Can you purchase a mat that will ring or chime when a dog steps on it to place at the back...?

I had the same problem because my dog is a breed that rarely barks. When he was a puppy I hung a bell from the doorknob and he quickly learned to ring it- well really, he rang it because he's very jumpy and when he jumps at the door, he also hits the bell. That really helped me train him because there were no more puddles at the door when he was too small to have bladder control. When I was upstairs I could hear him ring the bell and then we'd run outside!