All three of our dogs have learned to ring a bell (it's actually a windchime) that hangs by the back door to signal when they want to go outside (for potty or otherwise). The teaching is actually pretty simple and straightforward:
1) Hang a bell by the door you'll use to take your dog out for potty.
2) EACH and EVERY time you take your dog out, help him ring the bell by touching it with his nose. If yours is a pup it's much easier because you can lift them and make them touch it.
3) Praise puppy and take puppy outside (helps if a puppy in the middle of potty training to keep puppy on leash for this so he knows it is a business-only outing).
Repeat each time you take puppy outside and soon he'll make the connection. When he makes the connection, he'll likely abuse this and ring RING RING more often than he needs to potty. BUT...do not break the connection by restricting outdoor access. Continue to praise him and take him outside. Eventually the novelty will wear off and only ring if he really needs to go out.
For those with big houses or multiple floors I suppose you could hang one on each floor (puppy shouldn't be floors away from where YOU are anyway) so that where you are you can hear puppy ring. There is also this big door bell button I've seen advertised that is intended to be used outside for your dog to ring to come IN--I think it rings like a doorbell.
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Suppose you could use this INSIDE and teach your dog to paw at it.
If your dog is big and you can't just lift it and help it touch the bells with its nose, then hold the bells out for him or entice him to touch them and soon he'll WANT to even though they are just hanging.
You can purchase fancy "designer" bells at
www.poochie-pets.net but you can also visit a craft store and by the cord and bells and attach them together. Or buy a little windchime like we did!
Anyone else do this with their dog(s)?