Guys I'm confused, and maybe I'm confusing you too.
From the book, I read:
"[...] for this project we could use a number of buttons for every different tone, but now we are going to use a ladder resistor network."
Last year I studied "electronics and electrotechnology" at University, but the teacher has never mentioned this kind of network, so I was trying to understand it all by myself, but I think I'm having troubles whith this..
I would follow what my book says, so I would try to apply this ladder resistor network to my circuit; the values of the resistences and the position of the buttons is all showed in the book, so I'm applying to my circuit what I see in the book (image on the attachment).
I know I should learn more about the ladder resistence network, but I was trying to do this applying it to my project (sometimes I think it's better learning by practice than learning by reading a book).
So, please, don't be angry with me if my book is giving me wrong resistence values or something else.. 
Thank you so much for the patience..!!
