Shopkeeper’s Bell - used the world over let stores know when a customer has entered.  Or, hang it down low and train your dog to ask to be let out.  Put it in a office so you know when someone arrives. It is alow tech, low cost alarm system for someone entering a house. Or have it just because it makes a nice sound every time your door opens and closes. 
 
Materials: Metal
Made in: USA
Specifications: 4" X 6" x 2"