it to trace the outside and inside. Anybody have a clue to what i’m talking about?