When I was just a wee pup, about 12 weeks old, Mum started training me with a clicker to let her know when it was time for doing my 'business'. How did she do it?
First she found some old jingle bells in our Christmas decorations box, put them in a clear biodegradable plastic baggie (that's so I wouldn't get my nails caught in them, but could still see them), tied them to the back door knob and we were set.
As a pup I was still being potty trained. So every time she thought it was about time for me to go potty, she would lead me to the back door, have me touch the bag of bells (either with my nose or paw), then she would click the clicker and say 'do you have to go potty'? It was only a matter of a few days that I went to the back door all on my own, paw'd the bells to let her know it was 'time'.
Today we found some great instructions from Instructables on making your own doggie bell doorbell; so all you pups out there can let your pawrents know when 'it's time'. (Just a side note, we would suggest that you wrap the bells in a clear biodegradable material so that your pup doesn't get his/her nails caught.)